Friday, 2 May 2008

What is creatinine clearance? (Part 2)

Hmm, I've been getting a lot of follow-up questions about my last post on this topic. On reflection, I don't think I answered it that well - I concentrated on deriving the formula from first principles, and neglected to give you a bird's eye view.

So, here's basically what it's about:

  • 'Creatinine clearance' is the amount of creatinine filtered by the kidneys per minute.

  • Since creatinine is freely filtered and not reabsorbed by the kidneys, the creatinine clearance is an estimate of the glomerular filtration rate (GFR), which is the amount of blood filtered by the kidneys every minute.

  • The glomerular filtration rate is a useful number to know, since it provides us with a measure of renal function that precedes changes in the serum values of certain markers for renal dysfunction. For instance, in end-stage chronic renal failure, the serum creatinine will be raised, and this is all we need to assess renal function. However, at least 50% of the nephrons must be destroyed before the serum creatinine will rise, and so if we want to know about renal function before this point, we need to estimate the glomerular filtration rate, usually by measuring the creatinine clearance.

  • Since creatinine is actively secreted by the renal tubules (in addition to being filtered), creatinine clearance overestimates the GFR by about 10-20%. In particular, when the GFR is very low, this secretion is proportionately very high, and so creatinine clearance isn't a good way to estimate GFR then. There are other things that can be measured instead of creatinine (e.g. inulin) that avoid this problem, but they are usually far less convenient or simple to administer.

  • The formula for calculating the creatinine clearance is:

  • Nomal values for this figure are 120 ± 25 for men, and 95 ± 20 for women. There are ways of increasing the accuracy of this figure by taking into account one's muscle bulk (from which creatinine is derived). See the Wikipedia entry on this for more detail on this point, but also bear in mind that this will alter the normal values.

  • Finally, there are also formulae for estimating the creatinine clearance without collecting urine. These are less accurate, but much more convenient. One such common formula is:

where the constant is 1.23 for males and 1.04 for females. Note that this formula assumes that the creatinine is being measured in ┬Ámol/L. If it's being measured in mg/dL, use a different formula.

There, that's better!

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