Creatine and Creatinine Cheat Sheet

Creatine is synthe­sized primarily in the liver. It is then transp­orted through the blood and taken up by tissues with high energy demands, such as the brain and skeletal muscle, where it is phosph­ory­lated to phosph­ocr­eatine, a high-e­nergy compound.
During times of increased energy demands, ATP is rapidly resynt­hesizes from ADP with the use of phosph­ocr­eatine through a reversible reaction catalysed by the enzyme creatine kinase (CK).

Creatinine is formed irreve­rsibly in the muscles from the breakdown of creatine and phosph­ocr­eatine. This process is non-en­zymatic and occurs at a relatively constant rate depending on muscle mass. Therefore, creatinine concen­tra­tions in blood and urine may be used to calculate the creatinine clearance (CrCl), which correlates approx­imately with GFR.

Creatinine is removed from the blood primarily by glomerular filtra­tion, but also by proximal tubular secretion. Little or no tubular reabso­rption of creatinine occurs. If filtration in the kidney is deficient, blood creatinine concen­tra­tions rise.

In cases of severe kidney dysfun­ction the CrCl rate will overes­timate the GFR, because hypers­ecr­etion of creatinine by the proximal tubules will account for a larger fraction of the total creatinine cleared.