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Venom from the Russell's Viper snake causes blood to clot more quickly, whilst antiphospholipid syndrome slows down the speed at which blood clots. This test compares the time taken for a patient's blood sample to clot when it is mixed with Russell's Viper venom, and compares it to the speed for a control blood sample to clot. By dividing the time taken for the patient's sample to clot by the time taken for the control sample to clot a ratio is created, this helps to correct for the variability that can happen in reference ranges across laboratories. People who have antiphospholipid syndrome will clot more slowly, so the DRVVT ratio will be increased.

What might a high result mean?

An elevated DRVVT ratio may be an indication that antiphospholipid syndrome is present. The likelihood of this is increased if there are also beta 2 glycoprotein antibodies, and anti-cardiolipin antibodies.