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What are kidneys and kidney function tests?
Blood urea nitrogen (BUN) is one of the parameters measured to ascertain kidney function. BUN indicates the urea nitrogen produced in the body during protein breakdown. There is no definite value of BUN that would diagnose kidney failure.
Kidneys are vital organs performing various functions such as maintaining fluid and electrolyte balance, controlling blood pressure, removing waste products from the body, maintaining healthy bones (by activating vitamin D) and keeping healthy levels of red blood cells (RBCs) in the body. Various tests are performed to know about the health of the kidneys such as
- Blood urea
- Serum creatinine
- Blood urea nitrogen (BUN)
- Glomerular filtration rate (GFR)/eGFR
- Creatinine clearance
- Urine proteins
- Urine microalbumin
- Serum electrolytes
- Complete blood counts (CBC)
What is BUN?
Blood urea nitrogen (BUN) is one of the parameters measured to ascertain kidney function. A BUN test is also known by other names such as a urea nitrogen test and serum BUN. BUN indicates the urea nitrogen produced in the body during protein breakdown. It is removed from the body through urine. A decline in kidney function due to a disease or kidney damage can cause an increase in BUN. It is, however, a less specific indicator of compromised kidney function than glomerular filtration rate (GFR) and creatinine clearance. This is because BUN values can vary independent of kidney function. Your BUN values may be low if you eat a low protein diet, whereas they may increase with a high-protein diet. BUN can also increase in case of internal bleeding (hemorrhage), injuries and glucocorticoid therapy. Liver diseases can also cause a lower BUN value.
What level of BUN indicates kidney failure?
The normal range of blood urea nitrogen (BUN) is between 7 and 20 mg/dL or 2.5 and 7.1 mmol/L. There may be slight variations between labs. A decline in kidney function can cause an increase in BUN levels. There is no definite value of BUN that would diagnose kidney failure. BUN and creatinine tests can be used together to find the BUN-to-creatinine ratio (BUN:creatinine), which is more specific than the BUN test alone. More specific tests such as glomerular filtration rate (GFR) and creatinine clearance may be performed further to diagnose kidney failure. A high BUN value may be due to reasons unrelated to the kidneys such as
- A high protein diet
- Burns
- Dehydration
- Hemorrhage
- Shock
- Heart diseases such as congestive heart failure or a recent heart attack
- Certain medicines
- Older age
If your BUN values are consistently outside the normal limits, talk to your doctor to know the reason behind the variation.
Why is a BUN test done?
A blood urea nitrogen (BUN) test may be performed as part of your routine checkup. Your doctor may also advise a BUN test if you have any factors that may increase the risk of kidney diseases. The risk factors include
- A family history of kidney problems
- High blood pressure
- Diabetes
- Kidney diseases
- Long-term medications
The doctor may also advise the BUN test if you have any signs and symptoms of kidney diseases such as
- Swelling of the feet, legs or arms
- Increased or decreased urine frequency
- Having trouble sleeping
- Dry and itchy skin
- Abnormal appearance of urine (foamy or blood-tinged urine)
- Muscle cramps
- Excessive fatigue
- Reduced appetite
What happens during a BUN test?
A blood urea nitrogen (BUN) test is a quick and simple test that generally takes less than five minutes. You do not need to prepare for it. During the test, a healthcare professional takes a blood sample from a vein in your arm using a small needle. They will withdraw a small amount of blood and collect it in a special test vial. You may feel a little sting when the needle is inserted or withdrawn. You may get the results of the test within 24 to 48 hours depending upon the sample load of the lab.