 The most reliable test in the diagnosis of rhabdomyolysis is the level of creatine kinase ( CK ) in the blood . This enzyme is released by damaged muscle , and levels above 5 times the upper limit of normal ( <unk> ) indicate rhabdomyolysis . Depending on the extent of the rhabdomyolysis , concentrations up to 100 @,@ 000 U / l are not unusual . CK concentrations rise steadily for 12 hours after the original muscle injury , remain elevated for 1 – 3 days and then fall gradually . Initial and peak CK levels have a linear relationship with the risk of acute kidney failure : the higher the CK , the more likely it is that kidney damage will occur . There is no specific concentration of CK above which kidney impairment definitely occurs ; concentrations below 20 @,@ 000 U / l are unlikely to be associated with a risk of kidney impairment , unless there are other contributing risk factors . Mild rises without kidney impairment are referred to as " <unk> " . Myoglobin has a short half @-@ life , and is therefore less useful as a diagnostic test in the later stages . Its detection in blood or urine is associated with a higher risk of kidney impairment . Despite this , use of urine myoglobin measurement is not supported by evidence as it lacks specificity and the research studying its utility is of poor quality . 
