What laboratory tests are used to diagnose antiphospholipid antibody syndrome (APS) and when should they be performed in a 42-year-old woman with a prior cerebrovascular accident?

Laboratory Testing for Antiphospholipid Syndrome (APS)

For a 42-year-old woman with prior stroke, you should order concurrent testing for lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG and IgM, and anti-β2 glycoprotein I antibodies (aβ2GPI) IgG and IgM—and critically, any positive results must be confirmed with repeat testing at least 12 weeks later to establish persistence before diagnosing APS. 1

Which Tests to Order

The 2025 ISTH-SSC guidance is unequivocal about the testing panel 1:

First-line mandatory tests include:

• Lupus anticoagulant (LA) testing in citrated plasma
• Anticardiolipin antibodies (aCL) IgG and IgM isotypes
• Anti-β2 glycoprotein I antibodies (aβ2GPI) IgG and IgM isotypes

All three test types must be performed concurrently—not sequentially—as each detects different antibody populations and missing any one can result in underdiagnosis in up to 55% of triple-positive patients 1.

Technical Specifications for LA Testing

LA testing requires a specific 3-step methodology 1:

1. Screening with two parallel clotting tests: dilute Russell's viper venom time (dRVVT) AND activated partial thromboplastin time (APTT)
2. Mixing studies to differentiate inhibitors from factor deficiencies
3. Confirmatory testing with phospholipid-rich reagents

Both dRVVT and APTT must be performed together—omitting either test increases the risk of missing APS diagnosis in 31% of patients 1.

Solid-Phase Assays for aCL and aβ2GPI

These antibodies are measured using ELISA or other solid-phase assays, with results reported as levels above the 99th percentile of normal controls 1. The cutoff is set at the 99th percentile, not arbitrary fixed values 1.

When to Perform Testing

Timing is critical for accurate results:

• Ideally test when the patient is NOT acutely ill and at least 12 weeks after the thrombotic event (stroke in this case), as acute phase reactions can cause transient false-positive results 1
• If the patient is on anticoagulation, LA testing becomes complicated but not impossible:
  • LA testing during warfarin (VKA) therapy should be interpreted with extreme caution and ideally performed 1-2 weeks after discontinuation 1
  • For patients on direct oral anticoagulants (DOACs), pretest DOAC removal procedures can be used 1
  • Alternative tests like Taipan snake venom time (TSVT) or ecarin time (ET) may be used during anticoagulation but have reduced sensitivity 1

Critical Requirement: Confirmation Testing

A single positive test is NEVER sufficient for APS diagnosis. 1

The same antibodies that tested positive initially must be positive again on repeat testing at least 12 weeks apart to rule out transient antibody positivity, which does not constitute APS 1. This is non-negotiable—transient positivity can occur with infections, medications, or acute illness and does not carry the same thrombotic risk.

Interpreting Results: Risk Stratification

The antibody profile matters significantly for risk assessment 1:

• Triple-positive (LA + aCL + aβ2GPI): Highest risk for recurrent thrombosis
• Double-positive (aCL and aβ2GPI with concordant isotype): High risk
• Single-positive: Lower risk, with IgG isotypes more clinically relevant than IgM 1
• Persistent medium/high titers (>99th percentile) are most important for diagnosis 1

LA positivity is particularly critical—it identifies the highest-risk patients and is mandatory for diagnosing triple-positive APS 1.

Tests NOT Recommended for First-Line Diagnosis

The 2025 guidance explicitly recommends AGAINST including these in initial testing 1:

• Antiphosphatidylserine/prothrombin (aPS/PT) antibodies
• Anti-domain I (aDI) antibodies

These may be useful in specific situations (e.g., when LA testing is unreliable or to confirm aβ2GPI specificity) but should not be part of routine diagnostic workup 1.

Common Pitfalls to Avoid

1. Testing during acute illness or within 12 weeks of thrombosis: Can yield false-positive transient antibodies
2. Failing to confirm positive results: Single positive tests are insufficient and lead to overdiagnosis
3. Testing only one or two antibody types: Misses up to 55% of cases 1
4. Interpreting LA results during anticoagulation without appropriate caution: High risk of false negatives or positives
5. Confusing classification criteria with diagnostic criteria: Classification criteria (used for research) are more restrictive than what's needed for clinical diagnosis 1

Clinical Context for This Patient

Given this patient's prior cerebrovascular accident at age 42 (relatively young for stroke), APS should be strongly considered as a potential etiology. The testing should be performed now if she's beyond the acute phase, with careful attention to her anticoagulation status. If she's already on anticoagulation for secondary stroke prevention, coordinate with the laboratory about optimal timing or alternative testing strategies to ensure accurate LA detection.