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Blood Testing Guide

Everything you need to know about your blood and having a blood test.

Reviewed by Dr Sam Rodgers

6th December 2019

Contents

What is blood and what does it do?

Chapter 1

What is blood and what does it do?

What can you find out from a blood test?

Chapter 2

What can you find out from a blood test?

Understanding your blood test results.

Chapter 3

Understanding your blood test results.

What can influence your blood test results?

Chapter 4

What can influence your blood test results?

How does a blood test work?

Chapter 5

How does a blood test work?

Accuracy and reliability

Chapter 6

Accuracy and reliability

Glossary of blood testing terms

Chapter 7

Glossary of blood testing terms

Red blood cells in travel in an artery

Chapter 1

What is blood and what does it do?

We all have several litres of blood flowing through our veins, but how often do we stop and think about the important role blood plays in our wellbeing? Read on to learn about blood and what it does.

What are the main functions of blood?

Your blood has four main functions – it is responsible for transportation, fighting disease and infection, clotting and body temperature.

Transport system

Your blood is the main transportation system for your body, carrying oxygen, hormones and nutrients to the cells and tissues that need them. It is also responsible for carrying carbon dioxide back to the lungs where it is exhaled, and waste and toxins to the liver and kidneys where they can be filtered and cleared out.

Fighting infection

Your blood carries the white cells that fight infection to the site of injury. You have five different types of white cell and each one targets a different type of infection. For example, your neutrophils target bacteria and fungus, whereas your basophils target allergic reactions.

Clotting

Your blood forms clots at the site of an injury to ensure that blood loss is controlled.

Regulating body temperature

Your blood is responsible for maintaining body temperature by absorbing heat and moving it around the body.

What is blood made of?

Blood has four main components – plasma, red blood cells, white blood cells and platelets, each with their own roles.

Plasma

Plasma makes up around 55% of blood and is the liquid component containing over 700 proteins and other substances including hormones, fats, carbon dioxide, vitamins and glucose.

Red blood cells (also called erythrocytes and RBCs)

Red blood cells contain haemoglobin – a protein that distributes oxygen to the cells of the body and gives the blood its red colour. Red blood cells also carry waste carbon dioxide back to the lungs.

White blood cells (also known as leukocytes and WBCs)

White blood cells are essential for good health and protection against illness and disease. When the body is in distress and a particular area is under attack, white blood cells rush to help destroy the threat. There are a variety of different white blood cell types each with different roles in the immune response.

Platelets

Platelets are crucial in helping the blood to clot when the body is injured. If a blood vessel is damaged, it sends out signals to platelets. They respond to this signal by quickly finding their way to the site of damage and forming a blood clot.

What is a blood group?

Although all blood may look the same, its exact composition is not the same in everyone. The surface of each red blood cell is coated with a combination of sugars and proteins called antigens. The absence or presence of particular antigens (which is determined by the genes you inherit from your parents) determines your blood type. There are four main blood groups, referred to as the ABO system: O, A, B and AB. Blood groups are very important when it comes to blood transfusions – receiving blood from the wrong ABO group can be life-threatening.

The rhesus (Rh) system is also important in blood transfusions. There are five main rhesus antigens on red cells with Rh D being the most important. The D antigen on red blood cells is what gives individuals the positive (+) or the negative (-) after the letter A, B, AB or O of their blood group.

Blood test tubes in a rack

Chapter 2

What can you find out from a blood test?

Your blood is an invaluable window into your inner health. Let’s take a look at some of the things you can learn from a blood test.

Almost any molecule that is in your blood can be isolated and measured by a blood test. This means that we can study the different types of blood cells themselves as well as things that are being transported in the blood like nutrients, hormones and waste products.

A blood test gives you a snapshot of your health at any point in time. You may have a blood test as part of a regular check-up, to help investigate symptoms and diagnose a condition or to keep track of your risk of long-term chronic conditions like heart disease and diabetes. The NHS report that a laboratory blood test is involved in over 70% of the diagnoses made in a GP surgery or hospital.

