Choice of standard plasma for diagnosis and quantitation of lupus anticoagulants

We have recently developed a computer-assisted, semi-automated test for lupus anticoagulants (LA), based on the Activated Partial Thromboplastin Time (APTT) and the Russell's Viper Venom Time (RVVT). In this test, plasma samples mixed 1:1 with pooled normal plasma are tested at one low (screening procedure) and one high (confirming procedure) cephalin concentration, using an automated clot timer. The ratio of these two clotting times, divided by the corresponding ratio for normal plasma, was defined as the Lupus Ratio (LR). Based on the LR of several dilutions of a strong LA positive plasma, a standard curve was constructed for LA quantitation in both the APTT and the RVVT system. In the present article, we discuss the use of different LA positive plasma as standards for the identification as well as quantitation of LA. Due to the heterogeneity of LA, the shape and slope of the standard curves as well as the cut-off points between "normal" and "pathological" values varied from one plasma to another. Thus, the result of LA quantitation of a given plasma varied considerably, according to the choice of standard plasma. A strong LA plasma should be chosen as a standard, since any standard plasma only allows the quantitation of plasmas with a lower LR than its own. A pool of several plasmas is also suitable as a standard. In addition to LA quantitation, such a standard could be used to define the cut-off between "normal" and "abnormal" results in each assay, as a certain dilution of the standard plasma. The present study also confirms the greater sensitivity of the APTT as compared to the RVVT for LA detection.