- Last Update On : 2013-01-26
Antiphospholipid antibodies are autoantibodies directed against phospholipid-protein complexes. These antibodies are associated with an increased risk for venous and arterial thrombosis as well as miscarriage. The two main types of antiphospholipid antibodies are lupus anticoagulant and anticardiolipin antibodies. Testing for lupus anticoagulant should be limited to patients who have a significant probability of having the antiphospholipid syndrome or who have unexplained prolonged aPTT in the course of routine coagulation testing. The antiphospholipid syndrome is diagnosed in patients with a history of venous thrombosis, arterial thrombosis, recurrent pregnancy loss or thrombocytopenia in the presence of persistent antiphospholipid antibodies.
Lupus anticoagulants (LA) were first discovered the 1950’s in patients with systemic lupus erythematosis who had prolonged activated partial thromboplastin times (aPTT). Today we know that the name, lupus anticoagulant, is really a misnomer because these autoantibodies are not restricted to patients with lupus and they rarely cause bleeding. 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 or platelet dysfunction. LA may occur in the absence of any known underlying disease or be associated with autoimmune diseases, medications, infections, lymphoproliferative disorders, or malignancy.
Unfortunately, no single clotting test is available that detects the many types of antibodies that constitute LA. To circumvent this problem, the International Society on Thrombosis and Hemostasis (ISTH) has devised a set of diagnostic criteria for LA:
- Prolongation of a phospholipid dependent clotting test designed to be sensitive to LA
- Demonstration of an inhibitor by showing incomplete correction of the prolonged clotting time in a 1:1 mix of patient and normal pooled plasma
- Demonstration of phospholipid dependence by showing shortening of the clotting time by addition of more phospholipid
- Ruling out the possibility of a coexisting specific factor inhibitor
LA prolong various phospholipid dependent clotting times because they bind to phospholipid-protein complexes, which are an essential component of the coagulation cascade.
The most commonly used screening test for LA is the activated partial thromboplastin time (aPTT). When the aPTT is prolonged to >40 seconds, circulating anticoagulants are distinguished from factor deficiencies by mixing one part of patient plasma and one part of normal plasma and then repeating the aPTT on the mixture. Results are read immediately and after one hour incubation at 37o C. If the immediate result is >5 seconds of the normal plasma, the result is reported as no correction. If the immediate mix corrects within 5 seconds of the normal plasma, one hour incubation is performed. Incubated results that are 5 seconds or longer than the normal control are reported as no correction.
If the APTT does not correct, the hexagonal phase phospholipid test (HPPL) is performed as a confirmatory test for LA. This test derives its name from the use of a soybean extract of phospholipid that retains its hexagonal structure and avidly binds LA. If LA is present in a patient’s plasma, a shorter aPTT should be obtained after addition of this phospholipid. A difference of 8 seconds or greater between the tube containing phospholipid and the control tube is interpreted as positive. The combination of a prolonged aPTT that does not correct after a 1:1 mix and a positive HPPL is consistent with a lupus anticoagulant and no further testing is necessary. Direct thrombin inhibitors and Factor VIII inhibitors can cause a false positive reaction and should be ruled out.
If the hexagonal phase phospholipid test is negative, a dilute Russell viper venom time (dRVVT) can be run as an additional screening test. Russell viper venom activates factor X to initiate the common coagulation pathway. Patient plasma is tested with and without extra phospholipid. If LA is present, it binds to phospholipid, thereby prolonging the dRVVT clotting time. If the ratio of patient plasma to patient plasma plus extra phospholipid is 1.28 or greater, the result is reported as positive.
Occasionally, LA results meet some, but not all ISTH criteria for positive LA. When one confirmatory test is positive but another is negative (e.g. positive HPPL and negative DRVVT) a patient most likely has a LA because no single test is 100% sensitive. The positive test should be repeated 12 or more weeks later to determine if it is persistent.
Unlike LA, antiphospholipid antibodies are detected by enzyme immunoassays. APL include anti -cardiolipin and antiB2glycoprotein1 antibodies. They should be ordered at the same time as LA because results are discordant in ~30% of patients.
Specimen requirements include two 4.5mL blue top tubes and one 7.0 mL red top tube of blood.
Revised April 28, 2010