Detection of lupus anticoagulant by means of dilute Russell's viper venom time is affected by oral anticoagulant therapy.

The effects of oral anticoagulant therapy on the detection of lupus anticoagulant (LA) were studied in 110 patients with histories of thrombosis. LA was clearly positive in only two (2%) of the patients. Factor defects caused by warfarin therapy did not affect the kaolin clotting time (KCT) or recalcification time (RT) in mixing tests, but markedly affected the dilute Russell's viper venom time (DRVVT), resulting in 45 weak false-positive results. However, the problem was overcome in 93% of these cases by taking account of the basal prolongation of patients' DRVVT and the sensitivity of plasmas to dilution of the phospholipid reagent. A simple mathematical formula is suggested.