Blood Urea / BUN Test Explained
Blood urea is a commonly requested blood test that can provide information about how your body is handling a waste product called urea.
Urea is produced mainly in the liver when the body breaks down proteins. It travels in the blood to the kidneys, where much of it is removed from the body in urine.
Because the kidneys help remove urea, its level in the blood can provide useful information during an assessment of kidney function.
However, blood urea is not a kidney-function diagnosis by itself.
The result can also be affected by factors such as hydration, protein intake, bleeding in the digestive system, some illnesses and liver function.
For this reason, doctors usually interpret blood urea together with other information, particularly tests such as serum creatinine and eGFR.
What Is Urea?
Your body continuously breaks down proteins from food and from normal body processes.
This produces nitrogen-containing waste.
The liver converts much of this waste into urea.
Urea then enters the bloodstream.
The blood carries it to the kidneys, and the kidneys remove much of it into the urine.
A small amount of urea normally remains in the blood.
A blood urea test measures how much is present.
What Is BUN?
BUN stands for:
Blood Urea Nitrogen
BUN is closely related to blood urea, but the two terms do not describe exactly the same numerical measurement.
A BUN result measures the nitrogen contained in urea, rather than reporting the entire urea molecule.
This is why a laboratory report showing urea and another report showing BUN may have different numerical values even when they relate to the same underlying substance.
Why Is Blood Urea or BUN Tested?
A doctor may request blood urea or BUN as part of an assessment of your kidneys or your general medical condition.
It may be requested:
- together with serum creatinine and other kidney-related blood tests;
- when kidney function needs to be assessed;
- when a person has an illness that could affect kidney function;
- when monitoring someone with a known medical condition;
- as part of a broader renal profile / blood-test panel;
- when a doctor wants to assess changes in hydration or other aspects of a patient's condition.
The reason for requesting the test can differ from one patient to another.
How Is the Test Done?
Blood urea or BUN is measured from a blood sample.
A healthcare worker usually takes a small amount of blood from a vein in your arm.
The sample is then analysed by the laboratory.
The test itself does not require a urine sample.
However, your doctor may request urine tests at the same time if a broader kidney assessment is needed.
Do I Need to Fast for a Blood Urea or BUN Test?
If blood urea or BUN is being tested by itself, fasting is not always required.
However, it is often requested together with other blood tests, and some of those tests may have their own preparation requirements.
Therefore, follow the instructions given for the complete set of blood tests you are having, rather than assuming that no preparation is needed.
If you are unsure, ask the laboratory or the healthcare professional who requested the tests.
What Can Cause High Blood Urea or BUN?
A high blood urea or BUN result can occur when the kidneys are not removing urea from the blood as effectively as expected.
However, kidney disease is not the only possible explanation.
Blood urea or BUN may also increase in situations such as:
- dehydration or reduced body fluid volume;
- increased protein breakdown;
- a high protein intake;
- bleeding in the stomach or intestines;
- some serious illnesses;
- reduced blood flow to the kidneys;
- blockage affecting urine flow;
- some medicines.
The importance of the result depends on the individual situation.
This is why a high blood urea or BUN result should not be interpreted in isolation.
What Can Cause Low Blood Urea or BUN?
A low blood urea or BUN result is usually interpreted in the context of the person's overall health.
Possible reasons can include:
- low protein intake;
- poor nutrition;
- having a large amount of fluid in the body;
- reduced production of urea in some liver conditions.
A low result does not automatically mean that there is a kidney problem.
Your doctor will decide whether the result is important based on the rest of your medical information.
Does Blood Urea Tell You How Well Your Kidneys Are Working?
Blood urea can provide useful information during a kidney assessment, but it is not the best test to interpret alone.
Several factors outside the kidneys can change the blood urea level.
Doctors therefore commonly consider blood urea together with tests such as:
- serum creatinine;
- eGFR;
- urine tests when appropriate.
Serum creatinine and eGFR provide additional information about kidney filtration.
Urine tests such as urine albumin or ACR can provide information about kidney damage.
These tests answer related but different questions.
Blood Urea and Serum Creatinine: What Is the Difference?
Both urea and creatinine are waste products that can be measured in the blood.
However, they are produced differently and are affected by different factors.
Urea is produced mainly from the body's handling of protein and nitrogen-containing waste.
Creatinine is produced from normal processes involving muscles.
The kidneys help remove both substances from the blood.
However, blood urea can be strongly influenced by factors such as hydration, protein intake and some illnesses.
