Background Inclusion Body Myositis (IBM) pathogenesis is incompletely understood. Anti-cytosolic 5′-nucleotidase 1A (cN1A/NT5c1A) autoantibodies have recently been identified in ∼1/3 of patients. We investigated the potential utility of anti-cN1A testing in IBM.

Methods Data from four European IBM registries (UK, Netherlands, France, Sweden) were pooled. Anti-cN1A serotyping by ELISA was performed. Cases were stratified by anti-cN1A serotype and associations investigated. Mortality analysis was performed using Cox proportional hazards regression.

Results Data from 311 IBM patients were analysed, 102 (32.8%) were anti-cN1A seropositive. At disease onset, fewer seropositive patients had proximal arm weakness (8.3% vs. 23.2%, OR 0.30, 95% CI=0.11–0.73, p=0.004). A higher proportion of seropositive patients had excess cytochrome oxidase deficient fibres on muscle biopsy (86.9% vs. 71.8%, OR 2.61, 95% CI=1.08–6.97, p=0.022). During follow-up, 70 patients died at a mean age of 77.8 years (SD=8.2). Adjusted mortality risk was higher in the seropositive group (HR 1.95, 95% CI=1.17–3.27, p=0.011)

Conclusions Anti-cN1A seropositivity was associated with an increased mortality risk, suggesting a more severe disease phenotype. Stratification of IBM by anti-cN1A serotype may prove important in future treatment decisions if disease modifying therapies for IBM become available.