[muscle Bx & comprehensive muscle lab & path link] CPK is a cell cytoplasm enzyme (chemical) leaked normally into the blood in low amounts, the greatest cell amounts being present in muscle. Muscle injury tends to give serum elevations of myoglobin, CPK, LDH, AST (SGOT), and aldolase. By itself, CPK does not distinguish increased leakage into blood from injured but intact muscle cells vs. cells undergoing "bland" lysis vs. lysis with myositis. When high enough, myoglobin may spill into urine and may trigger a positive urine dipstick test for hemoglobin (by a cross-reaction).

• hyperthyroidism (overactive or overtreated thyroid gland).
• heredity (RBC) spherocytosis.
• connective tissue diseases.
• markedly malnourished persons.
• medications (analytical artifact vs. real decreases).
• from 8 to 20 weeks pregnancy is not unusual.

• "clinically important" rhabdomyolysis is when the patient's CPK result is 10 times higher than the upper limit of the normal range is for that lab method¹.
• cases of "agitated delirium" (usually summer time, very hyperthermic, DIC & 66% die at the scene or during transport to hospital).
• myoglobinemia from any cause [check causes]; medications or toxins injuring muscle cells (such as "statin" medications¹).
• alcohol intake/alcoholism; cocaine intake; emetine (Ipecac...as in bulemics...Munchausen); toxic chemicals (usually raise level very high and, when acute, with elevated LDH [as with Tylenol...acetaminophen])² .
• all diseases that injure muscle...including critical illness myopathy.
• hypothyroidism (underactive thyroid gland).
• idiopathic ( "essential"/asymptomatic.. no aches and pains) creatine phosphokinasemia.
• myositis (muscle inflammation); dermatomyositis or polymyositis.
• muscular dystrophy.
• last/third trimester of pregnancy.
• rare instances of stable elevated levels due to circulating serum enzyme-immunoglobulin complexes (macroenzyme) in otherwise-healthy persons.
• moderate hemolysis gives mild elevations sometimes².
• chronic crack cocaine use.

Other names for this exact or approximate agent are:   

• CPK, serum
• CK, serum
• Creatine kinase, serum

References:

1. Thompson PD, "Statin Associated Myopathy", JAMA 289(13):1681-1690, 2 April 2003.
2. Wallach, 7th Ed. Interp. of Diagnostic Tests., 2000 (EBS's offi8ce).