Laboratory Diagnosis of Antiphospholipid Antibodies

Antiphospholipid antibodies may be associated with venous thromboembolism and/or arterial thromboses or miscarriages; their heterogeneity complicates the laboratory diagnosis. For a long time, they were believed to be directed against anionic phospholipids. More recently, it has been demonstrated that these antibodies are directed at plasma proteins bound most of the time to a phos-pholipid surface (anionic phospholipids). Several cardiolipin antibodies and lupus anticoagulant belong to this variety of antibodies. An extremely rich literature is available on this subject. The laboratory diagnosis of the so-called "Anti-phospholipid Syndrome," is relatively difficult because of the heterogeneity of the antibodies, the large number of laboratory tests which have been advocated, and their standardization which has to be improved for many of them. z Laboratory diagnosis is usually considered in patients with unexplained venous and/or arterial thromboses, recurrent miscarriages, and thrombo-cytopenia, s,6 However, in several subjects the detection of a lupus or lupus-like circulating antico-agulant is made during preoperative laboratory testing, particularly in a child before tonsillectomy. The clinical significance of these latter antibodies is different than that of the previous group of symptomatic patients. The measurement of anti-cardiolipin (aCL) antibodies should be performed in parallel in every patient with a suspected anti-phospholipid syndrome. Many recommendations concerning the preanalytical variables, especially regarding the risk of platelet contamination which may suppress the anticoagulant activity, must be taken into account, z,4,7 The laboratory diagnosis of a lupus anticoagulant (LAC) relies on a four-step procedure for blood examination. 1. In the first step, the prolongation of the partial thromboplastin time with a ratio patient/control superior to 1:20 is detected. Several reagents are available, and the selection of a sensitive test is crucial. 7 2. In the second step, it is essential to demonstrate that the addition of patient's plasma to a control plasma prolongs the activated partial thromboplas-tin time of the latter. In few cases the clotting time is longer than that of the patient. The measurement of the APTT of the control plasma, the pa-tient's plasma, and their mixture (one volume of each) leads to the calculation of the Rosner index, the normal values of which are lower than 13 or 15, according to different investigators. 3. In the third step, a laboratory test is selected in order to demonstrate the antiphospholipid dependence of the LAC. Several reagents have been proposed: phospholipids present in a platelet preparation , hexagonal phospholipids, dilute tissue throm-boplastin reagent, or dilute Russel …