Causes of High Antiphospholipid Antibodies (aPL)

Introduction to aPL

Antiphospholipid antibodies arise primarily in autoimmune conditions, especially systemic lupus erythematosus, but also occur in infections, malignancies, drug exposures, and in otherwise healthy individuals without any identifiable underlying disease 1, 2

Systemic Lupus Erythematosus (SLE) is the most common autoimmune disease associated with aPL, with approximately 37% of SLE patients testing positive for these antibodies 2

Transient aPL positivity commonly occurs during acute infections, which is why confirmation testing at least 12 weeks apart is required for APS diagnosis 3, 4

Certain medications can induce transient aPL production, requiring repeat testing after drug discontinuation to confirm persistence 4
Drug-induced aPL typically resolve after medication cessation 4

Approximately 6% of pregnant women without diagnosed APS test positive for aPL and are at risk for pregnancy complications including recurrent fetal loss, preeclampsia, intrauterine growth restriction, and fetal distress 1
aPL levels may fluctuate during pregnancy, with 25% of LA-positive patients becoming negative in the second or third trimester 4

High-risk aPL profile includes lupus anticoagulant, double-positive, triple-positive, or persistently high titers, which is significant for risk stratification 5, 6
Low-risk aPL profile consists of isolated anticardiolipin or anti-β2-glycoprotein I antibodies at low-to-medium titers, particularly if transiently positive 5, 6
IgG isotype is generally more clinically significant than IgM, though IgM can be important in obstetric APS 3

Do not diagnose APS based on single positive test - persistence must be confirmed with repeat testing at least 12 weeks apart 3, 4
Timing of testing matters - aPL levels may decrease during acute thrombosis due to antibody deposition at the thrombotic site, and may fluctuate during pregnancy 4
Anticoagulation interferes with LA testing - consider testing for antiphosphatidylserine/prothrombin antibodies as a surrogate when LA testing is unreliable 7
Low positive results near cutoff values should be interpreted cautiously given assay imprecision of up to 10% 4