Antibodies to Domain 4/5 (Dm4/5) of β2-Glycoprotein 1 (β2GP1) in different antiphospholipid (aPL) antibody profiles

Determination of the three recommended tests for the diagnosis of antiphospholipid syndrome [Lupus Anticoagulant (LA), anticardiolipin (aCL) and anti β2-Glycoprotein 1 (aβ2GP1) antibodies] allow physicians to allocate patients into classification (risk) categories. OBJECTIVES: To measure antibodies of IgG isotype directed towards Domain 4/5 (Dm4/5) of β2GP1. PATIENTS/METHODS: In this cross-sectional study we measured IgG aβ2GP1-Dm4/5 in a group of individuals positive for IgG aβ2GP1 and classified as triple (LAC+, IgG aCL+, IgG aβ2GP1+, n=32), double (LAC-, IgG aCL+, IgG aβ2GP1+, n=23) or single positive (LA-, IgG aCL-, IgG aβ2GP1+, n=10). RESULTS: Geometric mean and standard deviation of IgG aβ2GP1 values expressed as Chemiluminescent Units (CU) in triple, double, single positive groups and in 40 healthy individuals were 1795±783, 321±181, 29±8 and 5.0±1.0, respectively (ANOVA p<0.0001). Geometric mean and standard deviation of IgG aβ2GP1-Dm4/5 expressed as Optical Density (OD) in triple, double and single positive groups and in 40 healthy individuals were 0.16±0.13, 0.16±0.15 and 0.26±0.15, 0.13±0,11, respectively (ANOVA p<0.002). Individuals in the single positive group, expressed significantly higher values with respect to triple (p=0.04) and double (p=0.03) positive groups. Approximate OD cut-off value (99° percentile) calculated in 40 normal control subjects is 0.404. Positivity to IgG aβ2GP1-Dm4/5 according to this cutoff was found in only 5 individuals, 3 in triple positive and 2 in single positive groups and was not associated with thromboembolism. CONCLUSION: Mean level of IgG aβ2GP1-Dm4/5 is higher in single positive group. There is no association between positivity to IgG aβ2GP1-Dm4/5 and thromboembolic events.