Two different incubation times for the activated partial thromboplastin time (APTT): a new criterion for diagnosis of lupus anticoagulant.

We describe a test for LA based on the specific APTT behaviour of LA plasmas when the incubation time with the APTT reagent is increased from 1 to 20 min. "1-10 APTT" test was defined as the difference (s) between results of the APTT performed with 1 and the one performed with 10 min incubation. A test value > 11 s (upper normal limit determined on 134 normal plasmas) was considered positive for a LA. The test distinguished all the LA patients studied (n = 40) from patients with factor VIIIc inhibitors, patients receiving heparin or warfarin therapy and also patients with congenital factor deficiencies, except those with prekallikrein and factor XII deficiencies. The test detected LA in warfarin (n = 3) and in heparin (n = 2) LA anticoagulated patients. Among 195 patients referred for LA screening, the test detected LA in 5 patients with normal standard APTT. This simple test, using a single reagent for screening and confirmatory procedures is sensitive and fairly specific for LA when combined with mixing studies. However, since the test was defined using one APTT reagent, the performances of other reagents have to be assessed.