What is the initial workup for a patient suspected of having anti-phospholipid syndrome (APS)?

Antiphospholipid Syndrome Workup

Initial Laboratory Testing

Perform all three core antibody tests simultaneously on the same blood sample: lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG/IgM, and anti-β2 glycoprotein I antibodies (aβ2GPI) IgG/IgM. 1 This comprehensive triple testing is mandatory because each test detects different antibody populations, and triple-positive patients carry the highest thrombotic risk. 1, 2

Core Laboratory Tests

Lupus Anticoagulant (LA):

• Use a standardized 3-step methodology: screening tests, mixing studies, and confirmatory tests 2, 3
• Perform both APTT-based and dilute Russell's viper venom time (dRVVT) assays in parallel, as omitting either test can miss up to 55% of triple-positive cases 3
• Report results as positive or negative (not quantitative) 2
• Critical caveat: LA testing is unreliable in patients on anticoagulation (warfarin, DOACs, heparin), so ideally obtain samples before starting anticoagulation or use specialized protocols 2, 4

Anticardiolipin Antibodies (aCL):

• Measure IgG and IgM isotypes by ELISA or validated automated solid-phase assays 1, 2
• Positivity threshold: values above the 99th percentile of normal controls 1, 2
• The 2023 ACR/EULAR criteria define moderate titer as ≥40 Units and high titer as ≥80 Units 1, 2
• Must be β2GPI-dependent to avoid false positives from infections or drugs 1

Anti-β2 Glycoprotein I Antibodies (aβ2GPI):

• Measure IgG and IgM isotypes by ELISA 1, 2
• Same positivity thresholds as aCL: >99th percentile, with moderate (≥40 Units) and high (≥80 Units) titer definitions 1, 2
• IgG isotype carries stronger clinical significance than IgM 5, 3

Confirmation Testing

Any positive test must be repeated after exactly 12 weeks (minimum) to confirm persistent positivity and rule out transient antibodies. 1, 2 This confirmation requirement applies only to positive results, not negative ones. 2 Single positive tests are insufficient for diagnosis regardless of titer level.

Risk Stratification Based on Antibody Profile

The antibody profile determines thrombotic risk and guides management intensity:

High-Risk Profile:

• Triple positivity (LA + aCL + aβ2GPI, especially with concordant IgG isotype) carries the highest risk 2, 5, 3
• Double positivity with concordant isotype (both aCL and aβ2GPI IgG or both IgM) 5, 3
• High titers (≥80 Units) of aCL or aβ2GPI 1, 2

Moderate-Risk Profile:

• Double positivity with mixed isotypes 5
• Isolated LA positivity 1, 3
• Moderate titers (40-79 Units) 1

Low-Risk Profile:

• Isolated IgM aβ2GPI at low-medium titers 5
• Single positivity for aCL or aβ2GPI at moderate levels 3

When to Retest Negative Results

Do not routinely repeat negative tests unless specific interfering conditions were present during initial testing. 2 Retest negative results only if:

• Initial testing was performed during acute thrombosis (antibodies may be consumed at thrombotic site) 2
• Patient was on anticoagulation during LA testing (causes false negatives) 2
• Testing occurred during pregnancy (Factor VIII elevation can mask LA by shortening APTT) 2
• Strong clinical suspicion persists with recurrent thrombosis or pregnancy morbidity despite negative standard testing 2

Additional Considerations

Emerging Tests (Not Yet Standard):

• Anti-domain I β2GPI antibodies (aDI) confirm higher thrombotic risk in already positive patients but do not replace standard testing 2, 4
• Antiphosphatidylserine/prothrombin (aPS/PT) antibodies may be considered only in patients with strong clinical suspicion who are negative for all three standard tests 1, 2
• Thrombin generation assays remain investigational 1, 4

IgA Isotype Testing:

• IgA aCL and aβ2GPI remain controversial and are not included in current diagnostic criteria 1
• Consider testing only in highly selected cases with strong clinical suspicion and negative IgG/IgM results 1

Critical Pitfalls to Avoid:

• Never diagnose APS on a single positive test—confirmation at 12 weeks is mandatory 1, 2
• Isolated low-positive IgM results have limited clinical significance, especially in thrombotic APS 5
• Results near cutoff values (within 10% of threshold) should be repeated due to assay imprecision 2
• Always interpret results in clinical context with close laboratory-clinician collaboration 1, 2
• Classification criteria (2023 ACR/EULAR) are designed for research; clinical diagnosis should be broader to avoid underdiagnosis 1