Why are BUN and creatinine important to monitor in kidney patients?

This is a very good question and one that many patients ask. The best, although indirect, measure of kidney function is indeed the creatinine, not the BUN. Blood, urea, nitrogen is used by nephrologists often to determine whether or not the patient is dehydrated. Generally speaking, the relationship between your creatinine and BUN is 1:10 relationship. If indeed the BUN is higher than this, say 1:20 relationship, that suggests the patient may be dehydrated. So, back to the original question, creatinine is a breakdown product of muscle metabolism which is secreted at the glomerulus and for all intensive purposes not reabsorbed or further secreted by the renal tubule. Therefore, it makes an excellent measure of kidney function. A normal creatinine is 1 and generally speaking, each time your creatinine doubles your renal function halves. There are now more sophisticated calculations such as the MDRD (The Modified Renal Dietary Determination) that is used to convert this into creatinine clearance. Monitoring renal function is important because currently we stage chronic kidney disease in five stages, the lowest being I with a creatinine clearance of 90 to the highest being V with a creatinine clearance less than 15. We do this so we can take appropriate actions so we can prepare patients for either dialysis or transplantation and to monitor the impact of our treatment. Very soon serum creatinines will be reported with the actual creatinine clearance (next to it) so that these hand calculations will no longer be necessary.

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2009 Colorado Springs Nephrology