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The lupus anticoagulant. High incidence of 'negative' mixing studies in a human immunodeficiency virus-positive population.

We identified 100 patients (51 males and 49 females) as having the lupus anticoagulant. The following diagnoses were found in the patient population: human immunodeficiency virus positivity, 20%; systemic lupus erythematosus, 10%; prolonged preoperative activated partial thromboplastin time (APTT), 10%; procainamide hydrochloride-induced inhibitor, 9%; deep vein thrombosis, 6%; seizure disorders/epilepsy, 5%; and miscellaneous conditions, 40%. Identification was based on a prolonged APTT (> 40 seconds) that normalized with increased phospholipid concentrations and/or a prolonged Russell viper venom clotting time patient-control ratio of 1.20 or greater. In 68 cases (group 1), patient plasma prolonged the APTT of normal plasma in a 1:1 mixing study. However, in 32 cases (group 2), no such prolongation was observed. There was a significant difference between presenting APTTs in patients from group 1 (mean +/- SD, 58.29 +/- 13.30 seconds) compared with that in group 2 (mean +/- SD, 47.93 +/- 5.09 seconds). Furthermore, 66% of group 1 patients had elevated anticardiolipin antibody titers compared with only 41% in group 2. Of the 32 patients in group 2, 16 (50%) were positive for human immunodeficiency virus. We concluded that the investigation of a lupus anticoagulant should not be abandoned because patient plasma does not prolong the APTT of normal plasma in a mixing study, especially in a human immunodeficiency virus-positive population.