Background Anti-dsDNA autoantibodies are highly specific for systemic lupus erythematous (SLE). They are an important factor in determining subject eligibility into SLE clinical trials as an abnormal titer is a criterion of the ACR classification for the disease. The Farr assay has long been considered the gold standard for detection of anti-dsDNA antibodies but is labor intensive and requires radiolabelled dsDNA. Alternative assays have been developed and are commercially available. The performance characteristics of two commercially available assays were compared to the FARR in adults with active moderate to severe SLE.
Objectives Compare the Athena Multi-Lyte II multiplex assay and Quantalite ELISA to the Farr in patients with SLE.
Methods Baseline data was collected from a subset of subjects (N=92) with moderate-severe active SLE (98% female, mean SLEDAI2k: 11) in an ongoing international, multi-center, double-blind, randomized, placebo-controlled trial. Anti-dsDNA levels were determined by the Farr radioimmunoassay, Quantalite ELISA, and the AtheNA Multi-Lyte ANA-II Plus Test System® (AMLII). All assays were performed according to manufacturers' instructions in Clinical Laboratory Improvement Amendments (CLIA) certified laboratories. The sensitivity and specificity of the recommended cutoff values for positive (>120) and indeterminate (>100) AMLII assay were assessed using the recommended value for a positive (>10) cutoff of the Farr assay. The sensitivity and specificity of the recommended cutoff values for positive (>75) and indeterminate (>30) cutoff values of the Quantalite ELISA were assessed using the recommended value for a positive (>10) cutoff of the Farr assay.
Results Using the manufacturer's recommended positive cutoff values for all three assays; AMLII and Quantalite ELISA showed a 64% and 82% concordance with the Farr, respectively. The sensitivity of the AMLII was 35% compared to 75% with the Quantalite ELISA. When the positive cutoff values of the AMLII and Quantalite ELISA were changed to include the indeterminate range for both, the concordance of the AMLII dropped to 48% while the Quantalite ELISA increased to 91%. The sensitivity of the AMLII dropped to 28% while the Quantalite ELISA increased to 96%. The increase in sensitivity of the Quantalite ELISA can be explained by a number of samples which were indeterminate for this assay but positive for the FARR. At the higher assay cutoff of 75, these indeterminate samples would be considered negative by Quantalite ELISA. When the cutoff was decreased to 30, the samples were positive and therefore, in agreement with the FARR.
Conclusions The AMLII is inferior to the ELISA as a replacement to the gold standard Farr assay. Further investigation may be needed in order to determine the optimal positive cutoff value for the ELISA.
Disclosure of Interest None declared