Antibodies to Extractable Nuclear Antigens in Antinuclear Antibody–Negative Samples

Antibodies to extractable nuclear antigens (ENAs)—SSA, SSB, U1RNP, Sm, Scl-70, and Jo-1—are clinically important in patients with systemic rheumatic diseases. Indirect immunofluorescence (IIF) with HEp-2 cells is a common initial screening test for detection of antinuclear antibodies (ANAs) and antibodies to ENAs. IIF-positive samples are further screened with more specific assays, but few studies have addressed the value of this cascade testing (1)(2). Although screening with conventional HEp-2 cells may miss some antigens, such as SSA (3) and Jo-1(4), false-negative ANA results are infrequent (1)(2). SSA-transfected cells in particular, which overexpress SSA (60 kDa), are considered highly sensitive for detection of anti-SSA antibodies (5). Some antibodies to ENAs can be missed by IIF, however (6). Hoffman et al. (6) found that of 291 ANA-negative samples, 12 were positive for antibodies to ENAs, including antibodies to SSA (Ro52 and Ro60), SSB, RNP-A, RNP-C, RNP-70, SmD, Scl-70, and Jo-1. We performed a prospective study to …