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Antiphospholipid Antibodies

Test code(s) 11344, 11345, 36552, 36553, 36554, 4661, 4662, 4663

Question 1. What is antiphospholipid syndrome (APS)?

APS is a clinical, pathologic disease state that includes at least one clinical event and one persistently positive (>12 weeks apart) antiphospholipid antibody test, as shown below.

Clinical Event and Laboratory Test Results table

Question 2. Why isn’t the 99th percentile reported with the β2-glycoprotein (β2-GPI) IgG and/or IgM results so that I will know if the patient result is clinically significant?

The cutoff for positivity in each antiphospholipid antibody assay represents the 99th percentile.

Question 3. Why can't I always get titers to endpoint so I can follow a patient's disease state?

After a certain point, serial dilutions are nonlinear and therefore inaccurate. Consequently results are reported only to the upper limit of the clinically reportable range.

Question 4. What does a positive cardiolipin (ACA) IgG or IgM result ≤40 GPL or MPL mean?

This is a low-positive result and as such is of uncertain significance. Depending on the clinical index of suspicion for APS, repeat testing to confirm the titer may be a consideration.

Question 5. Does a cardiolipin antibody titer >150 GPL always correlate with a positive lupus anticoagulant test?

Not always, although the correlation is higher with increasing cardiolipin IgG antibody titers.

In addition, studies have shown that triple antiphospholipid positivity (ie, cardiolipin antibody β2-GPI antibody, and lupus anticoagulant positivity) correlates more strongly with both thrombosis and pregnancy morbidity than single or double positivity.1

Question 6. What is the significance of a positive phosphatidylserine antibody result?

Phosphatidylserine antibodies have been associated with antibody-mediated thrombotic events, particularly stroke, but have not been incorporated into the classification criteria for the APS.

Question 7. What is the significance of a positive phosphatidylserine IgM antibody when all other tests are normal?

Isolated phosphatidylserine IgM antibodies that are near the cutoff for positivity are often reactive (ie, underlying acute phase response, infectious response) rather than a true positive.

Question 8. Are test results affected if a patient is on an anticoagulant (warfarin, heparin, dabigatran, rivaroxaban, etc.)?

Anticoagulants do not interfere with results of the antiphospholipid antibody assays.

Question 9. My patient has just had a massive thrombosis. Does this affect β2-GPI or ACA antibody results?

A thrombotic event will not likely mask an antiphospholipid antibody, because they are tested on an ELISA-based platform. However, a positive antibody titer may develop secondary to the thrombotic event; thus, testing should be repeated ≥12 weeks after the event to demonstrate antibody persistence.

Question 10. What does it mean when only the IgA isotype is positive?

The IgA isotype has been implicated in smaller studies, but has not yet been incorporated into the antiphospholipid diagnostic criteria. IgA has been reported in African American patients with systemic lupus erythematosus. It may indicate a subgroup of patients with a nonspecific autoimmune disorder who are at risk for specific clinical manifestations.2



  1. Semin Thromb Hemost. 2012;38:348-352.
  2. J Thromb Haemost. 2006;4:295-306.
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
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