Simplified screening procedure for detecting lupus inhibitor.

The lupus inhibitor is an autoantibody with anticoagulant properties, and is found in a high proportion of patients with 'systemic lupus eryth-ematosus (SLE) as well as in patients with a wide variety of other disorders.'3 It is characterised by prolongation of phospholipid dependent coagulation tests due to its binding to phosphodiester linkages on the phospholipid portion of the factor Xa, factor Va, phospholipid calcium complex (prothrombinase).' The clinical importance ofthe lupus inhibitor lies in its strong correlation with recurrent venous and arterial thrombosis and fetal loss due to recurrent abortion.4 In contrast, it is not associated with abnormal bleeding unless a coincident haemostatic defect such as throm-bocytopenia, abnormal platelet function, or hypofibrinogenaemia is present. Historically, a lupus inhibitor was suspected when the activated partial thromboplastin (APTT) was prolonged and the prolongation was not corrected by mixtures of patient and normal plasma.5 More sensitive assays were, however, necessary following the demonstration that a small percentage ofpatients with lupus inhibitor had a normal APTT (or partial thromboplastin time with kaolin, PTTK).367 An example of such a test is the assay developed by Exner et at in which the kaolin clotting time using mixtures of pooled normal and test platelet poor plasma has proved a sensitive system for detecting lupus inhibitors. Like other sensitive tests, however, it is relatively time consuming and, as originally described, the distinction between samples positive and negative for lupus inhibitor depended on interpretation of a graph and was not precisely defined. In view of the increasing number of requests for the lupus inhibitor test this survey was undertaken to re-evaluate the diagnosis of the lupus inhibitor in our laboratory. Our aim was to determine the rate of a positive test, the number of positive tests associated with prolongation ofthe APTT or PTTK, or both, and whether an abbreviated and sensitive screening procedure could be devised so that the laboratory work load could be reduced. Material and methods All samples referred for investigation over a period of 12 months (April 1986 to March 1987) were included in this survey. There were four main clinical indications for the lupus inhibitor test: suspected or diagnosed SLE or other connective tissue disorders; recurrent abortions; recurrent thromboses; and investigation of a prolonged APTT or PTTK. Blood was collected into sodium citrate anticoagulant and centrifuged at 1900 g for 10 minutes to produce platelet poor plasma which was then filtered through a 0-2 ,um …