What should be looked for in a patient's antiphospholipid antibody panel (with a blue top tube) results, particularly those with a history of recurrent miscarriages or thrombotic events?

Antiphospholipid Antibody Panel Interpretation (Blue Top Tube)

Core Components to Evaluate

When reviewing an antiphospholipid antibody panel, you must assess three mandatory tests: lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG and IgM, and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG and IgM—all performed on the same sample. 1

1. Lupus Anticoagulant (LA) Results

• LA is the strongest single predictor of thrombotic risk and is mandatory for identifying high-risk patients 1, 2
• LA testing requires a 3-step methodology: screening test (typically dRVVT and sensitive aPTT), mixing study, and confirmatory test with phospholipid correction 1, 2
• Report should indicate positive or negative LA status with documentation of any interferences 2
• A positive LA with prolonged screening test, negative mixing study, and positive confirm step indicates presence of phospholipid-dependent inhibitor 1

Critical pitfall: LA testing is unreliable during anticoagulation therapy (warfarin, DOACs, heparin)—if the patient was anticoagulated at time of testing, results may be falsely negative and repeat testing off anticoagulation is essential 1, 3, 2

2. Anticardiolipin Antibodies (aCL)

• Look for both IgG and IgM isotypes measured by ELISA or automated platforms 1
• Under the 2023 ACR/EULAR criteria, moderate titers are ≥40 Units and high titers are ≥80 Units 1
• Any result above the 99th percentile cutoff should be regarded as positive for clinical diagnosis 1, 3
• The presence of aCL and aβ2GPI of the same isotype (both IgG or both IgM) reinforces clinical probability of APS 1

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

• Measure both IgG and IgM isotypes using ELISA or automated solid-phase assays 1
• Same titer thresholds apply: moderate ≥40 Units, high ≥80 Units 1
• aβ2GPI antibodies must be β2GPI-dependent to avoid detection of infection-related antibodies 1

Risk Stratification Based on Antibody Profile

Triple positivity (positive LA + positive aCL + positive aβ2GPI of the same isotype) carries the highest risk for thrombotic and obstetric complications and warrants the most aggressive management 1, 3, 2

Risk Categories:

• High risk: Triple positive (LA + aCL + aβ2GPI, same isotype) 1, 2
• Moderate risk: Double positive (typically LA negative with positive aCL and aβ2GPI) 1
• Lower risk: Single positive (isolated LA, aCL, or aβ2GPI) 1
• Special consideration: In obstetric APS and arterial thrombosis, LA positivity alone (independent of other aPL) is the main predictor of adverse outcomes 1

Confirmation Requirements

Any positive result must be confirmed with repeat testing at least 12 weeks after the initial test to distinguish persistent from transient antibody positivity 1, 3, 2

• The 12-week confirmation requirement applies only to positive results, not negative results 3, 2
• This prevents over-diagnosis from transient antibodies related to infections or other temporary conditions 3

When to Consider Repeat Testing for Negative Results

If initial testing is negative but clinical suspicion remains high (recurrent thrombosis, pregnancy morbidity), consider retesting if: 3

• Initial sample was obtained during anticoagulation therapy 3, 2
• Testing occurred during acute thrombosis (antibodies may be consumed at thrombotic site) 3
• Testing occurred during pregnancy (Factor VIII increases can mask LA by shortening aPTT) 3
• Results were low positive or equivocal near cutoff values (assay imprecision up to 10% can affect classification) 3

Additional Considerations

IgA Isotypes

• IgA aCL and aβ2GPI are not recommended for routine diagnostic testing as their significance remains controversial 1
• There is some association with APS manifestations, especially in SLE, but isolated IgA positivity is rare 1

Emerging Markers (Not for Routine Use)

• Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies show strong association with thrombosis but are not yet recommended for first-line diagnostic workup 1
• Anti-domain I β2GPI antibodies (aD1) are mainly present in triple-positive patients and confirm higher thrombotic risk but do not replace standard aβ2GPI testing 1

Essential Clinical Correlation

Laboratory results must be interpreted in collaboration between clinical pathologist and clinician with full knowledge of: 1, 3, 2

• Anticoagulation status at time of blood draw
• Timing relative to thrombotic event or pregnancy
• Clinical context (recurrent miscarriages, thrombosis at unusual sites, young age)
• Presence of autoimmune diseases (SLE, rheumatoid arthritis)

Be aware of inter-assay and inter-laboratory variability—a sample positive in one assay may not test positive in another laboratory's assay 1