Creatinine is not protein bound and is freely filtered by glomeruli. All of the filtered creatinine is excreted in the urine.The renal clearance of creatinine provides an estimate of glomerular filtration rate. Since creatinine, for the most part, in the urine only comes from filtration, the concentration of creatinine reflects overall urinary concentration. Therefore, creatinine can be used to normalize other analytes in a random urine specimen