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Antiphospholipid Antibody Syndrome

Antiphospholipid antibodies (APL) are a family of autoantibodies that recognize various combinations of phospholipids, phospholipid-binding proteins or both. These antibodies include lupus anticoagulants, detected by coagulation assays, and anticardiolipin antibodies (ACA), detected by immunoassays. Despite frequent concordance between these types of antibodies, they are not identical. The antiphospholipid antibody syndrome (APS) refers to the clinical association between these antibodies and a syndrome of hypercoagulability. The syndrome may be “primary”, in patients without another autoimmune disease, or “secondary”, associated with an autoimmune disorder, especially systemic lupus erythematosus. Between 20 and 30% of patients with systemic lupus erythematosis have persistent antiphospholipid antibodies that are associated with clinical sequelae.

Diagnosis of this syndrome can be challenging, as APL are relatively common among healthy control subjects (1-5% prevalence for both ACA and lupus anticoagulants), and this prevalence increases with advancing age. Clinically insignificant APL may occur in association with other conditions such as infections or drug use. Correct diagnosis of APS has important implications regarding the use, choice and duration of anticoagulant therapy.

Stroke and transient ischemic attack are the most common arterial events in patients with the antiphospholipid syndrome. Patients with venous thromboembolism most commonly present with lower extremity deep vein thrombosis, pulmonary embolism, or both. Antiphospholipid-antibody related complications of pregnancy generally develop after 10 weeks of gestation. 

Antiphospholipid antibody syndrome should be included in the differential diagnosis if a patient presents with thrombosis at a young age, with an unusual site of thrombosis, recurrent thrombosis, with late pregnancy loss, with early or severe preeclampsia, or with the HELLP syndrome.

The revised Sapporo criteria for classification of the antiphospholipid syndrome state that the disease is characterized by thrombosis, pregnancy complications, or both in patients with persistent antiphospholipid antibodies (lupus anticoagulant, anticardiolipin antibodies, or anti-beta-2 glycoprotein 1 antibodies). However, these criteria do not include the full spectrum of clinical findings seen in patients with APS. Other clinical manifestations include: thrombocytopenia, prolonged aPTT, hemolytic anemia, thrombotic microangiopathy, cardiac valve vegetations, livedo reticularis or racemosa, and cognitive dysfunction.

Laboratory testing for APS includes coagulation assays for lupus anticoagulant (LA) and immunoassays for anti-cardiolipin (ACA) and anti-beta2 glycoprotein (anti-B2GPI) autoantibodies. LA tests correlates better with clinical events than do anti-cardiolipin anti-beta2 glycoprotein tests. 

No single clotting test is available that detects the many types of antibodies that constitute lupus anticoagulants (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 37oC. 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 (ACA) and anti-Beta 2 glycoprotein1 antibodies (anti-B2GPI). They should be ordered at the same time as LA because results are discordant in ~30% of patients.

High titer ACA IgG or IgM antibodies (40 GPL or MPL) and high titer anti-B2GPI IgG or IgM (99th percentile) antibodies correlate better with clinical events than do lower titer 20-39 GPL or APL) antibodies. IgG antibodies are more strongly associated with clinical events than is IgM. Isolated moderate to high-titer IgA ACA or anti-B2GPI antibody is rare and of unknown clinical significance.

Assessment of the entire APL profile has diagnostic implications and helps risk stratify patients who are persistently positive for APL. Persistence is defined by the revised Sapporo criteria as the presence of APL on two or more occasions at least 12 weeks apart.  A high risk APL profile is more likely to remain positive when repeated, independent of the timing and provides more confidence in the diagnosis. However, both high and moderate risk APL profiles are clinically important.

  • A high-risk profile is defined as a positive LA test with or without a moderate to high titer of ACA IgG or IgM and/or anti-β2GPI IgG or IgM
  • A moderate-risk profile is defined as a negative LA test with a moderate-to high titer of ACA IgG or IgM or anti-β2GPI IgG or IgM
  • A low-risk profile is defined as a negative LA test with a low titer of ACA or anti-β2GPI IgG or IgM

False positive LA results may occur in patients treated with warfarin, heparin, or direct oral anticoagulants. LA tests should not be ordered for patients on these anticoagulants or should be interpreted with caution if performed.


Garcia D and Erkan D. Diagnosis and management of the antiphospholipid syndrome. New Engl J Med 2018;378:2010-2021

Miyakis S et al. International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). J Thromb Haemost 2006; 4: 295-306.

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