Cut-off levels based on deviation from standard negative control is better than moderate level based on fixed cut-off for ACA assessed using ELISA for the diagnosis of antiphospholipid syndrome.

Aim: To assess the consistency of the standard negative control of IgG and IgM ACA levels within runs and batches of tests, and levels of ACA agreement between those established according to deviation from standard negative control and those established based on a fixed level cut off.

Methods: Serum samples of 148 patients who presented an INR < 0.9 or prothrombin activity of > 130% or aPTT below 0.8 times control or thrombosis with aPTT below 1.2 times control were tested in a 22-time running test to determine IgG and IgM ACA levels using Quanta Lite ACA IgG (HRP) and Quanta Lite ACA IgM (HRP) commercial reagents.

Results: Coefficients of variant within runs and batches of standard negative control IgG and IgM ACA levels were 19.30% and 29.17% respectively. Using kappa statistics to determine degree of agreement between cut-off levels by deviation from standard negative control and fixed cut-off level of ACA identified using ELISA, the disagreement in IgM and IgG were k 0.30, and 95% CI of k 0.27 to 0.34 (z = 1.033, p = 0.3015), and k 0.63, and 95% CI of k 0.53 to 0.73 (z = 1.411, p = 0.1584) for cut-off levels based on deviations from standard negative control and fixed cut-off levels respectively. Cut-off levels based on deviation from standard negative control was more sensitive, with a 92% predictive true positive value, compared to a 69% predictive true positive value by fixed cut-off levels of IgM ACA detected using ELISA, and nearly equivalent to IgG ACA, with 84.4% and 87.1% predictive true positive values respectively.

Conclusion: Cut-off points based on fixed levels of ACA detected using ELISA cannot be applied, because both IgG and IgM ACA levels of standard negative control were inconsistent among runs and batches. Cut-off points based on the deviation of 3 standard negative control levels for IgG ACA and based on deviations of 2.5 times from standard negative control levels for IgM ACA were better than cut off by fixed levels of ACA in producing true positive results.