BUN Levels: What to Look for in a Blood Test

SimpleNursing Editorial Team Jan 24, 2023
Young female Nurse preparing blood test for BUN levels on a young female patient

When learning about blood test components, blood urea nitrogen (BUN) levels are guaranteed to come up.

Jump to Sections


  1. What Does BUN Mean in a Blood Test?
  2. BUN Normal Range
  3. What Does High BUN Mean?
  4. What Does Low BUN Mean?

BUN levels are a measurement of how much urea has been processed by the kidneys. Urea is a waste product that is excreted in urine.

A patient’s high BUN levels may indicate that their kidneys aren’t functioning properly, while lower BUN levels indicate overhydration, liver disease, or malnutrition.

What Does BUN Mean in a Blood Test?

BUN results can reveal important information about how well a patient’s kidneys function. It measures the amount of urea nitrogen that’s in the blood.

A blood test group that includes BUN levels can be used to help diagnose several other conditions, such as liver damage, urinary tract obstruction, congestive heart failure, or gastrointestinal bleeding. However, an abnormal BUN result alone doesn’t confirm any of these conditions.

Urea is a byproduct of protein waste. Think of a protein bar wrapper — it’s trash that the body tosses out. Body-wise, this “trash,” is in the form of ammonia. 

The liver converts this ammonia into urea and pushes it into the blood, and the kidneys excrete it from the body. Hence the name blood urea nitrogen.

BUN Normal Range

BUN level results are measured in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

The normal range for BUN levels is about 6 to 20 mg/dL or 2.1 to 7.1 mmol/L. Any amount over that usually means a patient is dehydrated.

But normal ranges can vary, depending on a lab’s reference range and the age of the patient being tested.

Find more normal range lab values here.

High BUN Levels – What Does High BUN Mean?

A BUN level over 20 mg/dL is considered abnormal and can point to some harmful conditions. But a dangerously high BUN level, often starting at 50 mg/dL, indicates kidney damage that needs immediate medical attention.

A patient with a BUN between 100 mg/dL and 250 mg/dL may have severe kidney dysfunction, resulting from a problem with the organ’s ability to filter waste products out of the bloodstream.

A high BUN level typically indicates some dysfunction with a patient’s kidneys. This can be caused by:

  • Certain medications (such as some antibiotics)
  • Dehydration
  • Urinary tract obstruction
  • Congestive heart failure
  • Recent heart attack
  • Gastrointestinal bleeding
  • Shock
  • Severe burns
  • Too much protein in diet

High BUN Levels Symptoms

  • Muscle cramps
  • Joint and bone pain
  • Restless legs
  • Hand and feet swelling
  • Frequent urination
  • Discolored urine
  • Extreme fatigue
  • Skin itching 

Low BUN Levels – What Does Low BUN Mean?

Low BUN levels are uncommon, and in most cases aren’t a cause for concern. However, severe health conditions can cause BUN levels to be unusually low.

A low BUN level may be normal during the second or third trimester of pregnancy. They could also indicate:

  • Overhydration
  • Liver failure
  • Severe lack of protein 
  • Malnutrition

What level of bun indicates kidney failure?

Health care providers typically don’t rely solely on blood urea nitrogen (BUN) levels to diagnose kidney failure. However, if a patient’s BUN levels exceed their normal baseline and their creatinine levels are elevated, it indicates potential kidney failure. A BUN level higher than 20 mg/dL suggests that the kidneys may not be functioning at optimal capacity.

BUN/Creatinine Ratio

In general, the normal range for the BUN/Creatinine ratio is between 10:1 and 20:1. However, it’s important to note that the interpretation of the ratio can vary depending on the individual patient’s circumstances, medical history, and the reference ranges used by the specific laboratory. 

Therefore, it’s essential for nurses to refer to the laboratory’s reference range and consult with healthcare providers for a comprehensive understanding of the results.

Memorize Lab Values in Less Time

BUN is just one of many lab values you have to memorize as a nurse.

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