What’s in a routine blood test?

A routine blood test is one that your doctor is most likely to order to investigate a wide range of symptoms and conditions. The two main components of a routine blood test are a haematology profile and a biochemistry profile. The NHS performs an incredible 500 million biochemistry and 120 million haematology tests every year. And yet most patients have no idea what they’re being tested for and why. Below we explain what is included in these profiles and why they are so commonly tested.

Haematology profile

The main purpose of a haematology blood test is to look at the different types of blood cells – namely your red cells, white cells and clotting cells. This is also commonly known as a full blood count (FBC) or complete blood count (CBC).

Red Blood Cells

This test looks at the size, shape and distribution of your red blood cells. One of the primary roles of the red blood cells is to transport oxygen around the body. An examination of your red blood cells is used primarily to diagnose anaemia – a very common condition which can be caused by insufficient dietary iron or vitamin B12 and folate. The results of this test might explain your lack of energy and general unexplained fatigue.

White Blood Cells

Your white blood cells are a measure of your ability to fight infection – it is common to see your white blood cell count rise in response to a viral or bacterial infection and then normalise once the problem is resolved.

Clotting Cells

Your clotting cells are essential to prevent uncontrolled bleeding, but sometimes they don't work as they should. If you bruise easily, this may be due to an abnormality with your clotting cells.

Biochemistry profile

The main purpose of a biochemistry blood test is to identify how well your major organs and body systems are functioning.

Liver Function Test (LFTs)

Your liver is one of the most important organs in your body and, depending on your lifestyle, potentially one of the most abused. Your liver performs many vital functions, from storing glycogen to manufacturing cholesterol, to eliminating toxins. Your test results will show whether your liver enzymes are raised which could indicate liver damage, and may be the first signs of liver disease.

Kidney function

Your kidneys are responsible for filtering waste products from your body. The kidney function test, which also measures your level of electrolytes, examines how well they are doing their job. If you have raised blood sugar or diabetes then it is especially important to keep a check on your kidney function.

Proteins

This test looks at the amount of the proteins albumin and globulin that are in your blood which can help in the diagnosis of liver and kidney disease as well as malnutrition.

Uric acid

Uric acid is a waste product which is formed when the body breaks down the purines found in food. Having high levels of uric acid raises your risk of kidney stones and gout.

Diabetes

A diabetes test can either be a snap-shot of the amount of glucose in your blood at a point in time (a blood glucose test), or can look at the average amount of glucose in your blood over a 2-3 month period (HbA1c test). Raised blood sugar is a sign that you are becoming insulin resistant, which means that your body is failing to get the sugar you eat into your cells for energy. Eventually, if you don't bring your blood sugar under control, this will lead to type 2 diabetes, which raises your risks for a host of diseases including cardiovascular disease, cancer and Alzheimer’s.

Lipid profile (cholesterol test)

Your blood lipids are fats which circulate in your blood – you will probably know them as LDL (bad) cholesterol, HDL (good) cholesterol and triglycerides. Having high levels of LDL cholesterol and triglycerides in your blood raises your risk of heart disease, especially if you have low levels of protective HDL cholesterol.

Iron status

Iron status tests measure the amount of iron in your blood as well as your body's capacity to absorb iron. The aim to is to diagnose anaemia or haemochromatosis (iron overload syndrome) an inherited condition which means that the body absorbs too much iron from your food.

Bone health

A routine blood test will look for levels of calcium and phosphate, essential minerals needed for strong bones and teeth as well as muscle (including heart) function.

What else can be measured in a blood test?

There are many other types of blood test that look at different body systems and markers of health.

Endocrinology

Endocrinology tests measure the levels of hormones in your blood. Hormones are secreted by glands into the blood which transports them to the tissues they act upon. The type of test you are taking will depend on what you or your doctor is hoping to find out, but can include tests for thyroid function, reproductive hormones and pituitary hormones.

