Variable distributions of serum creatine kinase reference values. Relationship to exercise activity.

Although the effect of exercise on serum creatine kinase (CK: EC 2.7.3.2) activity is well known, no standard protocol for blood collection conditions has been developed. Variable frequency distributions of CK activities have been reported for reference range samples from different laboratories and have been attributed to laboratory differences. In this study the frequency distributions of creatine kinase activities (CK: EC 2.7.3.2) in serum samples obtained from the same groups of volunteers varied from a normal Gaussian distribution, to skewed distributions, on different occasions. In one collection extremely high CK values followed the performance of an eccentric exercise step test 5-8 days previously. In 2 students serum CK activities had not returned to resting values 12 and 15 days later. Peak serum CK values occurred within 24 h of other types of exercise. Serum CK frequency distributions in blood samples obtained from a group of nurses was skewed, whereas the distribution in mothers of school children was Gaussian. When repeat samples were obtained from the nurses after instruction to avoid exercise, the distribution became Gaussian. In view of the increasing levels of physical activity in the community, a standard protocol for serum CK analysis is required, in which exercise activity over the previous three weeks should be documented. Test subjects should be counselled to avoid unusual eccentric exercise and sampling should include 3 tests at least 1 week apart.