Anti-HMGCR antibodies can be accurately detected by indirect immunofluorescence on HEp-2000 substrate.

Detection of myositis-specific autoantibodies (MSAs) is important in the diagnosis and subtyping of idiopathic inflammatory myopathies (IIMs). Widespread use of the line immunoassay (LIA) for MSA detection has significant limitations due to a high false positive rate. Confirmatory testing is therefore vital to minimise reporting of false positive results; however, anti-3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (anti-HMGCR) antibody confirmatory assays are not widely available in Australasia. Indirect immunofluorescence (IIF) on the HEp-2000 substrate for anti-HMGCR detection was assessed. Anti-HMGCR antibodies lead to a characteristic cell cycle-dependent cytoplasmic pattern on the HEp-2000 substrate. Sensitivity was 93.1% [95% confidence interval (CI) 77.2-99.2], and specificity was 95.8% (95% CI 78.9-99.9) when compared to immunoprecipitation (IP). The use of IIF for confirmatory anti-HMGCR testing is an accessible and accurate option for diagnostic immunology laboratories when the gold standard IP assay is unavailable.