By Rajnish Mago, MD (bio)
Ideally, filtration of blood by glomeruli of the kidney would be evaluated by giving an external substance and seeing how well it gets filtered into the urine. But, this is inconvenient and costly (Steffl et al., 2012). If only there was a substance produced naturally in the body at a steady rate and entirely filtered by the kidneys. Its level would be a good surrogate measure of the glomerular filtration rate. Well, our bodies do produce such a substance—creatinine.
What is creatinine?
Skeletal muscles metabolized creatine phosphate as part of energy production; creatinine is a waste byproduct of the breakdown of creatine phosphate. The other source of creatinine is red meat and fish in the diet.
Why serum creatinine a good marker of renal function
There are several reasons why serum creatinine is usually an excellent marker of kidney function (Williamson and New, 2014):
1. Its rate of production is reasonably constant
2. It is eliminated only by the kidneys
3. It is not protein-bound, so it is freely filtered by the kidneys.
4. Its rate of elimination is almost the same as the glomerular filtration rate. This is because it is filtered into the urine by the glomeruli and after that is secreted/ reabsorbed by the tubules only to a limited extent.
This why by measuring the serum creatinine level, we can calculate the “estimated glomerular filtration rate” (eGFR), the most commonly used measure of renal function. The eGFR is explained in the following article: What is estimated GFR (eGFR)?
Under most circumstances, there is a direct relationship between GFR and serum creatinine. For example, when the GFR decreases by 50%, the serum creatinine level doubles.
5. Its rate of elimination remains relatively constant even if the person is dehydrated. This is why when a person is dehydrated, the blood urea nitrogen (BUN) goes up but the serum creatinine is relatively unchanged and, so, the BUN/ creatinine ratio is increased (above 20). This is explained in this article: How to interpret the BUN/Creatinine ratio when you get labs back
But, sometimes, an elevated serum creatinine level can be caused by factors other than impaired renal function. Please see the following articles:
Perrone RD, Madias NE, Levey AS. Serum creatinine as an index of renal function: new insights into old concepts. Clin Chem. 1992 Oct;38(10):1933-53. Review. PubMed PMID: 1394976.
Steffl JL, Bennett W, Olyaei AJ. The old and new methods of assessing kidney function. J Clin Pharmacol. 2012 Jan;52(1 Suppl):63S-71S. doi: 10.1177/0091270011420260. PubMed PMID: 22232755.
Williamson L, New D. How the use of creatine supplements can elevate serum creatinine in the absence of underlying kidney pathology. BMJ Case Rep. 2014 Sep 19;2014. pii: bcr2014204754. doi: 10.1136/bcr-2014-204754. PubMed PMID: 25239988; PubMed Central PMCID: PMC4170516.
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