What level of BUN indicates kidney failure?

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.
Blood urea nitrogen (BUN) is one of the parameters used to ascertain kidney function. There is no definite value of BUN that would diagnose 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:

If your BUN values are consistently outside the normal limits, talk to your doctor to know the reason behind the variation.

What are kidneys and kidney function tests?

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)

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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.

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:

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 5 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.

What are signs and symptoms of kidney failure?

Symptoms of kidney failure typically begin in the later stages, and there may be very few symptoms in the early stages.

Early symptoms kidney failure may include:

  • Decreased urine output
  • Shortness of breath
  • Disorientation
  • Fluid retention (swelling in the limbs)

Other possible symptoms of kidney failure may include:

What is the first stage of kidney failure?

first stage of kidney failure
Kidney failure symptoms can be difficult to detect in early stages. Learn about the 5 stages of kidney failure

Kidney failure, also called renal failure or end-stage kidney disease, occurs when at least 85% of kidney function has been lost. 

When your kidneys fail, it causes waste products to build up in the body, which can lead to chemical imbalances in the blood and be fatal if left untreated. People with chronic kidney failure may develop low blood counts or weak bones over time, requiring dialysis or a kidney transplant

Kidney failure can happen suddenly or gradually. But many people don’t experience any symptoms until their kidneys are already close to complete failure.

QUESTION

The only purpose of the kidneys is to filter blood. See Answer

Understanding the 5 stages of kidney failure

The five stages of kidney failure range from very mild damage (stage I) to complete kidney failure (stage V). Stages are based on declining kidney function and glomerular filtration rate (GFR) measurements, which is the rate at which kidneys filter waste from the body. 

GFR is calculated based on a blood test that estimates the amount of creatinine (waste product) in the blood. A higher GFR indicates a healthy kidney, whereas lower GFR indicates a suboptimal kidney function. A normal GFR is about 90 to 120 mL/min/1.73 m2. 

Symptoms and complications increase as the stages progress.

Stage I (normal or minimal loss of kidney function)

Kidney damage is very mild with GFR 90 mL/min/1.73 m2 or above (normal values). No symptoms are present, but there might be indications of kidney damage in tests, such as the presence of protein in the urine or physical changes of kidneys on a sonogram.

Stage II (mild or moderately reduced kidney function)

Mild kidney damage with GFR between 60 and 89 mL/min/1.73 m2. The filtration rate is slightly subpar. No symptoms are present, but certain indications may be more obvious, such as protein in the urine or physical damage to the kidneys.

Stage III (moderate to severe loss of kidney function)

Kidneys do not work as efficiently as they should, and GFR is between 30 and 59 mL/min/1.73 m2. Symptoms may become apparent at this stage and may include fatigue, swelling in hands and feet, back pain, frequent or infrequent urination, and high blood pressure.

Stage III kidney disease is divided into IIIA and IIIB.

  • Stage IIIA (early) refers to GFR between 45 and 59 mL/min/1.73 m2.
  • Stage IIIB (late) refers to GFR between 30 and 44 mL/min/1.73 m2.

Stage IV (severely reduced kidney function)

Kidneys are severely damaged with GFR between 15 and 29 mL/min/1.73 m2. This is the last stage before complete kidney failure. Symptoms such as fatigue, swelling, and back pain, may worsen and cause health complications such as anemia, high blood pressure, and bone disease.

Stage V (kidney failure or end-stage renal disease or ESRD)

Kidneys are close to or in complete failure with GFR less than 15 mL/min/1.73 m2. Symptoms of kidney failure become evident and include loss of appetite, vomiting and nausea, muscle cramps, swelling in hands and feet, back pain, urinating more or less than normal, trouble sleeping, breathing trouble, and itchy skin. Patients with kidney failure will need dialysis or a kidney transplant to survive.

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Medically Reviewed on 3/4/2022
References
Medscape Medical Reference

National Kidney Foundation


American Kidney Fund. Kidney Failure (ESRD) Causes, Symptoms, & Treatments. https://www.kidneyfund.org/kidney-disease/kidney-failure/

Workeneh BT. Acute Kidney Injury. Medscape. https://emedicine.medscape.com/article/243492-overview

Arora P. Chronic Kidney Disease (CKD). Medscape. https://emedicine.medscape.com/article/238798-overview