Creatinine - Creatinine is the best routine blood test for measuring how well kidneys are working. It is a waste product produced by muscles and put out through the kidneys. This means that 'normal' levels depend on how much muscle you have, and this can cause some problems in spotting kidney disease.
Creatinine comes from muscle. The little girl has normal kidney function and a creatinine level less than 60. The body-builder has normal kidney function and a creatinine of 120. For the girl, a creatinine of 120 would be very poor kidney function.
Because of the problem of differing amounts of muscle in different people, most labs now also report an estimated GFR (eGFR) with creatinine measurements, and this can be very useful.
Big changes in creatinine are likely to be important, whatever the creatinine level. Although different labs may give significantly different results for the same creatinine sample, because there are different ways of doing the test.
Normal blood creatinine is 60-120 micromol/litre (0.7-1.4mg/dl) - particularly to small, disabled, or elderly people. In many people, 'normal' creatinine can sometimes conceal 50% loss of kidney function.
Urea - Urea is a small molecule that is produced in the liver from protein that you have eaten. It is normally put out by the kidneys, so blood levels rise as kidneys fail. However other things change the level of urea in your blood too, so that it is not a simple guide to kidney function. Here are some of the things:
• Fluid - if you are short of fluid (e.g. drinking very little), your kidneys keep more urea in the blood
• How much protein you have eaten
• Liver disease can stop urea being produced normally
Urea is still a very useful test when used together with creatinine. It can also be used to measure how well dialysis is working to clear waste products (see Is my dialysis good enough?).
Normal blood urea is 3.5-6.5 mmol/litre (20-30mg/dl)
Creatinine clearance - You can get round many of the problems of blood creatinine measurements by collecting urine for 24 hours and measuring how much creatinine is in the urine at the same time as finding out how much is in the blood. This is called creatinine clearance. If any urine produced during the 24 hours is not collected the result will not be accurate.
Normal creatinine clearance is about 100ml/minute
Glomerular filtration rate(GFR) and estimated GFR (eGFR)
GFR is a test of how much the kidneys are filtering. It is normally about 100 mls/minute. This means that the kidneys are removing all the creatinine found in 100mls of blood every minute - almost 150 litres per day! Most of this is absorbed back into the body, so that only 1-2% of the filtrate appears as urine. Creatinine clearance (see above) gives quite a good measure of GFR, but requires a 24 hour urine collection for measurement. Two other ways of measuring GFR are:
• eGFR - Using blood tests, age, sex, and sometimes other information to estimate the GFR from the MDRD equation (eGFR). This isn't as good as measuring it, but is much simpler as it requires just one blood test. It is being used increasingly to spot kidney disease earlier than previously, and earlier than would be possible using just creatinine measurements (see above)
• Measured GFR - Injecting a tiny amount of a radioactive substance and measuring how quickly it disappears from the blood, or appears in the urine, is used to calculate GFR.
Knowing your GFR is very useful. It can give you an approximate % kidney function, and be used to show your CKD stage - more info about low GFR and CKD stages.
You need to know your creatinine.
Normal GFR is about 100ml/minute/1.73m2. eGFR provides a rough estimate of the percentage function of your kidneys
Normal kidneys filter 150 litres per day. That's 100 mls per minute. It also adds up to 33 Imperial gallons, 40 US gallons per day.