What is the appropriate diagnostic and management approach for a patient suspected of having Antiphospholipid Syndrome (APS) with a history of thrombosis or recurrent miscarriages, considering the use of the APAS (Antiphospholipid Antibody Syndrome) panel?

APAS Panel: Diagnostic Testing and Interpretation

For suspected Antiphospholipid Syndrome, order concurrent testing for lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG and IgM, and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG and IgM—all three tests are required to characterize the antibody profile and risk stratification. 1, 2

Required Laboratory Tests

The complete APAS panel must include:

• Lupus anticoagulant (LA) measured using a 3-step methodology (screening, mixing, and confirmation) with parallel testing in both APTT and dRVVT as first-choice clotting tests 1, 2
• Anticardiolipin antibodies (aCL) of IgG and IgM isotype measured by solid phase assays (ELISA or automated systems), with levels above the 99th percentile of normal controls considered positive 1, 2
• Anti-β2-glycoprotein I antibodies (aβ2GPI) of IgG and IgM isotype measured by solid phase assays, with levels above the 99th percentile considered positive 1, 2

All three tests should be performed preferably on the same sample to accurately characterize the patient's antibody profile 1, 2

Critical Timing Requirements

Positive results must be confirmed with repeat testing after at least 12 weeks to distinguish persistent from transient antibody positivity. 1, 2 This confirmation requirement applies exclusively to positive test results and is designed to avoid over-diagnosis from transient antibodies that can occur with infections or drugs 1

Interpretation of Results

High-Risk Profile (Triple-Positive)

Patients positive for LA + aCL + aβ2GPI of the same isotype have the strongest association with thrombotic and obstetric APS and indicate high risk of recurrence or development of first thrombosis 1, 2

Cutoff Values and Reporting

• Each laboratory must use the 99th percentile of a local normal population to determine cutoff values for both aCL and aβ2GPI 1
• LA is reported qualitatively (positive/present or negative/absent) without grading 1
• High-risk patients with definite APS usually have aCL/aβ2GPI values far exceeding 40 U, measured with any method 1
• The 2023 ACR/EULAR criteria use moderate (40 Units) and high (80 Units) titer thresholds rather than the 99th percentile cutoff 2

Special Testing Circumstances

Anticoagulation Interference

• During warfarin (VKA) therapy: LA testing is unreliable and should be interpreted with extreme care; ideally assess 1-2 weeks after discontinuation (with or without bridging to LMWH) 1, 2
• During DOAC therapy: Pretest DOAC removal procedures can be used, with LA testing performed pre- and post-removal 1
• Alternative during anticoagulation: TSVT/ET can be used in patients on anti-Xa DOAC or VKA therapy, though it does not have 100% sensitivity 1

Pregnancy Testing

Factor VIII increases gradually during pregnancy, which can mask LA presence by shortening APTT 1 Additionally, IgG aPL levels may be significantly lower during second and third trimesters 1 Test results obtained during pregnancy should be repeated postdelivery 1

Acute Thrombosis

Antibody levels may decrease during acute thrombosis due to antibody deposition at the thrombotic site, with subsequent increase after the event 1 Testing should be repeated at a distance from the acute thrombotic event 1

When to Retest Negative Results

Repeat testing of initially negative results is warranted when:

• Strong clinical suspicion persists (recurrent thrombosis, pregnancy morbidity) despite negative standard aPL testing 2
• Initial testing was performed during anticoagulation therapy, acute thrombosis, or pregnancy—conditions that can produce false negative results 2
• Results were equivocal or low positive near the cutoff value, as assay imprecision (up to 10%) can affect classification 1, 2

Clinical Application Algorithm

1. Order complete panel: LA, aCL IgG/IgM, aβ2GPI IgG/IgM on same sample 1, 2
2. Assess timing: Avoid testing during anticoagulation, acute thrombosis, or pregnancy when possible 1, 2
3. If positive: Confirm with repeat testing after minimum 12 weeks 1, 2
4. Characterize profile: Determine if single, double, or triple positive 1, 2
5. Correlate clinically: Assess for thrombosis history or pregnancy morbidity to determine if APS criteria are met 2

Important Caveats

• IgA isotypes of aCL and aβ2GPI remain controversial and are not included in standard classification criteria 2
• Test results must always be interpreted in clinical context with close laboratory-clinician interaction 1, 2
• LA conveys the greatest risk for adverse pregnancy outcomes, with relative risk of 12.15 in aPL-positive women 1
• Emerging biomarkers like anti-domain I antibodies (aD1) and antiphosphatidylserine/prothrombin antibodies (aPS/PT) show promise but are not yet standard diagnostic tests 1, 2