Routine kidney function is one of the most commonly performed blood tests.
The kidneys regulate the amount of water and salts that we have in our bodies. They do this by filtering the blood through millions of structures called nephrons. The kidneys also pass out certain waste products from the body. Urine is made up of the excess water, salts and waste products passed out by the kidneys down to the bladder.
Routine blood test of kidney function
The usual blood test which checks that the kidneys are working properly measures the level of urea, creatinine and certain dissolved salts.
Urea is a waste product formed from the breakdown of proteins. Urea is usually passed out in the urine. A high blood level of urea ('uraemia') indicates that the kidneys may not be working properly, or that you have a low body water content (are dehydrated).
Creatinine is a waste product made by the muscles. Creatinine passes into the bloodstream, and is usually passed out in urine. A high blood level of creatinine indicates that the kidneys may not be working properly. Creatinine is usually a more accurate marker of kidney function than urea. The effect of muscle mass needs to be taken into account. A person with a lot of muscle and little fat on their body is likely to have a higher creatinine than a person who has a lot of fat and little muscle.
Estimated glomerular filtration rate (eGFR) provides a guide to kidney function. Although the level of creatinine in the blood is a useful guide to kidney function, the eGFR is a more accurate measure. Blood creatinine can be used to estimate the eGFR using age, sex and race. This is often calculated by computer and reported with the creatinine blood test. The normal value for eGFR is 90-120 ml/min. An eGFR below 60 ml/min suggests that some kidney damage has occurred. The value becomes lower with increasing severity of kidney damage.
Dissolved salts that are routinely measured are sodium, potassium, chloride and bicarbonate. They are sometimes referred to as 'electrolytes'. Abnormal blood levels of any of these may be due to a kidney problem. (Some other conditions may also alter the salt balance in the blood.)
Who has a blood test of kidney function?
Routine kidney function is one of the most commonly performed blood tests. It may be done:
- As part of a general health assessment.
- If you have suspected low body water content (dehydration), when the urea level increases.
- If you have suspected kidney failure. The higher the blood levels of urea and creatinine, the less well the kidneys are working. The level of creatinine is usually used as a marker as to the severity of kidney failure. Creatinine in itself is not harmful but a high level indicates that the kidneys are not working properly. So, many other waste products will not be cleared out of the bloodstream. You normally need treatment with dialysis if the level of creatinine goes higher than a certain value.
- Before and after starting treatment with certain medicines. Some medicines occasionally cause kidney damage as a side-effect. Therefore, kidney function is often checked before and after starting treatment with certain medicines.
Other tests of kidney function
The routine kidney blood test is a general marker of kidney function. If the blood test is abnormal it cannot say what is causing the kidney problem. Therefore, if you have an abnormal result you may need further tests to find the cause of a kidney problem. For example: urine tests, other blood tests, scans, X-rays, kidney biopsy, etc.
Further reading and references
Blann A; Routine blood tests 1: why do we test for urea and electrolytes? Nursing Times 110: 5, 19-21, 2014.
Chronic kidney disease: early identification and management of chronic kidney disease in adults in primary and secondary care; NICE Clinical Guidelines (July 2014)
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