Kidney function blood test – in-depth information

A kidney function blood test usually measures the level of the waste product creatinine and certain dissolved salts (electrolytes) in your blood. The test is done to check for a number of aspects of kidney function. 

• 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 your urine (pee). A high level of creatinine in your blood indicates 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. 

In some cases urea and uric acid are also tested which are other waste products.

• Urea is produced when protein is broken down by your body. Healthy kidneys get rid of more than 90% of the urea your body produces. A high level of urea in your blood may indicate that you are dehydrated, bleeding in your bowel, or that your kidneys may not be working properly.
• Uric acid is produced when purines are broken down. High uric acid is a risk factor for gout. This is usually measured when helping to diagnose gout and monitoring the response to treatment of gout.

A urine test is sometimes done to check for excess protein leaking from your urine. Albumin is a type of protein and a high ratio indicates that your kidneys are leaking protein. This can be elevated in certain conditions such as kidney damage from diabetes mellitus and pre-eclampsia in pregnancy.

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. It may be done to: 

• check if you have acute or chronic kidney disease
• check for early detection of kidney disease if you have risk factors such as diabetes mellitus
• assess your risk of heart attack and stroke
• check for dehydration
• assess how your kidneys are functioning before and after starting certain medicines.

Chronic kidney disease is increasingly common as you get older and doesn't have symptoms in the early stages. Therefore a kidney function test at various stages can be helpful to find problems sooner. Read more about chronic kidney disease. 

No preparation is needed and you can have this blood test at any time of the day. The urine albumin or protein creatinine ratio test is best done in the early morning if possible.

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.

Interpreting blood test results can be difficult. Your doctor or nurse will contact you if there is anything that needs discussing or if further action is needed.

A low eGFR suggests impaired kidney function. This may be due to acute or chronic kidney disease. Some of the causes of reduced kidney function include:

• diabetes mellitus (type 1 or type 2)
• high blood pressure
• pyelonephritis (infection of your kidneys)
• urinary tract obstruction such as from a kidney stone
• reduced blood flow to your kidneys due to shock, dehydration, congestive heart failure, atherosclerosis or complications of diabetes
• certain medicines such as non-steroidal anti-inflammatories, antibiotics, lithium, certain Chinese herbs, or some poisons
• polycystic kidney disease
• glomerulonephritis lupus.