For this reason, doctors do not usually decide how well the kidneys are working from the urea result alone.
For a detailed explanation of creatinine, read Serum Creatinine Test Explained.
Blood Urea and eGFR Are Not the Same Thing
Blood urea measures the amount of urea or urea nitrogen present in the blood.
eGFR is an estimate of how well the kidneys are filtering the blood.
eGFR is commonly estimated using information that includes the serum creatinine level.
Therefore, a blood urea result and an eGFR result should not be treated as interchangeable measurements.
For a detailed explanation, read eGFR Explained.
Can Blood Urea Be High When Creatinine Is Normal?
Yes.
Because blood urea and creatinine are affected by different factors, they do not always rise and fall together.
For example, blood urea may increase because of dehydration, increased protein breakdown, high protein intake or bleeding in the digestive system even when the creatinine result does not show the same degree of change.
The opposite pattern can also occur.
This is one reason doctors interpret the combination of results and the clinical situation, rather than relying on one number.
Can Dehydration Increase Blood Urea or BUN?
Yes.
Dehydration or reduced body fluid volume can increase blood urea or BUN.
However, you should not assume that every high result is caused by dehydration.
A high result may have other explanations, including kidney-related and non-kidney-related causes.
If your result is abnormal, the appropriate next step depends on your overall medical situation.
Should I Drink Extra Water Just Because My Urea Is High?
Do not make major changes to your fluid intake simply to try to change a laboratory number without understanding why it is abnormal.
Some people may need more fluid when dehydrated, while other patients — including some people with kidney, heart or other medical conditions — may have been advised to limit their fluid intake.
If your blood urea or BUN is high, the important step is to understand why it is high.
Follow the advice given for your individual medical condition.
Can Food and Protein Intake Affect Blood Urea?
Yes.
Because urea is produced during the body's handling of protein, protein intake can influence the blood urea or BUN level.
However, an abnormal result should not automatically be blamed on food.
Do not make major dietary changes or restrict protein solely because of one blood urea result unless this has been recommended for your medical condition.
What Is the Normal Range?
The normal range depends on:
- whether the laboratory reports urea or BUN;
- the units used;
- the laboratory's testing method and reference interval.
For this reason, use the reference range printed next to the result on your own laboratory report.
Do not compare a urea result directly with a BUN reference range.
Also remember that a value slightly outside the laboratory range does not by itself establish a diagnosis.
What Should I Do If My Blood Urea or BUN Is Abnormal?
An abnormal result should be interpreted according to the reason the test was requested and your overall health.
Your doctor may consider:
- whether the result is only mildly or significantly abnormal;
- serum creatinine and eGFR;
- previous blood-test results;
- hydration and current illness;
- medicines;
- urine-test results;
- other medical conditions.
Sometimes a repeat test or additional investigations may be needed.
Do not diagnose kidney disease from a blood urea or BUN result alone.
When Should I Seek Medical Advice?
Arrange medical review if you have an abnormal kidney-related blood test and have been advised to discuss the result, particularly if the abnormality is new or persistent.
Seek prompt medical attention if you are unwell and develop symptoms such as:
- markedly reduced urine output;
- difficulty passing urine;
- repeated vomiting or inability to keep fluids down;
- significant swelling;
- unusual drowsiness or confusion;
- difficulty breathing;
- severe weakness or a rapidly worsening general condition.
Remember
A high result does not automatically mean kidney failure.
A normal result does not by itself prove that the kidneys are completely healthy.
Urea can change because of kidney-related and non-kidney-related factors.
Doctors interpret it together with tests such as serum creatinine, eGFR and urine tests, as well as the patient's medical condition.
Related kidney health topics
Key Points
- Urea is a waste product produced mainly during the body's handling of protein.
- The kidneys remove much of the urea from the blood.
- BUN means Blood Urea Nitrogen.
- Blood urea and BUN are closely related but are not numerically the same measurement.
- Always check whether your laboratory reports urea or BUN and which units it uses.
- A high result can occur with kidney problems, but kidney disease is not the only possible cause.
- Dehydration, protein-related factors, digestive-system bleeding and some illnesses can also increase the result.
- Low values can occur for reasons such as low protein intake, poor nutrition, excess body water or reduced urea production in some liver conditions.
- Blood urea should not be interpreted alone.
- Creatinine, eGFR, urine tests and the clinical situation provide additional information.
- Do not make major fluid or dietary changes solely to alter one laboratory value without appropriate medical advice.