Determination of the 99^th percentile of anticardiolipin and anti-β2 glycoprotein I antibodies in an institution in the city of Medellin, Colombia

Introduction: Antiphospholipid syndrome diagnosis requires abnormal results of lupus anticoagulant and high titles of anticardiolipin (aCL) and β2glycoprotein I (anti-β2GPI) antibodies. The latter immunological tests lack a standard threshold in clinical practice. Objective: To determine the 99th percentile of aCL and anti-β2GPI in healthy volunteers. Materials and methods: This cross-sectional study reviewed antibody titles of anticardiolipin and β2glycoprotein I (IgG and IgM by enzyme-linked immunosorbent assay) in forty-nine healthy blood donors in Medellin, Colombia. Sociodemographic and immunological variables are also assessed. Antibody titles are described in median and interquartile range and the 99th percentile was estimated. Results: We analysed samples from 16 men and 33 women. We found that the upper limits of the reference range (99th percentile) of aCL and anti-β2GPI were: aCL IgM: 18.0, aCL IgG: 16.1, anti-β2GPI IgM: 16.4, and anti- β2GPI IgG: 6.9. Conclusions: The upper limits obtained differ greatly from the arbitrary classification values suggested in the international guidelines, taking into account that values greater than 40 international units are usually required, and the values identified in this study are between 6.9 and 18 international units. We suggest conducting additional studies that validate cut-off points according to the percentiles explored in this work.