Diagnostic Testing for Antiphospholipid Syndrome
The diagnostic testing for antiphospholipid syndrome (APS) requires concurrent testing for lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG and IgM, and anti-beta2 glycoprotein I antibodies (aβ2GPI) IgG and IgM, with positive results persisting for at least 12 weeks. 1
Primary Diagnostic Tests
Lupus Anticoagulant (LA) Testing
- Must be measured in citrated plasma using a 3-step methodology: screening, mixing, and confirmation 1
- Requires parallel testing with both activated partial thromboplastin time (APTT) and dilute Russell's viper venom time (dRVVT) as first-choice clotting tests 1
- Omitting either APTT or dRVVT increases risk of underdiagnosis in up to 55% of triple aPL-positive samples 1
- Results should be reported as positive or negative with warnings about potential interferences 1
Solid Phase Assays
- Anticardiolipin antibodies (aCL) IgG and IgM isotypes 1
- Anti-beta2 glycoprotein I antibodies (aβ2GPI) IgG and IgM isotypes 1
- Both should be measured in plasma or serum by solid phase assays (ELISA or non-ELISA systems) 1
- Results should be reported with their level; positivity defined as values above the 99th percentile of normal controls 1
Interpretation Guidelines
- Persistence requirement: Two consecutive positive tests at least 12 weeks apart are required to rule out transient positivity 1
- Triple positivity (LA, aCL, and aβ2GPI) or double positivity (aCL and aβ2GPI with concordant isotype) significantly increases confidence in APS diagnosis 1
- IgG isotype antibodies are considered clinically more relevant than IgM 1
- Low positive results around the cutoff value should be interpreted with caution due to potential 10% imprecision of solid phase methods 1
Special Testing Considerations
Testing During Anticoagulant Therapy
- LA testing during anticoagulation may be unreliable but sometimes necessary 1
- For patients on direct oral anticoagulants (DOACs), pretest DOAC removal procedures can be used 1
- For patients on vitamin K antagonists (VKAs), Taipan snake venom time/ecarin time (TSVT/ET) can be used, though it doesn't have 100% sensitivity 1
- Ideally, LA should be assessed 1-2 weeks after discontinuation of VKA (with or without bridging to LMWH) 1
Second-Line Tests
- Antiphosphatidylserine/prothrombin antibodies (aPS/PT) are not recommended as first-line tests but can be useful when LA testing is unreliable or uncertain 1
- Anti-domain I antibodies (aDI) can be used to confirm the specificity and pathogenicity of aβ2GPI 1
- aPS has shown high specificity (87%) and may detect additional APS cases when other aPL tests are negative 2
Common Pitfalls and Caveats
- Inappropriate use of classification criteria may lead to misdiagnosis or underdiagnosis of APS 1
- Classification criteria (designed for research) are stricter than diagnostic criteria (used in clinical practice) 1
- Single positive IgM antibody is considered less clinically relevant than IgG 1
- LA testing can be affected by anticoagulant therapy, acute phase proteins, and other interferences 1
- Laboratory results must be interpreted in clinical context with knowledge of anticoagulation status 1
- Close interaction between the laboratory and clinician is essential for proper interpretation 1