Assessing the diagnostic utility of baseline creatine kinase levels in patients with suspected idiopathic inflammatory myopathies and neuromuscular disorders.

Objectives: Serum creatine kinase (CK) is widely used in the diagnostic evaluation of neuromuscular disorders, including idiopathic inflammatory myopathies (IIM). As most laboratories define reference ranges using the central 95% of values from white populations, elevated CK levels are common in otherwise healthy individuals. We investigated the predictive value of baseline CK at defined cutoffs for diagnosing an underlying myopathic illness including IIM. Methods: A retrospective chart review was performed of consecutive adult patients attending a tertiary neuromuscular clinic from January 2018 to June 2022. We investigated the test characteristics of CK based on several a priori defined thresholds. Results: From a total of 488 patients, we identified 276 IIM cases, 89 with a non-inflammatory myopathy and 123 had an alternative/reassuring aetiology. Baseline CK levels were significantly higher in the IIM and non-inflammatory myopathy groups vs the alternative/reassuring group. The laboratory defined cutoffs for CK yielded a sensitivity of 77.0%, a specificity of 66.7% and a positive predictive value (PPV) of 87.3% for identifying IIM or other neuromuscular disorders, which only marginally improved when evaluating the 97.5th percentile (75.6, 67.5 and 87.3%) or the European Federation of Neurological Societies (EFNS) thresholds (68.8, 78.0 and 90.3%). Conclusion: The current thresholds defined by the EFNS guidelines compared with laboratory defined cutoff to pursue investigations for elevated CK only marginally improve specificity. A baseline CK >1000 IU/l is highly predictive of an IIM or other neuromuscular disorders. This study highlights the role of baseline CK levels in suspected IIM and neuromuscular disorders, clarifying its diagnostic value.