Background: Kidneys play a key role in maintaining homeostasis within the body and glomerular filtration rate (GFR) is the best measure of kidney function. Direct measurement of GFR using exogenous substances is the gold standard, but it is not practical to do this in clinical practice, and instead clinical practice is heavily dependent on the presence of a metabolic byproduct of muscle, serum creatinine, which is low cost and widely available.
Key points: This research examines the value and usability of serum creatinine as a diagnostic tool in kidney failure patients and draws conclusions based on the results.
Methods: This includes a detailed review of the biochemistry and physiology of creatinine and a discussion of the methods for measuring the concentration in the blood, along with a detailed review of available equations to estimate GFR (eGFR) including the MDRD and CKD-EPI equations.
Results: It has been found that not all the factors influencing the results of serum creatinine reflect its renal origin, as age, sex and muscular mass are all important. A "diagnostic paradox" can also occur in advanced kidney function stage such that the secretion of creatinine into the tubules can be responsible for up to 50% of the loss of creatinine and muscle wasting, or "uremic sarcopenia," may lead to surprisingly stable creatinine levels despite a life-threatening reduction in real GFR.
Conclusions: Serum creatinine is a useful routine marker of renal function, but is not ideal for use in the advanced stages of renal dysfunction, due to biological variation and mathematical inaccuracy in eGFR equations at low levels of filtration. Diagnosis of kidney failure should be based on the context of the interpretation of the actual diagnosis, and other indicators such as Cystatin C may be available for inclusion.