Test Overview

A blood urea nitrogen (BUN) test measures
the amount of nitrogen in your blood that comes from the waste product
urea. Urea is made when protein is broken down in your
body. Urea is made in the
liver and passed out of your body in the urine.

A BUN test is done to see how well your
kidneys are working. If your kidneys are not able to
remove urea from the blood normally, your BUN level rises.
Heart failure,
dehydration, or a diet high in protein can also make
your BUN level higher. Liver disease or damage can lower your BUN level. A low
BUN level can occur normally in the second or third
trimester of pregnancy.

Blood urea nitrogen to creatinine ratio (BUN:creatinine)

A BUN test may be done with a blood
creatinine test. The level of creatinine in your blood
also tells how well your kidneys are working-a high creatinine level may mean
your kidneys are not working properly. Blood urea nitrogen (BUN) and creatinine
tests can be used together to find the BUN-to-creatinine ratio
(BUN:creatinine). A BUN-to-creatinine ratio can help your doctor check for
problems, such as dehydration, that may cause abnormal BUN and creatinine
levels.

Why It Is Done

A
blood urea nitrogen (BUN) test is done to:

  • See if your kidneys are working
    normally.
  • See if your kidney disease is getting
    worse.
  • See if treatment of your kidney disease is
    working.
  • Check for severe dehydration. Dehydration
    generally causes BUN levels to rise more than creatinine levels. This causes a
    high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine
    from your kidney causes both BUN and creatinine levels to go up.

How To Prepare

Do not eat a lot of meat or other
protein in the 24 hours before having a blood urea nitrogen (BUN) test.

How It Is Done

The health professional drawing blood
will:

  • Wrap an elastic band around your upper arm to
    stop the flow of blood. This makes the veins below the band larger so it is
    easier to put a needle into the vein.
  • Clean the needle site with
    alcohol.
  • Put the needle into the vein. More than one needle stick
    may be needed.
  • Attach a tube to the needle to fill it with
    blood.
  • Remove the band from your arm when enough blood is
    collected.
  • Put a gauze pad or cotton ball over the needle site as
    the needle is removed.
  • Put pressure to the site and then a
    bandage.

How It Feels

The blood sample is taken from 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.

Risks

There is very little chance of a problem from
having a blood sample taken from a vein.

  • You may get a small bruise at the site. You can
    lower the chance of bruising by keeping pressure on the site for several
    minutes.
  • In rare cases, the vein may become swollen after the blood
    sample is taken. This problem is called phlebitis. A warm compress can be used
    several times a day to treat this.

Results

A blood urea nitrogen (BUN) test measures
the amount of nitrogen in your blood that comes from the waste product
urea.

Normal

The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Blood urea nitrogen (BUN) footnote 1

Adults:

10-20 milligrams per deciliter (mg/dL) or 3.6-7.1 millimoles per liter (mmol/L)

Children:

5-18 mg/dL

BUN-to-creatinine ratio footnote 1

Adults:

6-25 with 15.5 being the best value.

High values

  • A high BUN value can mean kidney injury or
    disease is present. Kidney damage can be caused by
    diabetes or
    high blood pressure that directly affects the kidneys.
    High BUN levels can also be caused by low blood flow to the
    kidneys caused by
    dehydration or
    heart failure.
  • Many medicines may cause a
    high BUN. Be sure to tell your doctor about all the nonprescription and
    prescription medicines you take.
  • A high BUN value may be caused by
    a high-protein diet,
    Addison’s disease, tissue damage (such as from severe
    burns), or from bleeding in the gastrointestinal tract.
  • High
    BUN-to-creatinine ratios occur with sudden (acute) kidney problems, which may be
    caused by
    shock or severe dehydration. A very
    high BUN-to-creatinine ratio may be caused by bleeding in the
    digestive tract or
    respiratory tract.

Low values

  • A low BUN value may be caused by a diet very
    low in protein, malnutrition, or severe liver damage.
  • Drinking
    excessive amounts of liquid may cause overhydration and cause a low BUN
    value.
  • Women and children may have lower BUN levels than men
    because of how their bodies break down protein.
  • A low
    BUN-to-creatinine ratio may be caused by a diet low in protein, a severe muscle
    injury called
    rhabdomyolysis, pregnancy,
    cirrhosis, or syndrome of inappropriate antidiuretic
    hormone secretion (SIADH).

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

  • Taking medicines, such as amphotericin B (such
    as Fungizone), nafcillin, aminoglycosides, kanamycin (Kantrex), tobramycin
    (Nebcin),
    corticosteroids, tetracycline antibiotics, and, rarely,
    the antibiotic chloramphenicol (Chloromycetin).
  • Taking
    diuretics, which can cause
    dehydration.

What To Think About

  • A BUN test may be done with a blood
    creatinine test. Blood urea nitrogen (BUN) and
    creatinine tests can be used together to find the BUN-to-creatinine ratio
    (BUN:creatinine). To learn more, see the topic
    Creatinine and Creatinine Clearance.
  • BUN
    levels may be measured regularly in people who have
    kidney dialysis. BUN levels are used to see how well
    dialysis is working.
  • A
    glomerular filtration rate may be done for people with
    chronic kidney disease to regularly check how well the kidneys are
    working.

References

Citations

  1. Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Credits

ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Kathleen Romito, MD – Family Medicine
Specialist Medical Reviewer Caroline S. Rhoads, MD – Internal Medicine

Current as ofMay 3, 2017