What are the problems associated with the use of serum creatinine as a filtration marker?
Creatinine is a 113 dalton amino acid derivative that is generated from the breakdown of creatine in muscle, distributed throughout total body water, and excreted by the kidneys primarily by glomerular filtration. Although the serum level is affected primarily by the level of GFR, it is also affected by other physiological processes, such as tubular secretion, generation and extrarenal excretion of creatinine. Due to variation in these processes amongst individuals and over time within individuals, particularly the variation in creatinine generation, the cutoff for normal versus abnormal serum creatinine concentration differs among groups. In addition, assays for serum creatinine vary across clinical laboratories, leading to differences in GFR estimates for the same patient when creatinine is measured in different labs. Because of the wide range of normal for serum creatinine in most clinical laboratories, GFR must decline to approximately half the normal level before the serum creatin
