Kidney function blood test

Also known as renal function

A kidney function blood test is a blood test which checks that the kidneys are working properly.

Key points:

  1. A kidney function blood test usually measures the level of urea, creatinine, and certain dissolved salts in the blood.
  2. The test is done to find out if your kidneys are working normally, to check if your kidney disease is changing, to find out if medicines that you take is causing kidney damage, and to check for severe dehydration. 
  3. To perform a kidney function test, a blood sample is drawn from a vein in the arm, and collected in a tube. This tube is sent to the laboratory for analysis.

What is a kidney function blood test?

The kidney function blood test measures the level of creatinine, urea and certain dissolved salts.

  • Creatinine is a waste product in the body that is made by the muscles. It passes into the bloodstream, and is usually passed out in urine. A high blood level of creatinine indicates that your kidneys may not be working properly. 
  • Urea is produced when protein is broken down by the body. Healthy kidneys eliminate more than 90% of the urea the body produces. A high blood level of urea may indicate that you are dehydrated or that your kidneys may not be working properly.
  • 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 sometimes be due to a kidney problem.

When is a kidney function test done?

A kidney function test may be requested as a routine blood test to find out about your general health. It is also requested for a variety of other more specific situations, for example:

  • To assess if you have any sign of acute or chronic kidney disease.
  • To assess for dehydration.
  • To check how your kidneys are functioning before and after starting certain medicines.

Chronic kidney disease is common as we get older and has no warning signs until too late. Therefore a kidney function test at various stages can be helpful to find problems sooner. Read more about chronic kidney disease

How to prepare for the test

No preparation is needed and you can have this blood test at any time of the day.

How is the sample collected for testing

A blood sample taken by a needle placed in a vein in the arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.The blood sample is collected in a tube, which is sent to the laboratory for analysis.

Results

Interpreting blood test results can be difficult. Your doctor or nurse will contact you if there is anything that needs discussing or further action. Increased creatinine levels in the blood suggest diseases that affect kidney function such as:

  • glomerulonephritis 
  • diabetic kidney disease 
  • pyelonephritis (infection of the kidneys)
  • urinary tract obstruction, or reduced blood flow to the kidney due to shock, dehydration, congestive heart failure, atherosclerosis, or complications of diabetes.
  • High urea levels suggest impaired kidney function. This may be due to acute or chronic kidney disease. However, there are many things besides kidney disease that can affect urea levels such as decreased blood flow to the kidneys as in congestive heart failure, shock, stress, recent heart attack or severe burns; bleeding from the gastrointestinal tract; conditions that cause obstruction of urine flow; or dehydration.

Learn more

The following is further reading that gives you more information on kidney function tests. Be aware that websites from other countries may contain information that differs from New Zealand recommendations.

Blood test safety information Labtests NZ
Routine kidney function blood test Patient Info, UK
Lab tests Online  Australasian Association of Clinical Biochemists

Credits: Editorial team.