Serum creatinine (a blood measurement) is an important indicator of renal health because it is an easily measured byproduct of muscle metabolism that is excreted unchanged by the kidneys. Creatinine itself is produced via a biological system involving Creatine, Phosphocreatine (also known as Creatine Phosphate), and Adenosine Triphosphate (ATP, the body’s immediate energy supply).
Creatine is synthesized primarily in the liver from the methylation of glycocyamine (guanidino acetate, synthesized in the kidney from the amino acids arginine and glycine) by S-adenosyl methionine. It is then transported through blood to the other organs, muscle, and brain, where, through phosphorylation, it becomes the high-energy compound phosphocreatine. During the reaction, Creatine and Phosphocreatine are catalyzed by Creatine Kinase, and a spontaneous conversion to creatinine may occur.
Creatinine is removed from the blood chiefly by the kidneys, primarily by glomerular filtration, but also by proximal tubular secretion. Little or no tubular reabsorption of creatinine occurs. If the filtration in the kidney is deficient, creatinine blood levels rise. Therefore, creatinine levels in blood and urine may be used to calculate the creatinine clearance, which correlates with the glomerular filtration rate. Measuring serum creatinine is a simple test, and it is the most commonly used indicator of renal function.