Influence of different IgG anticardiolipin antibody cut-off values on antiphospholipid syndrome classification

Background: While medium to high titers of anticardiolipin (aCL) antibodies, defined as > 40 GPL units or > 99th percentile, is a laboratory criteria for the 'definite' diagnosis of antiphospholipid syndrome (APS), agreement between the two cut-offs has not been validated. Objective: To validate the current aCL laboratory criterion by verifying the effect of the two cut-offs on APS classification. Patients/methods: Ninety aCL positive APS patients were selected on the basis of their GPL values above the 99th percentile (17.4 GPL), which was calculated by testing 100 age- and sex-matched healthy subjects. Results: A significant difference in the IgG positivity (P < 0.0001) was found between the APS laboratory profiles as 20 out of the 24 (83.3%) patients with single positivity (aCL alone), six out of the 23 (26.1%) with double positivity (aCL plus lupus anticoagulant or anti-beta(2)glycoprotein I), and none out of the 43 with triple positivity (aCL plus lupus anticoagulant and anti-beta(2)glycoprotein I) had titers between the 99th percentile and 40 GPL units. Moreover, the rate of aCL values between the 99th percentile and 40 GPL units was significantly higher (P < 0.0001) in patients with pregnancy morbidity (73.7%) as compared to those with vascular thrombosis (16.9%) and those with both conditions (16.7%). Conclusion: The 99th percentile cut-off level seems more sensitive than the > 40 GPL value for APS classification, as it includes subjects with aCL positivity alone as well as patients with pregnancy morbidity.