How is antiphospholipid syndrome diagnosed?

Diagnosis of Antiphospholipid Syndrome

The diagnosis of antiphospholipid syndrome (APS) requires a comprehensive panel of tests including lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG/IgM, and anti-β2 glycoprotein I antibodies (aβ2GPI) IgG/IgM, with positive results confirmed on two separate occasions at least 12 weeks apart. 1

Laboratory Testing Protocol

First-Line Testing

• Complete aPL Panel: All three tests must be ordered concurrently:
  • Lupus anticoagulant (LA)
  • Anticardiolipin antibodies (aCL) IgG/IgM
  • Anti-β2 glycoprotein I antibodies (aβ2GPI) IgG/IgM 1

Lupus Anticoagulant Testing

• Methodology: Requires a 3-step approach:
  1. Screening tests
  2. Mixing studies
  3. Confirmatory tests 2
• Required Tests: Two parallel phospholipid-dependent coagulation tests:
  • Dilute Russell's viper venom time (dRVVT) - first choice
  • LA-sensitive activated partial thromboplastin time (APTT) - first choice 2
• Important Note: Using only one test increases risk of underdiagnosis by up to 31% in APS patients 1

Solid Phase Assays

• aCL and aβ2GPI Testing:
  • Method: ELISA or automated systems
  • Isotypes: Both IgG and IgM must be measured
  • Positivity threshold: >99th percentile of normal controls 1
  • Titer classification:
    • Moderate titer: >40 Units
    • High titer: >80 Units 2, 1

Confirmation of Diagnosis

Persistence Testing

• Mandatory Interval: Repeat positive tests after at least 12 weeks
• Purpose: To rule out transient antibody positivity due to infections or medications 1
• Diagnostic Requirement: Same antibody must be positive on both occasions 2

Risk Stratification

• Triple Positivity: Positive for all three tests (LA, aCL, aβ2GPI) indicates highest risk for thrombosis and pregnancy morbidity 1
• Double Positivity: Especially aCL and aβ2GPI with concordant isotype increases confidence in diagnosis 1
• Single LA Positivity: Carries lower thrombotic risk than triple positivity 2, 1
• Antibody Isotypes: IgG isotypes generally have greater clinical relevance than IgM 1

Special Testing Considerations

Anticoagulant Interference

• Timing of Testing:
  • Ideally test before anticoagulation is started
  • If patient is already on anticoagulants:
    • For VKA: Assess LA 1-2 weeks after discontinuation (if clinically feasible)
    • For DOACs: Consider pretest DOAC removal procedures 1
• Documentation: If testing during anticoagulation cannot be avoided, clearly document this in the laboratory report 2

Result Interpretation

• Cutoff Values: Interpret according to local laboratory cutoff values
• Low Positive Results: Exercise caution with results near cutoff (consider 10% imprecision of solid phase methods) 1
• Clinical Correlation: Always interpret results in relation to clinical symptoms 1

Common Pitfalls and Caveats

1. Premature Testing: Testing too soon after clinical event may lead to false positives due to acute phase reactions
2. Single Test Reliance: Using only LA or only solid phase assays significantly increases risk of missed diagnosis 2
3. Anticoagulant Interference: LA testing can be falsely positive or negative in patients on anticoagulant therapy 1
4. Transient Positivity: Single positive test is insufficient for diagnosis; persistence must be confirmed at ≥12 weeks 2, 1
5. Assay Variability: Significant variability exists between laboratories and assays, emphasizing need for standardized methods 1
6. Classification vs. Diagnosis: Classification criteria (used for research) are stricter than diagnostic criteria used in clinical practice 2

By following this structured approach to laboratory testing and interpretation, clinicians can accurately diagnose APS and appropriately stratify patients according to their thrombotic risk.