How is antiphospholipid antibody syndrome diagnosed?

How is Antiphospholipid Antibody Syndrome (APAS) Diagnosed?

APAS diagnosis requires both a clinical event (thrombosis or pregnancy morbidity) AND persistent laboratory evidence of antiphospholipid antibodies confirmed by repeat testing at least 12 weeks apart. 1

Mandatory Laboratory Testing Panel

You must test for all three antibody types in parallel—never test for just one or two: 2, 1

• Lupus anticoagulant (LA)
• Anticardiolipin antibodies (aCL) IgG and IgM
• Anti-beta2 glycoprotein I antibodies (aβ2GPI) IgG and IgM

Lupus Anticoagulant Testing Methodology

LA testing requires a rigorous 3-step approach performed with TWO parallel screening assays—you cannot skip either one: 2, 1

1. Screening step using BOTH:

   • Dilute Russell's viper venom time (dRVVT)
   • LA-sensitive activated partial thromboplastin time (APTT)

2. Mixing step (performed when screening is prolonged)

3. Confirmation step (phospholipid-dependent correction)

Critical pitfall: Omitting either dRVVT or APTT increases the risk of missing the diagnosis in up to 55% of triple-positive patients and 31% of all APS patients. 2, 3 Many laboratories make this error, leading to significant underdiagnosis.

Never test for LA during anticoagulation therapy—this is the most common source of false-positive and false-negative results. 3 If the patient is already anticoagulated, LA testing becomes unreliable and should be deferred or interpreted with extreme caution.

Anticardiolipin and Anti-β2GPI Antibody Testing

Both aCL and aβ2GPI must be measured using solid-phase assays (ELISA or validated automated platforms) for IgG and IgM isotypes. 2, 1

Positivity thresholds: 2, 1

• Values must exceed the 99th percentile of healthy controls
• Moderate titer: >40 Units
• High titer: >80 Units (per 2023 ACR/EULAR criteria)

The 2023 ACR/EULAR classification criteria abandoned the 99th percentile cutoff calculation in favor of fixed thresholds at 40 and 80 Units. 2

Confirmation of Persistence

All positive antibody results must be repeated after at least 12 weeks (and no later than 5 years) to confirm persistence. 2, 1 This repeat testing serves two purposes: 2

• Excludes transient antibody positivity (which does not constitute APS)
• Ensures reliability of the positive test result, particularly important given poor standardization and potential interferences

Even though triple-positive patients typically maintain persistently abnormal antibody profiles, repeat testing at 3 months remains mandatory. 2

Risk Stratification Based on Antibody Profile

The antibody profile directly determines thrombotic risk and informs management: 1, 4

• Triple positivity (LA + aCL + aβ2GPI all positive): Highest thrombotic risk, requires most aggressive management 1, 4
• Double positivity with concordant isotype (both aCL and aβ2GPI of same isotype, especially IgG): High-risk feature 1
• Single LA positivity: Lower thrombotic risk compared to triple positivity 2, 1
• IgG isotype: Clinically more relevant than IgM 1, 4
• Medium/high titers (>40 Units): Associated with higher risk 1, 4

Clinical Criteria Required for Diagnosis

Laboratory positivity alone is insufficient—you need at least one of these clinical manifestations: 1

Thrombotic events:

• Arterial thrombosis (any site)
• Venous thrombosis (any site)

Pregnancy morbidity:

• Three or more consecutive pregnancy losses before 10 weeks' gestation
• One or more fetal losses at or after 10 weeks' gestation
• Delivery before 34 weeks due to preeclampsia, intrauterine growth restriction, or fetal distress

Classification vs. Clinical Diagnostic Criteria

Important distinction: Classification criteria (used for research) are more stringent than clinical diagnostic criteria. 1, 3 Using classification criteria for routine clinical diagnosis leads to underdiagnosis and may deny appropriate treatment to patients who actually have APS. 1, 3 Clinical diagnosis should be broader to optimize patient management.

Ongoing Monitoring

For patients with confirmed APS, annual retesting of LA, aCL, and aβ2GPI is recommended to evaluate titer fluctuation and changes in antibody profile over time. 3, 4 Close interaction between the laboratory and clinician is essential for proper interpretation of results, particularly regarding anticoagulation status. 2, 1