Creatine and Creatinine

Key points

• • Creatine is made from the amino acids glycine, arginine and methionine, beginning in the kidney and completed by methylation in the liver, and is then taken up by muscle.
• • In muscle, creatine is charged up to creatine phosphate, a store that quickly regenerates ATP during the first seconds of intense effort.
• • Creatine and creatine phosphate slowly and spontaneously break down to creatinine at a fairly steady daily rate that depends on muscle mass.
• • Creatinine is a waste product filtered by the kidneys and passed in the urine, so blood creatinine is used to gauge kidney filtration, though it is affected by muscle mass, diet and some drugs and is refined into the estimated GFR (eGFR).
• • A rising blood creatinine points to falling kidney function, which is why it is a standard test of the kidneys.
• • Because production depends on muscle mass, a very muscular person tends to have a higher creatinine and a frail person a lower one, for the same kidney function.