Urinary levels of creatine and other metabolites in the assessment of polymyositis and dermatomyositis

Background. A simple and reliable method is needed to assess disease activity and monitor the efficacy of therapy in polymyositis (PM) and dermatomyositis (DM). This study used in vitro proton (H-1) magnetic resonance spectroscopy (MRS) to explore whether excretion of urinary metabolites can be used as a reliable marker of disease in PM and DM patients. Methods. Urine samples were obtained from PM/DM patients (n = 34), healthy controls (50) and subjects with known muscle-wasting conditions including adult-onset muscular dystrophy (8), stroke patients (10), rheumatoid arthritis (RA) patients on steroids (13) and not on steroids (16) and patients with alcoholic myopathy (12). Levels of urinary metabolites were then correlated with creatine kinase (CK) activities and quadriceps; muscle strength. Results. Creatine was detected in the urine in 26 of 35 patients with PM/DM, four of 60 cases with other medical disorders (including one with adult-onset dystrophy, one with a stroke and two with RA who were not on steroids) and 10 of 50 healthy controls. The urinary creatine/creatinine ratio exceeded 0.4 in 20 patients with PM/DM but no patients with other medical disorders and no healthy controls. These differences were highly significant (P < 0.001) by Kruskal-Wallis test (comparing all groups) and by Mann-Whitney U-tests (comparing individual groups with PM/DM cases). Citrate, glycine, choline-containing compounds and taurine levels were significantly increased in PM/DM when compared with controls. There were positive correlations between CK activities and choline-containing compounds (r = 0.78, P = 0.0006) and also between CK activities and betaine (r = 0.57, P = 0.026). Conclusions. This study shows significant differences in the urinary levels of creatine, choline-containing metabolites, betaine and citrate in PM/DM subjects compared with controls, although further work is required to elucidate the underlying metabolic processes.