A kidney function blood test is a blood test that checks your kidneys are working properly.
- A kidney function blood test usually measures the level of urea, creatinine, and certain dissolved salts (electrolytes) in your blood.
- The test is done to check for a number of aspects of kidney functioning, such as whether they are working normally, your kidney disease is changing, any medicines are causing damage or to check for severe dehydration.
- To perform a kidney function test, a blood sample is drawn from a vein in your 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 your body that is made by your muscles. It passes into your bloodstream, and is usually passed out in urine (pee). A high blood level of creatinine indicates that your kidneys may not be working properly.
- Urea is produced when protein is broken down by your body. Healthy kidneys eliminate more than 90% of the urea your 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.
- eGFR stands for estimated glomerular filtration rate. Although the level of creatinine in your blood is a useful guide to kidney function, the eGFR is a more accurate measure of how well your kidneys are filtering your blood. Using your blood creatinine, and your age and sex, your eGFR can be calculated by computer and reported with the creatinine blood test.
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, eg:
- 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 you get older. It 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 a kidney function 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 your 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.
What do the results of my kidney function test mean?
Interpreting blood test results can be difficult. Your doctor or nurse will contact you if there is anything that needs discussing or further action is needed. Increased creatinine levels in your blood suggest diseases that affect kidney function such as:
- diabetic kidney disease
- pyelonephritis (infection of your kidneys)
- urinary tract obstruction, or reduced blood flow to your kidneys 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 dehydration, decreased blood flow to your kidneys as in congestive heart failure, shock, stress, recent heart attack or severe burns, bleeding from your gastrointestinal tract, conditions that cause obstruction of urine (pee) flow.
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.