Clinically Relevant Cutoffs for Anti-Extractable Nuclear Antigen Line Immunoassays

Anti-extractable nuclear antigens (ENAs) are a form of antinuclear antibody test in the diagnostic laboratory that plays a crucial role in the diagnosis of systemic autoimmune diseases. The line immunoassay (LIA) is one of the most popular assays used to measure these autoantibodies. However, LIAs suffer from limited diagnostic specificity, and numerous attempts have been made to improve the cutoffs.

To readjust LIA autoantibody cutoffs, using the clinical diagnoses of a large and heterogeneous group of patients.

During a 12-month period, 667 discrete patients received an LIA test that had adequate clinical records to determine a diagnosis. Autoantibodies on the Euroimmun LIA (anti-Ro52, anti-Ro60, anti-dsDNA, anti-La, anti-mitochondrial antibodies, anti-Sm, anti-RNP, anti-histone, anti-nucleosome, anti-ribosomal P, anti-centromere protein B, and anti-Scl70) were evaluated. Two laboratory physicians independently rated whether the LIA density for each autoantibody was consistent or not consistent with the diagnosis. Receiver operating characteristic curves were constructed for each autoantibody and cutoffs were reestablished to maximize diagnostic specificities of 85% to 90%.

New cutoffs were proposed at the diagnostic specificities of 85% and 90%. Overall, there were similar rates of autoantibody detections, using the new cutoffs, compared to the manufacturer's defined cutoff of 10 units for each autoantibody.

We have used a novel approach to redefining anti-ENA LIA cutoffs by using clinical diagnoses across a range of pathologic conditions. This ensures that results are clinically relevant to a general laboratory cohort and, when used as a characterizing assay after an initial anti-ENA screen, maximizes diagnostic specificity. Longitudinal evaluation of the new cutoffs is required after implementation to examine clinical impact.