Creatinine is a measure of kidney functions to determine how dilute a urine sample is. The creatinine has to be below 20 mg/dL for the sample to be considered too dilute. Creatinine levels above the 20 mg/dL are considered acceptable.

Creatinine is found in all muscle tissue and begins life as creatine. Creatine passes through the body in two ways. First is consumption as it is available as a supplement, and is also found in all types of red meat and fish. The second is production by the body. As muscles are broken down and rebuilt, creatine is metabolized into creatinine. It takes 2 days for the body to metabolize creatine into creatinine. Creatinine in high levels is toxic, therefore the kidneys filter out the creatinine from the blood and pass it on to the bladder. Creatinine extraction from the blood is at a very constant rate and the level in the urine is constant, varying only with the length between urinations. Therefore, a creatinine level under 20 mg/dL indicates a bladder being continuously emptied in an attempt to pass a U/A.