Normal Creatine Kinase in Idiopathic Inflammatory Myopathies: Disease Characteristics and Outcomes

Creatine kinase (CK) is an important biomarker for monitoring idiopathic inflammatory myopathies (IIM). Data regarding normal CK in IIM are limited. We aimed to characterize patients with normal CK levels and investigate the association between CK and disease activity and outcomes.

Data were collected from the Rituximab in Myositis (RIM) trial, a randomized, double-blind, placebo-controlled trial of refractory adult IIM. Myositis core set measures (CSMs) were collected every 4 weeks for 44 weeks. Normal/ near-normal was defined as < 2 times the upper limit of normal at baseline.

Of 143 patients, 53 (37%) had normal CK. These patients were more frequently White (85% vs. 62%, p<0.01), female (83% vs. 70%, p=0.08), and older (52±10 vs. 49±12, p=0.07), and were most frequent in dermatomyositis, whereas elevated CK was common in necrotizing myopathy (p< 0.001). At baseline, CK correlated with physician assessment (p=0.01) and muscle disease activity (p=0.001) only in patients with elevated CK, with no baseline associations observed in those with normal CK, despite active disease. In linear mixed models, CK showed significant longitudinal associations with all CSMs, regardless of baseline CK status (p<0.001). However, associations were more robust in polymyositis and in elevated baseline CK, while attenuated in dermatomyositis and antisynthetase syndrome among patients with baseline normal CK.

Normal CK levels are common in IIM, especially in DM, and do not exclude active muscle disease. CK functions as a longitudinal biomarker across IIM, with performance varying by disease subset and baseline CK, supporting nuanced clinical interpretation.