Creatinine is a metabolite waste product of phospho-Creatine (p-Creatine), a molecule used as a store for high-energy phosphate that can be utilized by muscles and other tissues for the production of ATP. Dietary intake and de novo synthesis from the amino acids arginine, glycine, and methionine in the kidney and liver are the primary sources of Creatine. Creatine and p-Creatine are converted non-enzymatically to the metabolite Creatinine, which diffuses into the blood, is filtered through the kidneys, and is then secreted into urine. In vivo, this conversion appears to be irreversible and in vitro it is favored by higher temperatures and lower pH. Some Creatinine may form via p-Creatinine as well. Under normal conditions, Creatinine's formation occurs at a rate that is relatively constant. For instance, in humans approximately 2% of the Creatine/p-Creatine pool is converted to Creatinine daily. This predictability makes Creatinine a useful tool for normalizing the levels of other molecules found in urine. In addition, altered Creatinine levels can be used as an indicator of kidney dysfunction, or may be associated with other conditions that result in decreased renal blood flow. Some examples include diabetes and cardiovascular disease. The RayBio® Creatinine assay is a 7 minute chemical analysis designed to measure Creatinine in Serum/Plasma.

The RayBio Creatinine Assay in an in vitro quantitative assay for the detection of creatinine based on the Jaffe reaction. In this method, creatinine is treated with an alkaline picrate solution to yield a bright yellow/orange complex. The rate of color development is directly proportional to the concentration of creatinine in the sample and is measured at an absorbance at 492nm. Unknown samples are compared to the standard curve. The kinetic nature of the assay eliminates interference from extraneous serum contaminants, such as lipids and bilirubin.