Evaluation of a line immunoblot assay for detection of antibodies recognizing extractable nuclear antigens

Sera (n = 90) giving positive results in a screening test for antibodies to extractable nuclear antigens (ENAs) were tested in a line immunoblot assay that measures antibody reactivity with individual ENAs in a single test field. Results were then compared to those obtained in monospecific ENA antibody enzyme immunoassays (EIAs). Discordant results were resolved by immunodiffusion. Of 540 result pairs (90 sera tested for 6 ENAs [Sm/RNP, Sm, SSA, SSB, Scl-70, Jo-1]), 509 (94%) showed concordance. Immunodiffusion resolved 28 of 31 discordant result pairs in favor of the immunoblot result. After resolution of discordant data, the immunoblot assay exhibited 100% sensitivity for all ENA antibodies except those recognizing Scl-70, for which the sensitivity was 89%; specificity was over 96% for all 6 ENA antibodies. These findings show that a line immunoblot assay for the characterization of ENA antibodies yields results comparable to those obtained using monospecific ENA antibody EIAs. The immunoblot assay is easier and less expensive to perform due to its utilization of a single test field.