According to past and current guidelines from the International Society on Thrombosis and Hemostasis on the laboratory criteria for antiphospholipid antibody syndrome (APS), antiphospholipid antibody (anti-cardiolipin (aCL) and anti-beta2-glycoprotein 1 (aB2GP1)) concentrations are an integral component of the diagnostic criteria for APS. However, external quality control exercises have highlighted high variability between commercially available aCL and aB2GP1 assays. Due to lack of both international standards and common human monoclonal antibodies, guidelines recommend that test results be considered positive if greater than the 99th percentile of a particular population. The objective of this study was to determine the 99th percentile of a New Mexico reference population and compare those against cutoffs recommended by one manufacturer of aCL and aB2GP1 assays.
Whole blood samples in sodium citrate were collected from 120 healthy donors and stored at -70 degrees C for up to 12 months prior to testing. Concentrations of aCL and aB2GP1 were determined using the BioPlex 2200 System (Bio-Rad Laboratories, Hercules, CA). Results were combined with those from 37 samples previously used to verify manufacturer proposed cutoffs of greater than or equal to 20 units for all assays. The results were ranked and the 99th percentile determined. Newly derived cutoffs were applied to 1,118 aCL and 1,140 aB2GP1 results retrieved from our laboratory’s data warehouse over a 12-month period.
See Table. Based on our population’s 99th percentile cut-off values, 27 previously negative individuals would now be labeled positive, whereas only 3 previously positive individuals would now be labeled as negative; the majority of patient results did not change.
Table. 99th percentile determinations from validation and verification studies, and number of positive results using manufacturer’s cut-off (≥20 units) versus 99th percentile cut-offs. (see attachment).
Given guideline recommendations that a local population be used to establish cut-off values, we have changed our cut-off values to the 99th percentile of our population. Interestingly, these cut-offs were similar to those established by Bio-Rad Laboratories during their validation. It is unclear why a uniform value of greater than or equal to 20 units was applied as the FDA-cleared cut-off.
Sosnovske, Dennis; D G. Grenache; P Lowrey; D Rospopo; K Riley; R A. Marlar; and M A. Rollins-Raval. "Establishing new positive ranges for anti-phospholipid antibody tests based on the local population." (2020). https://digitalrepository.unm.edu/hsc_qips/15