Aims of the study: To evaluate the advantages and reliability of screening for antinuclear antibodies (ANA) by enzyme immunoassay (ELISA).
Methodology: Sera from 96 patients comprising 51 with systemic lupus erythematosus (SLE), 11 with other systemic rheumatological diseases (SRD) and 34 with various other diseases (non-SRD) were tested using a commercial ELISA kit (ANA-Ease, Genesis Biotechnology, U.K.). These sera consisted of 53 immunofluorescence assay (IF) ANA-positive and 43 IF ANA-negative samples
Results: We observed that when compared to the IF for ANA the sensitivity, specificity, predictive values for positives (PPV) and negatives (NPV) of ELISA were 90.7%, 85.7%, 89.1% and 87.8% respectively. Exclusion of borderline ELISA positive by slightly raising the cut-off optical density (OD) increased the specificity and PPV to 93.1%, and 94.1% respectively. Importantly, none of the non-SRD sera were positive when this higher cut-off was used. ELISA was noted to be strongly positive in three IF ANA-negative SLE patients. However there was no correlation between the ELISA ANA semi-quantitative index and the IF ANA titers.
Conclusions: ELISA appears to be suitable as a preliminary screening test for ANA. An appropriate cut-off should be identified to segregate low positive samples that could be false-positives. Nevertheless, IF will need to be performed to estimate the titers, identify patterns of ANA positive samples and confirm results of low positive "gray-zone" samples and ELISA negative sera from patients with a high index of clinical suspicion of SLE.