Immunology

Immunology tests look for evidence that your body is fighting an infection or disease (or has fought one in the past). They look for and measure antibodies produced by your immune system to overcome a certain infection. They can also screen for autoimmune disease, when the body starts to produce antibodies against its own tissues.

Microbiology

Microbiology tests are looking for bacteria or infections in your blood.

Do I have to be ill to have blood test?

Although blood tests are an invaluable tool to help diagnose a condition or investigate symptoms, you don’t have to be ill to benefit from a blood test. Lots of people are now using blood tests as a prevention tool, picking up health risks that can be managed through lifestyle interventions before they develop into symptoms.

Other reasons why people have blood tests include:

  • To investigate a family history
  • To understand their health risks
  • To optimise their nutrition
  • To optimise their sports performance
  • To understand their fertility
  • For occupational reasons (immunity tests and exposure to work-related toxins)

How often should I have a blood test?

This will depend on why you are having a blood test in the first place. Sometimes you will need only one test, at others you might need several or even multiple tests to get on top of a health issue.

Diagnostic tests

If your blood test has successfully diagnosed a condition for which you have been treated and you are no longer symptomatic, then you may not need any follow-up tests. If your test results were inconclusive, your doctor may recommend a repeat test in the next few weeks or months.

Monitoring tests

Blood tests are often used to monitor a condition, to ensure that it is being controlled through medication and/or lifestyle changes. Sometimes a monitoring blood test is looking at the impact of medication on other health markers like your liver function. It is normal for monitoring to be fairly frequent in the 12 months following a diagnosis. Once your doctor is satisfied that your condition is stable testing frequency is generally reduced.

Optimising tests

People who want to optimise their health are not usually ill, but are aiming to meet certain health goals through their diet say or a fitness programme. Someone seeking to optimise their sports performance may choose to test at different points in their training cycle to see how well they are adapting to the additional training stresses. People looking to optimise their diet and lifestyle may want to test before and after they make significant changes like becoming vegan, giving up alcohol or starting running. Some changes may show up in your blood quite quickly, and others may take longer. It is best to wait at least 3 months for any changes to be seen.

Understanding your blood test results.

Chapter 3

Understanding your blood test results.

Most laboratory results are a sea of numbers – which mean little if you are not medically-trained. Here we shine a light on how test results are reported and what they mean.

Some of your results will be reported according to whether they fall inside or outside of a normal range, whereas others (like detecting a virus) might be reported as “detected” or “not detected”.

How are laboratory ranges set?

The "normal" range for any given population is usually the range within which 95% of that population falls. This means that 2.5% of that population will be above the normal range and 2.5% will be below the normal range. At the edges of the normal range there are areas of overlap where healthy individuals will have results outside the "normal" range and unwell individuals will have results inside the normal range.

Some reference ranges aren’t based on the distribution of results in a given population, but on a level which is optimal for your health. For example, HbA1c – which is a marker for diabetes – is ideally kept below 42 mmol/mol. Above that, you will be classed as having pre-diabetes – and above 48 mmol/mol, full blown type 2 diabetes is diagnosed.

Who sets laboratory ranges?

Laboratory reference ranges are usually set by laboratory itself or by the manufacturer of the laboratory test. This means that ranges can be different between laboratories, especially if they test different populations of people with differing health characteristics or use analysers which are manufactured by different companies.

Can I compare results between different laboratories?

As different laboratories use different analysers and potentially have different reference ranges (and may test at different times), one result can look very different from another. The important thing is not to try to compare results directly, but to look at where your result lies in the range. A result mid-way through the range with one laboratory should also be about mid-way through the range with another. However, there are lots of things that might affect your result from one test to the next, including what time of day you took your sample, whether you have eaten, how hydrated you are and whether you have taken supplements or medication. We discuss what can influence your blood test results in the next chapter.

How will my blood test results be reported to me?

