What tests should be ordered to diagnose antiphospholipid antibody disorder?

Diagnostic Testing for Antiphospholipid Antibody Syndrome

Order all three tests simultaneously: lupus anticoagulant (LAC), anticardiolipin antibodies (aCL) IgG/IgM, and anti-beta2-glycoprotein I antibodies (aβ2GPI) IgG/IgM, and repeat positive tests at least 12 weeks later to confirm persistence. 1

Required Laboratory Tests

Comprehensive testing requires all three assays performed together, as triple-positive patients carry the highest thrombotic risk and missing any single test can lead to underdiagnosis in up to 55% of cases. 1, 2

1. Lupus Anticoagulant (LAC)

• Must use a 3-step methodology: screening tests, mixing studies, and confirmatory tests 3, 2
• Requires parallel testing with both activated partial thromboplastin time (APTT) and dilute Russell's viper venom time (dRVVT) as first-choice clotting assays—omitting either test significantly increases risk of missed diagnosis 2
• Report results as positive or negative with warnings about potential interferences 3
• Critical caveat: LAC testing is unreliable in patients on anticoagulation; ideally assess 1-2 weeks after discontinuing vitamin K antagonists (with or without LMWH bridging) 2

2. Anticardiolipin Antibodies (aCL)

• Test both IgG and IgM isotypes in plasma or serum 1
• Use solid phase assays (ELISA or automated systems) 1
• Positivity defined as values >99th percentile of normal controls 1
• Must be β2-glycoprotein I-dependent to avoid false positives from infections or drugs 1

3. Anti-Beta2-Glycoprotein I Antibodies (aβ2GPI)

• Test both IgG and IgM isotypes in plasma or serum 1
• Use solid phase assays (ELISA or automated systems) 1
• Positivity defined as values >99th percentile of normal controls 1

Confirmation Testing Protocol

All positive tests must be repeated at least 12 weeks apart to rule out transient positivity and confirm persistent antibody presence. 1, 3, 2 This temporal requirement is mandatory for diagnosis—a single positive test is insufficient.

Interpretation and Risk Stratification

High-Risk Profiles

• Triple positivity (LAC + aCL + aβ2GPI): Highest thrombotic risk 3, 2
• Double positivity (aCL and aβ2GPI with concordant isotype): Significantly increases diagnostic confidence 3, 2
• IgG isotype antibodies are clinically more relevant than IgM 3, 2
• Medium/high titers (>99th percentile) are most diagnostically important 3, 2

Lower Clinical Relevance

• Single positive IgM antibody alone is considered less clinically relevant than IgG positivity 3, 2
• Isolated LAC without ELISA positivity carries lower thrombotic risk than triple positivity 1, 2

Critical Pitfalls to Avoid

Anticoagulation interference: LAC testing can be unreliable in patients already on anticoagulants, but most patients with thromboembolism will already be anticoagulated before testing is ordered. 4 For patients on direct oral anticoagulants (DOACs), pretest DOAC removal procedures can be used; for vitamin K antagonists, Taipan snake venom time/ecarin time can be used, though sensitivity is not 100%. 2

Laboratory-clinician collaboration: Results must be reviewed jointly between a clinical pathologist and clinician skilled in interpreting the data, as laboratory results require clinical context including anticoagulation status. 1, 2

Low positive results near cutoff: Interpret with caution due to potential 10% imprecision of solid phase methods. 2

Assay variability: High variability exists between commercially available aCL and aβ2GPI assays in classifying samples as positive or negative. 1

Tests NOT Recommended for Routine Use

Other antiphospholipid antibody tests are not recommended yet for standard diagnostic workup. 1 Emerging tests like antiphosphatidylserine-prothrombin (aPS/PT) antibodies and thrombin generation assays may be considered only in research settings or when standard tests are negative despite high clinical suspicion. 3