Negative Antiphospholipid Antibody Testing in Recurrent Miscarriage and Thrombosis
These negative results do not exclude antiphospholipid syndrome (APS), and you must complete the diagnostic workup by testing for lupus anticoagulant (LAC), which is the third mandatory laboratory criterion and often the sole positive marker in APS patients. 1
Complete the Three-Test Panel Immediately
All three tests (LAC, anticardiolipin, and anti-β2 glycoprotein I) must be performed to diagnose or exclude APS, as each test detects different antibody populations and a single negative test does not rule out the syndrome. 1
Lupus anticoagulant alone can be positive in 26% of thrombotic APS cases where anticardiolipin and anti-β2 glycoprotein I are negative, making it essential for diagnosis. 2
LAC testing requires functional coagulation assays (dilute Russell's viper venom time and activated partial thromboplastin time with mixing and confirmation steps), which are fundamentally different from the solid-phase immunoassays used for anticardiolipin and anti-β2 glycoprotein I. 1
Critical Testing Considerations Before Interpretation
If the patient is currently on anticoagulation therapy (warfarin, DOACs, or heparin), LAC testing will produce unreliable results and should either be performed before starting anticoagulation or using specialized methods. 1, 3
During acute thrombosis or pregnancy, antibody levels may decrease due to antibody deposition at thrombotic sites or Factor VIII increases masking LAC, potentially causing false-negative results. 3
If initial testing was performed under these conditions, repeat the complete panel (all three tests) when the patient is off anticoagulation or after the acute event/pregnancy. 3
Confirm Persistence if Any Test Becomes Positive
Any positive result must be confirmed with repeat testing at least 12 weeks later to distinguish persistent pathogenic antibodies from transient positivity unrelated to APS. 1, 3
This 12-week confirmation requirement applies only to positive results; negative results do not require routine repeat testing unless clinical suspicion remains high or testing conditions were suboptimal. 3
Risk Stratification Based on Antibody Profile
If LAC testing is also negative (triple-negative):
Triple-negative results make APS highly unlikely as the cause of recurrent miscarriage and thrombosis, though non-criteria antibodies may rarely be present. 1, 4
Consider testing for other thrombophilias (Factor V Leiden, prothrombin G20210A mutation, protein C/S deficiency, antithrombin deficiency) and other causes of recurrent pregnancy loss. 4
If LAC is positive (isolated LAC positivity):
Isolated LAC positivity carries significant thrombotic and obstetric risk and meets laboratory criteria for APS when confirmed at 12 weeks. 1, 5
Isolated LAC is strongly associated with thrombosis independent of anticardiolipin and anti-β2 glycoprotein I detection, and is the strongest single risk factor for pregnancy morbidity. 1
The 2023 ACR/EULAR classification criteria assign high weight to persistent positive single LAC. 1
Common Pitfalls to Avoid
Do not assume APS is excluded based on only two of the three required tests—this is the most common diagnostic error. 1
Do not test for non-criteria antibodies (IgA anticardiolipin, IgA anti-β2 glycoprotein I, anti-domain I, antiphosphatidylserine/prothrombin) as first-line tests; these have controversial significance and are not part of diagnostic criteria. 1, 6
Be aware that different commercial assays show high variability in classifying samples as positive or negative, so if clinical suspicion remains high despite negative results, consider retesting at a different laboratory using alternative methods. 1
Low-positive or equivocal results near cutoff values should be repeated due to assay imprecision (up to 10%) that can affect classification. 3