The vast majority of blood test results will be reported according to whether they fall within the test's reference range. Normally, results which fall outside the range are marked by the laboratory with an asterisk. Many consumer health companies and some GPs are now presenting laboratory results in easy-to-read digital dashboards where you can visualise your results and see at a glance whether they lie inside or outside of the normal range.

Some GPs will not have the time to take you through your test results individually and you may not be told anything about the actual values tested, only that they are normal or abnormal. If you are interested in knowing more you have the right to ask your GP for a copy of your results. The NHS app may let you view your GP test results if your GP practice has enabled this feature.

Some of my blood tests results are outside the reference range – should I be worried?

Given that 5% of the universe of normal participants used to establish the range are outside the 95% distribution, it is perfectly possible to have a result outside that range and be considered "normal".

Any abnormal result, however small, can give rise to anxiety which is why it is important that a qualified doctor interprets your results. Most blood tests need to be looked at in the context of other results, your symptoms, your medical history and your lifestyle. One or two abnormalities in otherwise normal results is usually no cause for concern, but sometimes the pattern of results may indicate that a result needs further investigation or treatment.

Blood facts and figures

5 litres

Average amount of blood in the human body

500m

Biochemistry tests performed by the NHS every year

NHS

120 days

Average lifespan of a red blood cell

4.9 m

Results Medichecks has delivered to customers

What can influence your blood test results?

Chapter 4

What can influence your blood test results?

Answer – lots of things, some of which may surprise you!

There are numerous (literally thousands) of factors that can affect blood test results and laboratories have a great deal of experience in dealing with many of these "interferences". Some we can plan for, like whether you fast before collecting your sample, while others may only become apparent when we’re investigating an unexplained result.

Before you take your blood test you will told whether there are any special instructions you should follow so that you can be confident of an accurate result.

Things that can affect your result include:

Fasting

Some tests will be influenced by what you have just eaten. Certain tests for diabetes, like insulin and glucose, will be affected by a recent meal. Tests like this usually require that you take the test after fasting for 8 hours, usually overnight.

Hydration

Your level of hydration can also affect certain markers, especially those for kidney health – e.g. electrolytes such as sodium and potassium as well as urea. Certain red blood cell and liver markers can also be impacted by hydration.

Exercise

Strenuous exercise can cause certain liver and muscle enzymes to be released into your bloodstream and can also cause testosterone levels to be temporarily high or low. If you want a true picture of your liver and hormone health it is best to wait a few days after exercise to take a blood test.

Supplements

Taking vitamin or mineral supplements will almost always cause your blood levels of that vitamin or mineral to rise, unless you have a problem absorbing nutrients in food. Biotin (vitamin B7) is known to interfere with some laboratory tests as it is used in the testing process itself. If your test is affected by biotin then you will be advised to stop taking it for a week before your test so that you can be confident of an unaffected result.

Medications

Numerous drugs and medications can influence your blood test results in many ways. It is always best to let your doctor or testing company know what medications (prescribed or otherwise) you are taking so that they can put unusual results into context.

Age

Your age can influence things like your hormone levels and the laboratory will have different reference ranges for some tests according to the age range you fall into. For women, knowing where you are in your menstrual cycle or whether you are post-menopausal will also affect your reference ranges.

Time of day

Some hormones are very dependent on the time of day – both cortisol and testosterone levels are highest in the morning and will decline over the rest of the day. Both are best taken around 9am.

Diet

Some tests require you to have eaten specific foods to be able to give you an accurate result. This is particularly true of coeliac tests which are looking for an immune response to gluten. For the test to work properly, you need to have eaten gluten for at least 6 weeks before taking your test.

Illness

Being ill (or having just been ill) can affect lots of blood test results. Your white blood cells are likely to be temporarily high or low, inflammation and liver markers can be elevated and your thyroid hormones can be affected as your body adjusts your metabolism to help you conserve energy and recover. It is best to wait a few weeks (6 weeks for a thyroid test) after any acute illness before having a blood test.

