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Lupus anticoagulants are immunoglobulins that inhibit phospholipid dependent coagulation tests. These antibodies are not specific for any single coagulation factor, but recognize phospholipid epitopes, thereby inhibiting in vitro formation of phospholipid dependent, activated coagulation factor complexes. Lupus anticoagulants are a common cause of prolonged activated partial thromboplastin times (APTT). The prothrombin time (PT) is relatively insensitive to lupus anticoagulants. Approximately 30% of cases will have a prolonged PT. In vivo, platelets provide a phospholipid surface for coagulation factor complex formation, which is not inhibited by lupus anticoagulants, explaining why most patients do not bleed unless there is concurrent thrombocytopenia, platelet dysfunction, or prothrombin deficiency. Lupus anticoagulants may be associated with autoimmune diseases, drugs (including procainamide, quinidine, chlorpromazine, hydralazine, antibiotics and phenytoin), infections, lymphoproliferative disorders, malignancy, or no underlying disease. The "anti-phospholipid antibody syndrome" or "anticardiolipin antibody syndrome" includes patients with anti-cardiolipin antibodies or the lupus anticoagulant who have clinical features associated with these antibodies, namely venous thrombosis, arterial thrombosis, recurrent pregnancy loss or thrombocytopenia (see Antiphospholipid Antibodies section). Antiphospholipid antibodies and lupus anticoagulant are not always concordant; discordant results will be obtained in approximately 30% of patients, in whom either one of the tests will be positive, and the other negative. This discordance suggests that the two assays may not be detecting the same antibody to phospholipids. When the APTT is prolonged, circulating anticoagulants are distinguished from factor deficiencies by first mixing one part of patient plasma and one part of normal plasma and repeating the APTT on the mixture, both immediately and after one hour's incubation at 37o C (see Anticoagulants, Circulating section). If a lupus anticoagulant is present, the APTT on the 1:1 mix will fail to correct, usually immediately, but in some cases only after an hour's incubation, since lupus anticoagulants may be immediate acting or time dependent. Specific tests for lupus anticoagulants are then performed. The hexagonal phase phospholipid test is more sensitive than the DRVVT (98% vs 70%) for diagnosis of lupus anticoagulants. The DRVVT can be used to make the diagnosis in the occasional case in which the hexagonal phase phospholipid test is negative, but is especially useful in those cases in which the APTT is normal. Heparin and Factor V inhibitors may cause false positive results. To differentiate between a lupus anticoagulant and the presence of heparin, a thrombin time can be performed. It will be prolonged if heparin is present and normal with lupus anticoagulant. Specimen requirements include two 4.5mL blue top tubes (for APTT, mixing studies, hexagonal phase phospholipid test and/or DRVVT as appropriate, and thrombin time to exclude heparin), and one 7.0 mL red top tube for anticardiolipin antibody (IgG and IgM). An assay for anti-?2 glycoprotein I antibody, which is more closely associated with clinical features of the antiphospholipid antibody syndrome, is also available. See also: Factor X Chromogenic Assay - use in monitoring oral anticoagulant therapy in patients with lupus anticoagulants. |
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