Effect of Racial Variation in Serum Creatine Kinase on Interpretation: A Case Report

A 20-year-old West Indian student presented to the rheumatology clinic, with a 2 year history of incapacitating generalized muscle weakness and pain. On examination, he was a slim, fit-looking, muscular man, with no abnormal signs and, in particular, he had no muscle tenderness. Formal muscle strength testing was normal. Initial investigations, including urea and electrolytes, liver function tests and thyroid function were normal and the only abnormal finding was an elevated serum creatine kinase (CK) activity of 488 lUlL (upper limit of reference range: 195 lUlL). Further tests were undertaken to determine the nature and the cause of his reported muscle weakness and pain. These included electromyography and muscle biopsy, both of which were normal. He was admitted to hospital for 48 h of bed-rest in order to exclude vigorous physical activity as a cause of the raised CK. However, CK activity remained elevated. Electromyography and muscle biopsy were again normal when repeated. The search for a physical cause for the patient's illness was abandoned. Psychological assessment demonstrated learned illness behaviour in an underachiever, with academically successful siblings. He was referred for behavioural therapy.