What laboratory tests are recommended for a patient suspected of having antiphospholipid syndrome (APS)?

Laboratory Testing for Antiphospholipid Syndrome

For any patient suspected of having antiphospholipid syndrome, you must order all three core laboratory tests: lupus anticoagulant (LAC), anticardiolipin antibodies (aCL) IgG and IgM, and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG and IgM—all performed on the same sample and confirmed with repeat testing at least 12 weeks later if positive. 1, 2

Core Laboratory Panel (All Three Required)

1. Lupus Anticoagulant (LAC)

• Must use both APTT-based and dRVVT-based assays in parallel—omitting either test misses up to 55% of triple-positive cases 3, 4
• Requires a 3-step methodology: screening test, mixing study, and confirmatory test with phospholipid correction 1, 3
• Report as positive or negative with documentation of any potential interferences 3
• LAC is the strongest single predictor of thrombotic risk, particularly when combined with positive ELISA tests 1, 5

2. Anticardiolipin Antibodies (aCL)

• Test both IgG and IgM isotypes using solid-phase assays (ELISA or automated systems) 1, 2
• Positivity defined as >99th percentile of normal controls 1, 2
• Must be β2-glycoprotein I-dependent 1
• Report quantitative levels, not just positive/negative 3

3. Anti-β2-Glycoprotein I Antibodies (aβ2GPI)

• Test both IgG and IgM isotypes using solid-phase assays 1, 2
• Positivity defined as >99th percentile of normal controls 1, 2
• Report quantitative levels 3
• IgG isotype is clinically more relevant than IgM 3, 4

Critical Timing Requirements

• All positive tests must be confirmed on repeat testing at least 12 weeks after initial testing to distinguish persistent from transient antibody positivity 1, 2, 4
• This 12-week confirmation requirement applies only to positive results, not negative results 2
• Ideally, perform all three tests on the same blood sample to accurately characterize the antibody profile 2

Risk Stratification Based on Results

Highest Risk (Triple Positive)

• Positive LAC + positive aCL + positive aβ2GPI of the same isotype carries the strongest association with thrombotic and obstetric APS 2, 3, 4
• These patients warrant the most aggressive management 4

Moderate-High Risk (Double Positive)

• Positive aCL and aβ2GPI with concordant isotype significantly increases diagnostic confidence 3, 4

Lower Risk (Single Positive)

• Isolated LAC without positive ELISA tests carries lower thrombotic risk than triple positivity 1, 3
• Single positive IgM antibody is considered less clinically relevant than IgG 3

Common Pitfalls and How to Avoid Them

Anticoagulation Interference

• LAC testing is unreliable during anticoagulation therapy 2, 3
• For patients on DOACs: use pretest DOAC removal procedures 3
• For patients on warfarin: ideally test 1-2 weeks after discontinuation (with or without LMWH bridging), or use Taipan snake venom time/ecarin time (though sensitivity is not 100%) 3
• Heparin interferes with LAC testing 2

False Negative Results

• Factor VIII increases during pregnancy can mask LAC by shortening APTT 2
• Antibody levels may decrease during acute thrombosis due to antibody deposition at the thrombotic site 2
• If strong clinical suspicion persists with negative results obtained during anticoagulation, pregnancy, or acute thrombosis, repeat testing under optimal conditions 2

Borderline Results

• Low positive or equivocal results near the cutoff should be repeated, as assay imprecision (up to 10%) can affect classification 2, 3

Single Test Approach

• Never rely on a single test type—comprehensive testing with all three assays is mandatory because no single test has sufficient sensitivity and specificity 1, 2

Tests NOT Currently Recommended

• IgA isotypes of aCL and aβ2GPI remain controversial and are not part of classification criteria 2
• Anti-domain I β2GPI antibodies (aD1) may confirm higher thrombotic risk in triple-positive patients but do not replace standard aβ2GPI testing 2
• Antiphosphatidylserine/prothrombin antibodies (aPS/PT) show potential value but need further confirmation before routine use 2
• Other antiphospholipid antibody tests are not recommended yet 1

Essential Collaboration

• Laboratory results must be interpreted in collaboration between a clinical pathologist and a clinician skilled at interpreting the data, with full knowledge of anticoagulation status and clinical context 1, 2, 3