, an important part of the muscle. It is usually produced at a fairly constant rate by the body depending on muscle mass. It is a cyclic derivative of creatine which is spontaneously formed. It is chiefly filtered out of the blood by kidneys, though a small amount is actively secreted by kidneys into urine and there is little or no tubular reabsorption. Increase of its levels in the blood indicate problem in renal function.
Very little of creatinine is found in urine. About 1.0 to 1.8 g is excreted by and adult in 24 hours. Men have higher levels of it because they have more skeletal muscle mass than women. Vegetarians have been shown to have lower levels of it. Its levels in the urine also is said to be increased in conditions associated with increased muscular activity.
A decrease in its excretion may be observed in disorders associated with muscular weaknesses and kidney diseases.
A complete estimation or renal function can be made when interpreting blood creatinine levels along with that of urea. The ratio of urea to creatinine can indicated other problems besides those intrinsic to the kidney. It can indicate a pre-renal problem such as volume depletion.
Measuring serum creatinine is a simple test and it is the most commonly used indicator of renal function. Increased levels suggest damage to kidney cells however it cannot detect early stage kidney disease. Hence a creatinine clearance test is done. It can be calculated using serum creatinine concentration and variables like sex, age, weight, and race without of a 24 hour urine collection. Creatinine concentration levels in urine helps in standard urine drug tests. Low concentration of creatinine suggest a manipulated test, either through addition of water in sample or by drinking excessive amounts of water.
Creatinine is the breakdown product of creatine, an important part of the muscle.