Sensitivity and clinical utility of the anti-cytosolic 5′-nucleotidase 1A (cN1A) antibody test in sporadic inclusion body myositis: Report of 40 patients from a single neuromuscular center

•We report the sensitivity and clinical utility of the anti-cN1A antibody test in sporadic inclusion body myositis (IBM).•This is a retrospective review of 40 patients with clinico-pathologically defined or clinically defined IBM.•The anti-cN1A antibody test has limited diagnostic utility in IBM. Sporadic inclusion body myositis (IBM) is the most common acquired myopathy affecting patients over age 50. The discovery of an autoantibody directed against a 43–44 kD protein (anti-cytosolic-5′-nucleotidase 1A or anti-cN1A) has provided support for the hypothesis of an immune-mediated pathogenesis. Previous studies have reported variable test sensitivity and specificity, and inconsistent results on the predictive value. In our cohort of 40 patients with clinico-pathologically or clinically defined IBM, we found the sensitivity of the anti-cN1A antibody test to be 50%. Comparing characteristics for test positive and test negative groups, we found that patients in our cohort testing positive for the anti-cN1A antibody were significantly more likely to be older than age 60 years at symptom onset. We found no positive association between anti-cN1A reactivity and other clinical, laboratory, and muscle histopathologic findings. Based on all clinical studies published to date including the present, the anti-cN1A antibody test shows high diagnostic specificity, moderate sensitivity, and a low predictive value in regards to age of onset, disease severity and other associated clinicopathological findings.