[A case of anti-synthetase syndrome: negative ELISA/immunoblot, but positive RNA immunoprecipitation with multiple antibodies].

A 68-year-old woman presented with a one-month history of polyarthritis and fever, followed by myalgia and muscle weakness involving the proximal limbs, neck, trunk, and distal upper extremities. Myositis was suspected based on an elevated serum CK level. Chest CT revealed interstitial pneumonia. Commercial laboratory tests, including ELISA and immunoblot, were positive for anti-Ro-52 antibodies but negative for anti-aminoacyl transfer RNA synthetase (ARS) antibodies. However, muscle pathology revealed perifascicular necrosis and perimysial pathology, strongly suggesting anti-synthetase syndrome (ASS). Further investigation using RNA immunoprecipitation (RIP) assay identified positive anti-EJ antibodies, leading to the diagnosis of ASS. In addition, anti-Ku and anti-U1 RNP antibodies were co-positive. These findings highlight that, even when ELISA and immunoblot were negative for specific antibodies, RIP assay is necessary when ASS is suspected based on myopathologic findings.