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Subject: Creatinine, Urine
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Creatine is synthesized from amino acids in the kidney, liver, and pancreas. The creatine is then transported in the blood to other organs where it is synthesized into creatinine. In the absence of kidney disease, the urinary creatinine is excreted in rather constant amounts and represents glomerular filtration and active tubular excretion of the kidney. Because the creatinine is excreted from the body at a constant rate, there are expected values for creatinine in normal human urine. Specimen validity testing is the evaluation of the specimen to determine if it is consistent with normal human urine (creatinine values >20 mg/dL). Creatinine is made at a steady rate and is not affected by diet or by normal physical activities.
Normal range: see Table 16.25.
Urinary creatinine, in conjunction with serum creatinine, is used to calculate the creatinine clearance, a measure of renal function.
Animal meat diet
Decreased muscle mass
Advances renal disease
Urine creatinine is not ordered alone. Creatinine clearance, which requires a serum creatinine, offers useful renal function data. Serum creatinine alone is not an adequate index of glomerular filtration rate.
Twenty-four–hour urine creatinine levels are used as an approximate check on the completeness of 24-hour urine collection.