Routine Kidney Function Blood Test

The kidneys

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 are dehydrated (have a low body water content).

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.

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 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.