Hormone gels

Hormone gels can be absorbed into the skin of the fingers of the hand you use to apply the gel. If you are taking a hormone test with a finger-prick sample, it is common to see elevated results if you take the sample from the same finger you use to apply the gel. This is because the sample can become contaminated with the hormones absorbed in your finger.

Pets or farm animals

Yes, we’re not kidding. Having a pet or coming into contact with certain farm animals can, in a very small number of cases, affect your blood test results. Lots of blood tests are performed using immunoassays – tests which use antibodies to bind to the molecules in blood which are of interest so that they can be measured. Animal antibodies are often used in immunoassays – typically goat, sheep or mouse antibodies – but if you already have those antibodies in your blood through previous contact with the animal concerned, often when you were young, it can interfere with the blood test and produce an unreliable result.

Fortunately, this is very uncommon and laboratories have developed strategies to overcome antibody interferences. On very rare occasions it is not possible to eliminate the interfering antibody effect and doctors will revert to a good clinical history and examination to assess whether there is an underlying issue or will use alternative tests.

What should I do if I get an unexpected result?

If a result looks unusual the first thing your doctor will do is repeat the test. Often, an unusual result will revert to normal when it is tested again, especially if there were any factors (e.g. illness, exercise, medications) that may have interfered with the original result.

How does a blood test work?

Chapter 5

How does a blood test work?

Learn everything you need to know about preparing for and taking your blood test.

Before your blood can be tested it needs to be removed from your body before being sent to a laboratory. There are two main ways to collect a blood sample:

Venous sample

Collecting a venous sample uses a needle to puncture a vein, usually in your arm, to collect blood into a tube and it allows a larger volume of blood to be collected than by finger-prick. You should only have your blood taken in this way by a phlebotomist – someone who is trained and qualified to collect a venous sample.

Finger-prick sample

When taking a finger-prick sample a lancet is used to prick your finger and allows you to collect a small blood sample, by gently ‘milking’ your finger. You can do this type of test yourself and it’s less invasive than a venous sample. However, while it can be used to perform a wide range of blood tests it’s not possible to test every biomarker, especially if a larger volume of blood is required.

Where can I get a blood test?

NHS blood tests

Blood testing is one of the many services that the NHS provides and can be done at your GP surgery or your local hospital. Blood testing helps to investigate symptoms and diagnose a condition. A trained clinician will take your blood, usually from a vein in your arm, and the blood will generally be sent to an NHS laboratory to be analysed.

Private blood tests

There is a wide range of blood tests available privately that can be ordered online and taken at home or where you can go and have a sample collected by a phlebotomist. Many private clinics, hospitals and pharmacies now offer blood collection services. The laboratories used for private blood tests use the same methodologies, operate to the same standards and are accredited in the same way as NHS labs. In some cases they are the same labs that the NHS uses.

Preparing for a blood test

To make your sample collection a smooth process there are a few things you can do to help increase blood flow:

  • Drink plenty of water – staying hydrated will make it easier to collect your blood sample
  • Get warmed up – try having a hot shower before your collect your sample (or immerse your hands in hot water) or try jumping on the spot for a minute (this is especially useful for finger-prick blood collection)
  • Stand up – when taking a finger-prick sample let gravity help you by standing up

Does a blood test hurt?

Many people are nervous before they have a blood test and this is completely normal. The process itself is quick and straightforward. With a venous blood test, you will feel a little scratch and pinch when the needle goes in but drawing the blood isn’t really painful. It is normal for your arm to bruise afterwards at the site where the needle went in. For a finger prick blood test, as the name suggests, you will feel a scratch and pinch on your finger in the area where the lancet went in. This area may be slightly tender for a little while after taking your sample.

Will there be enough blood from a finger prick sample?

Some private blood tests require only a small finger prick sample. The option of a finger prick sample will only be available if the laboratory is able to analyse results from only a few drops of blood – usually 8-10 are required to fill a finger-prick tube. This method is now well established although occasionally the volume of blood collected is too small for it to be tested – usually when the individual concerned is taking their own blood for the first time.

