Serum creatine kinase activity is not a reliable marker for muscle damage in conditions associated with low extracellular glutathione concentration.

Creatine kinase (CK, EC 2.7.3.2) assays usually contain thiol-reducing compounds to restore the enzyme activity. In this study, we investigated the effect of endogenous extracellular glutathione on serum CK activity. We examined CK activity and glutathione concentrations in serum from 200 healthy subjects (107 males, 93 females) and 38 patients with multiple organ failure, muscle wasting, and low serum CK activity (<50 U/L) (24 males, 14 females). Muscle damage was further evaluated using serum myoglobin concentrations and aldolase activity. In the overall group, serum glutathione concentrations correlated with serum CK activity (r = 0.791) but not with myoglobin concentrations and aldolase activity. In patients with multiple organ failure, low serum CK activities were accompanied by extremely low serum glutathione concentrations (<0.5 ,micromol/L, P <0.001). Endogenous glutathione can be regarded as a CK-preserving agent during the lifetime of the enzyme in the circulation (22 h on average). Serum CK activity should be interpreted with caution in patients with liver disease and multiple organ failure. In these conditions, the loss of CK activity due to extracellular glutathione depletion cannot be restored by the presence of thiol-reducing compounds in the CK assays.