Creatinine forms in muscle as a waste product from creatine and phosphocreatine, and is excreted in urine at relatively constant rates. This allows creatinine excretion to be used as a check for the completeness of 24-hr urine collections, and to normalize, for urine flow rate, urinary concentrations of endogenous and exogenous substances primarily excreted by renal filtration. Because urinary creatinine levels are 10-50 fold higher than those measured in serum, urine samples are typically diluted prior to creatinine analysis. The Vitros 250 chemistry analyzer (Ortho Clinical Diagnostics, Rochester, NY, USA) measures creatinine using a single-slide enzymatic method comprised of a dry, multilayered analytical element coated on a polyester support. The Vitros employs three levels of standard to calibrate creatinine. The highest standard (17 mg/dL) corresponds to 340 mg/dL for urine diluted 20-fold, the dilution recommended by Ortho as a basis for reference accuracy.
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