What time of day should I take my blood test?

Unless stated otherwise, try and take your blood sample in the morning. For hormone tests this is really important because hormone levels are at their highest in the morning and this will give the best results. If you need to fast before taking your blood test it is often easiest to fast overnight while you are asleep.

Will a blood sample degrade in the post?

Generally, blood markers are stable for a period of several days before they start to deteriorate – so it is perfectly acceptable to test a sample that has been sent through the post. Markers that start to degrade more quickly (such as potassium and homocysteine) are not suitable for postal samples, nor are those like calcium and insulin which deteriorate quicker when the temperature rises (e.g. during the summer).

How long does it take to get blood test results?

Blood test results can take anything from a few hours to a few weeks to be processed, depending on the test. If you’ve had a blood test through the NHS or a private specialist, your results will be interpreted by your clinician and shared with you. If you have tested with a digital consumer health company, then your results will typically be interpreted by a doctor and then uploaded online where you can view them.

Do I need a doctor’s referral for a blood test?

Many private blood tests do not require a doctor’s referral. For more specialist tests a doctor’s referral may be required.

What causes blood sample errors?

Blood sample errors are a common occurrence. They can happen for a variety of reasons and usually mean that a sample cannot be tested, or the full range of tests cannot be performed. Although they are most likely to occur if a sample has been collected via finger-prick, errors can still arise with a venous sample. Here are the three most common causes of blood sample errors and how you can help to avoid them.

Clotted sample

The tubes used to collect a blood sample contain an anticoagulant which is coated onto the inside surface of the tube. Often it is invisible to the eye. Lavender-topped tubes for example contain EDTA which binds to the calcium in the blood and prevents it from clotting. Clotting in a sample can occur if your blood does not mix properly with the anticoagulant or if it takes too long to collect your sample and the blood clots before it can mix. After collecting your sample and putting the top on you should invert the tube 10 times to ensure the blood has mixed properly with the coating. Don’t shake the sample vigorously as this may lead to damage of the blood cells which can also affect testing.

Haemolysed sample

A haemolysed sample occurs when some of the red blood cells burst and the haemoglobin within the cells escapes and spoils the surrounding sample. With finger-prick samples, this can happen if you squeeze your finger too hard to get the blood out or if you scrape your finger on the side of the tube rather than letting the droplets drop down the tube gently. Venous blood tests can also be affected by haemolysis when there are difficulties in taking the sample, although this is far less common.

Insufficient sample

When your sample is received at the laboratory it is spun in a centrifuge and separates into two components – the blood cells and the liquid (plasma) that surrounds the cells. If the sample of blood taken is too small then the laboratory will struggle to obtain enough plasma from it for testing. This usually occurs if the blood tube has not been filled to the upper line.

Inside one of our laboratories

Chapter 6

Accuracy and reliability

How do I know that my test results are right?  Read on to find out how laboratories are regulated and monitored.

There are over a hundred NHS hospitals which provide pathology services as well as a large number of private laboratories providing testing services to private GPs as well as the NHS. Doctors treating patients have to be able to rely on the test results they see and consequently there is a rigorous process of internal and external quality control that laboratories must undergo to ensure the accuracy, reliability and comparability of test results.

The Department of Health is ultimately responsible for the quality control and accreditation processes that laboratories must undergo to ensure that they produce accurate and reliable results.

Staff

Regulations require that doctors and scientists working in laboratories are appropriately qualified. For doctors, this means that they should be registered with the General Medical Council (GMC) and for scientists with the Health and Care Professionals Council (HCPC).

UKAS Accreditation

For quality and assurance, laboratories aim for accreditation by UKAS (United Kingdom Accreditation Service) which ensures that the staff who process laboratory tests are qualified and competent, processes and methodologies followed are robust and transparent and that the equipment the laboratories use is safe and fit for purpose.

What is the process of accreditation?

