The creatine kinase reference interval. An assessment of intra- and inter-individual variation

Serum creatine kinase (CK) values in reference populations generally vary by greater than one order of magnitude. This large range of variation overlaps the range of values found in female carriers of Duchenne muscular dystrophy and limits the clinical usefullness of serum CK testing. In order to examine whether the source of this variation is intrinsic variability of individual CK values, or whether the variation is due to day-to-day variation in each individual's CK activities, we tested 15 female medical students at weekly and 22 female medical students on at least 3 occasions. Each venesection followed a 3-day exercise free interval, according to a standard protocol, designed to minimise the effects of exercise. Each individual tended to have consistent CK activities. The range of variation in the same individuals on different occasions was small (the mean coefficient of variation, CV was 19%). There was a much greater range of serum CK activities between individuals (CV 47%). Even when sporting activities were excluded slightly higher values were found after minor exercise was recorded in the three days prior to venesection. The conclusion that individuals have consistent serum CK activities was supported by the results from a larger population tested under standardised resting conditions on three different occasions which yielded similar frequency distributions on each occasion.