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

Summary.  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‐β2glycoprotein I), and none out of the 43 with triple positivity (aCL plus lupus anticoagulant and anti‐β2glycoprotein 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.