The accreditation of medical laboratories in the UK is handled by UKAS to ISO standard 15189. UKAS is the sole national accreditation body for the UK which is recognised by the government and accreditation ensures:

  • Conformity between laboratories
  • Laboratory staff are competent to carry out diagnostic tests
  • The equipment that is being used is fit for purpose
  • They provide a quality-led service with patient safety at its core

UKAS inspect accredited laboratories at regular intervals and any non-conformances found are raised and the laboratory is expected to rectify the issues and provide evidence that this has been done within three months. Between the main inspections, the laboratories are also subject to surveillance visits to ensure quality management is up to date.

How does accreditation work?

Accreditation is a review of internal quality control (IQC), external quality assessments (EQA), standard operating procedures, education and training. To gain accreditation, laboratories must participate in regular quality audits, clinical governance and accreditation inspections.

Internal Quality Control

Practically, IQC is performed on a daily basis and sometimes more than once per day depending on the test being performed. Samples containing quality control material (very similar to blood but with exact known amounts of test material in the sample) are measured on the analysers. The results produced by the analyser should fall within very strict pre-set limits that agree with the exact amount in the test sample at different levels – i.e. low, medium and high. A patient’s blood sample will only be analysed when the quality material results have passed all the control criteria.

External Quality Assessment

All accredited laboratories are required to participate in an external quality assurance scheme (EQAS) for all the tests that they provide. These schemes are national schemes – i.e. NEQAS (National EQA Scheme) or WEQAS (Wales EQA Scheme). The EQA schemes send out samples to all laboratories either fortnightly or monthly, depending on the tests being checked. These samples contain known amounts of the specific “test” that is being assessed, although the laboratories being assessed do not know the true “target” value and have to measure these samples and report the results to the EQA scheme. If these results are not within certain acceptable limits the laboratory will receive a warning or a “poor performance letter” and if the problem is not rectified, the laboratory may be required to cease testing for the marker in question by UKAS.

What is an acceptable difference between one set of blood test results and another?

All blood tests show a degree of variability even when the same sample is tested on the same analyser on the same day at consecutive times. This is called the analytical variation of a test and is calculated by determining a number called the 'coefficient of variation" (CV). The CV for every test also varies according to the level of the test being performed.

For example, sodium has a CV of 1.5% at a level of 140 which is an acceptable level of variation. (At a sodium level of 110 the CV may be 2% and at 170 it may also be 2%). Therefore, at a true sodium level of 140 the measured sodium may be anywhere between 137.9 and 142.1 – the true result plus or minus 1.5%. On the same day, one laboratory may report sodium on a sample as 142.1 and another laboratory may report the same sample as 137.5 – although there is a difference in the results, it is not considered to be clinically significant.

For some tests such as hormone tests, the variation is usually higher with CVs commonly between 3 - 10%.

Why did my blood test results from Medichecks differ from those my doctor performed?

There are several factors that affect blood test results which can be summarised under the following headings.

Biological variation (intra-individual variation)

In any individual, there are biological (diurnal) cycles that may affect blood results. For example, cortisol levels in the blood are highest at around 8–9 am in the morning and very low at midnight. Similarly, testosterone levels are highest at 8-9 am decreasing throughout the day, so the results may vary depending on the time of day a sample is collected.

Pre-analytical variation

These are variations or changes that may occur before a sample is tested – and may be dependent on the delay in reaching the laboratory, being left in a hot or cold environment during transport or the volume of blood collected. These changes are mostly small and in the majority of cases will not impact on treatment but may cause differences in the results. Occasionally results may be significantly altered by cold, heat, time delay or other factors.

Analytical imprecision

All laboratory analysers show some variation in a similar way that a size 10 dress from different manufacturers varies in size, or the speedometers in two cars may differ at the same true speed. These laboratory variations are termed analytical imprecision and every laboratory has set criteria for what is acceptable and what is not. If an internal quality control sample with a true testosterone level of 10 is measured on the same analyser for 20 times, on the same afternoon, the results may vary between 9.5 and 10.5 (therefore giving values of between 5% above and 5% below the true value). These variations are called the “coefficient of variation” (CV) of an assay and are a measure of the precision of the assay.

