Creatinine is a breakdown product of creatine phosphate. Creatinine is produced and excreted at a constant rate, and blood creatinine is used to determine glomerular filtration rate (GFR), a measure of kidney function. Blood creatinine levels increase only in cases of significant (>75%) damage to nephrons. Creatinine clearance is frequently used as a means of standardizing excretion of other compounds such as isoprostanes. Assay Genie's Creatinine Assay Kit provides an accurate, convenient measure of creatine concentration in biological fluids such as serum, urine or CSF. In the assay, creatinine is converted to creatine by creatininase, creatine is converted to sarcosine, which is specifically oxidized to produce a product which reacts with a probe to generate red color (ax = 570 nm) and fluorescence (Ex/Em = 538/587 nm). Unlike the picric acid assay, this kit is suitable for serum/plasma creatinine determinations, as well as for urine and other biological samples.

Figure. (a) Creatinine Standard Curve (colorimetric), (b) Creatinine Standard Curve (fluorometric). Fluorescence assay utilized 5X dilution of probe to reduce background. (c) Serum and urine samples were deproteinized using 10 kDa spin column (10000 x g, 10 min., 4°C). Sample filtrates (serum: 25 µl of 5-fold dilution; urine: 5 µl of 200-fold dilution) were spiked with known amounts of creatinine (serum: 0.6 nmol; urine: 4 nmol) and assayed according to kit protocol. Calculated concentrations: serum: 0.97 ± 0.06 mg/dl; Urine:197.1 ± 18 mg/dl.