Your body contains several different compounds related to creatine -- a nitrogen-containing organic compound produced in the kidneys and liver during the breakdown of proteins. An enzyme known as creatine phosphokinase stimulates the conversion of creatine into phosphocreatine. This reaction produces energy for your cells, and the phosphocreatine becomes a component of muscle cells. When muscle cells contract or break down, the phosphocreatine releases a waste product known as creatinine. Diagnostic tests can determine the level of creatinine and creatine phosphokinase to help doctors diagnose medical conditions.
Serum Creatinine
Your doctor may draw blood to perform a serum creatinine test as part of a routine metabolic test or to help diagnose nonspecific symptoms. MedlinePlus reports the acceptable creatinine level in the blood as 0.8 to 1.4 mg/dL. Muscle mass influences your creatinine level. Men typically have more muscle mass than women and exhibit a higher blood creatinine level.
Urine Creatinine
Your kidneys filter the creatinine in your blood and excrete it in the urine. Doctors can test a sample of urine collected over a 24-hour period to determine the amount of creatinine getting removed from the body. The acceptable range for a urine creatinine test is large because the level of creatinine in the urine varies based on age and muscle mass. MedlinePlus reports the acceptable range as 500 to 2,000 mg per day.
Creatine Kinase
Your doctor can test a sample of blood to determine the total amount of creatine phosphokinase enzyme in the blood. The majority of creatine kinase comes from the heart, the brain and the muscles, and each organ contains a slightly different form of the enzyme. The acceptable range for total creatine kinase is large, from 5 to 130 units per liter, according to Objectives for the Qualifying Examination. A high total creatine kinase result may prompt your doctor to determine the types of creatine kinase present, which will help him determine the cause of the abnormal result.
Significance
Creatinine and creatine kinase levels outside of the acceptable range may occur due to a variety of medical conditions. A serum creatinine level higher than normal or a urine creatinine level lower than normal may indicate a problem with the kidneys, such as kidney failure or diabetic nephropathy. Blood creatinine levels lower than the acceptable range may indicate muscular dystrophy or myasthenia gravis. A high creatine kinase level usually occurs due to damage to the heart, lungs or muscles. Doctors can use this test to diagnose a heart attack, evaluate chest pain, determine your risk for muscular dystrophy, or uncover a brain injury or stroke.
References
- Lab Tests Online: Creatinine
- "American Family Physician"; Clinical Practice Guidelines for Chronic Kidney Disease in Adults; C. Johnson, et al.; September 2004
- Objectives for the Qualifying Examination: Clinical Laboratory Test Normal Values
- MedlinePlus: Creatinine -- Blood
- MedlinePlus: Creatinine -- Urine
- MedlinePlus: Creatine Phosphokinase
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