RECOMBINANT THROMBOPLASTIN INHIBITION ASSAY FOR THE DETECTION OF LUPUS ANTICOAGULANT

Tissue thromboplastin inhibition assay (TTI) is a sensitive test for l upus anticoagulant (LA). We have performed TTI in 12 LA positive patie nts using a new recombinant human tissue factor (Innovin, IN) and comp ared it with Thromborel S (TH), a commonly used human placenta thrombo plastin. The effect of using two different dilutions of each thrombopl astin (1:10 & 1:26) was investigated. A 1:26 dilution discriminated be tter than the 1:10 and this was more evident for Innovin. The mean val ue obtained with a 1:26 IN dilution was statistically different from t hat observed with TH at the same dilution. Furthermore when PT and TTI ratios were considered, differences were statistically significant an d seemed to increase depending on thromboplastin dilutions, When we us ed IN at 1:26 all LA positive patients had a value >1.2. Then we compa red TTI at a 1:26 dilution with dilute Russell's Viper Venom Time (dRV VT) in 50 consecutive patients with suspected lupus anticoagulant not treated with warfarin or heparin. In these patients the diagnosis of l upus anticoagulant was carried out using dilute APTT mixing studies an d a platelet neutralization procedure: four out of 50 patients thus te sted were LA positive. When dRVVT or TTI-I 1 : 26 were used, five pati ents were positive for lupus anticoagulant. Innovin showed similar sen sitivity of dRVVT for detection of lupus anticoagulant. It is likely t hat higher dilutions of thromboplastins could further improve the spec ificity of this method.