My 9 year olds ck level is 638 how worried should i be ?

Ck: Why was a ck drawn on a 9y/o? Muscle damage or inflammation make ck rise, myocardial injury is unlikely unless myocarditis is a possibility. Ischemic cardiac disease is low probability in a 9 y/o.

How does he feel?: A bruise to a muscle, or a muscle strained from exercise, can raise the ck this high or higher. Common sense needs to govern the ordering and interpretation of such tests. If junior is simply building his young muscles playing sports, this may be the explanation as well.