Are NHS tests more accurate than private blood tests?

All accredited laboratories are subject to the same strict rules and quality control procedures as one another and therefore NHS blood tests are not inherently more accurate than private tests, or vice versa. Very often NHS laboratories use the same analysers and are part of the same quality assurance schemes as the laboratories used by Medichecks.

Can I trust a finger-prick blood test? Surely a venous sample is more accurate?

There is a misconception that venous samples are more accurate then finger-prick blood testing but this is not always true. Finger-prick samples have been proven to be as accurate as venous samples provided that the samples have been collected correctly and an adequate sample volume has been obtained. There are certain tests where a full venous sample collection is required and where a finger-prick sample would not give an accurate and true result.

How do I know whether the laboratory testing my results is UKAS accredited?

All laboratories that are accredited by UKAS are listed on their website (www.ukas.com). The principal laboratories that Medichecks uses to analyse samples (County Pathology and The Doctors Laboratory) are both accredited by UKAS.

Two women in a library researching

Chapter 7

Glossary of blood testing terms

Medical terminology can be confusing. This glossary explains in simple language the key terms used to describe blood testing

Anticoagulant

A substance which is usually coated onto the inside of blood collection tubes, which prevents the blood from clotting before it can be tested.

Analyser

The analyser is the machine in the laboratory that is used to analyse the sample. There are many manufacturers of laboratory equipment including Roche, Siemens and Abbott.

Analyte

An analyte is a general term that is used to describe the substance or component that is being measured in the sample.

Assay

An assay is the name of the test that is performed in the laboratory to test the components of a sample. Assays are performed on many sample types including blood, saliva, urine or stools.

Biomarker

Biomarker comes from “biological marker”. It is a natural molecule that is found in an organism and can be measured to give an indication of a biological condition or state. Every component (e.g. hormone, protein or vitamin) that is found in a blood sample is considered a biomarker.

Capillary blood sample

A capillary blood sample is collected by puncturing the skin, typically of the finger, to collect a small blood sample. Capillary tests have the advantage that they are more convenient and can often be taken by the patient themselves. However, because of the size of the sample, they are limited in the number of biomarkers they can test for.

Lancet

A lancet is a blade housed in a spring-loaded device which is used to puncture the skin to collect a capillary (finger-prick) sample.

Microtainer

A microtainer is a very small plastic test tube which can be used to collect a finger-prick sample. Microtainers usually have a yellow, lavender or green top. The different colours denote the different types of additive that have been added to the tube to preserve and ensure the quality of the sample when it reaches the laboratory.

Phlebotomy

Phlebotomy is the term used for taking a blood sample from a vein in your arm. Phlebotomy should be performed by a trained health professional (phlebotomist) who works for an organisation which is registered with the Care Quality Commission (CQC).

Reagent

Reagents are substances that are mixed with a blood when the sample is analysed at the laboratory in order for a chemical reaction to occur – the reaction is then measured to produce a test result.

Tourniquet

A tourniquet is a narrow strap that is used by a phlebotomist to find a suitable vein in the arm from which to draw blood. In many cases a tourniquet is not needed, but if veins are deep under the skin or the patient is overweight, using a tourniquet on the upper arm can cause the blood to pool in the vein making it more visible.

Vacutainer

A vacutainer is a glass or plastic test tube used for collecting a venous blood sample. Vacutainers have rubber stoppers on the top which create a vacuum in the tube to control the amount of blood that enters the tube to a predetermined amount. Different coloured stoppers denote the different additives used to preserve and stabilise the sample before it reaches the laboratory.

Venous blood sample

A venous blood sample means that the blood is drawn from a vein in your arm. A venous sample provides a larger volume of blood than a finger-prick capillary sample and can therefore be used to test a larger number of biomarkers.