Although clinicians often recognise the importance of elevated serum activity of a commonly measured enzyme, creatine kinase (CK), subnormal values are frequently overlooked or ignored by both laboratory staff and clinicians. We present two cases of low CK activity (Roche Cobas Pro). In the first case, an infectious diseases registrar contacted the laboratory to discuss the clinical significance of low CK (16–36 U/L) in a 28-year-old man observed on three separate occasions over the past 8 days. The patient was admitted to the ICU with complicated bacteraemia with a CRP of 338 mg/L. The low CK is probably caused by glutathione depletion observed in sepsis, which otherwise protects CK from degradation in the circulation. The second case involves falsely low CK activity (<7 U/L) in a 27-year-old man with significantly abnormal biochemistry due to drug-induced rhabdomyolysis, along with anuric renal failure requiring dialysis. A renal advanced trainee raised concerns with the laboratory about the possibility of falsely low CK results. The same result was obtained upon repeat testing of the neat sample; however, following sample dilution, the CK activity was determined to be 490,172 U/L. The initial falsely low CK result was likely due to substrate depletion caused by the high CK concentration in the sample before the lag phase. Subnormal values reported by the laboratory should be discussed between clinicians and laboratory staff to prevent missed diagnoses and interventions.
Gyawali et al. (Sun,) studied this question.