Creatinine is a chemical waste product of creatine phosphate, which is a very important molecule for energy production in muscles. Approximately 2% of the body's creatine is converted to creatinine every day. Creatinine is carried in the bloodstream to the kidneys where it is filtered out of the blood and excreted in the urine.
Depending on muscle mass, creatinine is usually produced at a fairly constant rate by the body. It is a spontaneously formed cyclic derivative of creatine. The kidneys filter creatinine out of the blood, with no tubular reabsorption of it. If the filtering of the kidney is deficient, blood creatinine levels rise. Thus the levels of creatinine contained in the blood and urine may be used to calculate the creatinine clearance (CrCl), which reflects the glomerular filtration rate (GFR), which is a measurement of renal function. However, in cases of severe renal dysfunction, the creatinine clearance rate will be "overestimated" because the active secretion of creatinine will account for a larger fraction of the total creatinine cleared. Ketoacids, cimetidine and trimethoprim reduce creatinine tubular secretion and therefore increase the accuracy of the GFR estimate, particularly in severe renal dysfunction.
Measuring serum creatinine is a simple test and it is the most commonly used indicator of renal function.A rise in blood creatinine levels can be observed only with marked damage to functioning nephrons. Thus, this test is not suitable for detecting early stage kidney disease. A better estimation of kidney function is given by the creatinine clearance test. Creatinine clearance can be accurately calculated using serum creatinine concentration and some or all of the following variables: sex, age, weight, and race as suggested by the American Diabetes Association without a 24 hour urine collection. Normal levels of creatinine in the blood are approximately 0.6 to 1.2 milligrams (mg) per deciliter (dl) in adult males and 0.5 to 1.1 milligrams per deciliter in adult females.