The RenalTracker Team
August 23, 2021

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A Blood Urea Nitrogen (BUN) test measures the urea nitrogen levels in the blood. Urea is a waste product that is produced in the body as a natural process of breaking down proteins. The liver breaks down protein and ammonia to produce urea, after which, it is taken up by the kidneys. The kidneys transfer the urea from the blood to the urine. Urea removes the extra nitrogen and nitrogenous products from the body.

Your BUN level increases if your kidneys couldn’t expel urea from your blood. Moreover, heart failure, dehydration, or a diet high in protein can also raise your BUN level. 

A BUN test is recommended when a patient experiences symptoms of kidney disease, such as: 

  • Frequent urination
  • Itching
  • Often feeling tired
  • Swelling in your legs, arms, and feet
  • Muscle cramps
  • Difficulty in sleeping

As kidney function deteriorates, the filtering process is affected, and waste products such as urea nitrogen accumulate in the blood, which causes elevated BUN results. 

What is BUN's normal range?

According to the American Board of Internal Medicine, the typical range for blood urea nitrogen is 8-20 mg/dL in adults. However, this may differ from one laboratory to another. 

High levels of Blood Urea Nitrogen are one sign of kidney disease or failure. But this may not be the only case. This may also indicate:

  • Dehydration
  • High-protein Intake
  • Medications such as prednisone or tetracycline antibiotics
  • Burns
  • Stomach and intestinal bleeding
  • Pre-existing conditions like pregnancy and liver disease
Hands holding a kidney frame

At times, age is also a factor. Studies show that elderly aged 65 and above often have high BUN levels. On the other hand, having a low BUN Value may be a result of a very low protein diet, malnutrition, or extreme liver damage. 

BUN vs. Creatinine

Blood Urea Nitrogen cannot be the only basis in determining kidney function. This is because it can be influenced by factors such as a high protein diet, variables in protein synthesis, and patient hydration status.

For these reasons, Serum Creatinine is also included in the series of tests in determining kidney function. 

Serum Creatinine is a test that checks the level of Creatinine in the blood. Creatinine is a waste product of muscle protein breakdown, and similar to BUN, it is also filtered by the kidneys. When it comes to Creatinine, the age and gender of a person are considered during interpretation, since male patients often have a larger body frame than women. Thus, their results are usually higher.

Factors that may elevate Serum Creatinine are: 

  • Glomerulonephritis - damage or swelling of blood vessels in the kidneys
  • Pyelonephritis - a bacterial infection in the kidneys
  • Acute Tubular Necrosis - the death of cells in the kidneys
  • Prostate disease or kidney stone - conditions that can block the flow of urine in the urinary tract
  • Reduced blood flow to the kidneys - may be due to Congestive heart failure, dehydration, atherosclerosis, or complications of diabetes.

Interpreting BUN and Creatinine levels

For accurate results, doctors refer to Serum Creatinine and BUN tests to see the status of the kidneys. There are times when the BUN is normal and may seem like your kidneys are functioning normally. Still, when the Serum Creatinine results show abnormally high levels, this may mean that there is a problem with your kidney function.

The results of these two tests may help interpret the capacity of the glomerulus (the filtering tube of the kidney). Usually, when either one or both have abnormal values, the kidney's filtration rate is measured through a Glomerular Filtration Rate (GFR) test. Glomerular Filtration Rate (GFR) is a test conducted to see the filtration of the kidneys' glomeruli.

The interpretation of BUN & Serum Creatinine levels may differ depending on the existing health conditions and other lifestyle factors. This is why you must consult a doctor to help discuss the factors which may affect your BUN results when interpreting them. 

What is BUN to Creatinine Ratio?

In some cases, doctors check the BUN to Creatinine Ratio to determine which caused the altered kidney function. The  BUN to Creatinine Ratio is a calculation that can be used together in verifying Glomerular Filtration Rate (GFR) results. It is assumed that when Creatinine and BUN levels are increased, the kidney's function is altered or slowed down, leading to a decreased GFR result. 

The BUN to Creatinine is usually between 10:1 and 20:1

A high ratio may confirm that it may be due to decreased blood flow to the kidneys. This may be caused by shock or dehydration. At times, it can also be linked to heart diseases such as Congestive Heart Failure. While, a very high ratio may mean that there is bleeding in the gastrointestinal tract. 

In contrast, a lower ratio may indicate liver problems or protein malnutrition. 

Although high levels of creatinine are indicative of potential kidney problems, other possible reasons could elevate your creatinine levels. These reasons may be one-time occurrences, such as dehydration or intake of large amounts of supplement creatine or protein.

How to lower BUN levels?

Here are three tips to help lower BUN levels:

1) Stay hydrated

Dehydration reduces blood volume and can increase blood solutes. This can elevate BUN levels and even Creatinine levels. Drinking more water can help flush out the urea and Creatinine from your body through urination.

However, this tip can be a challenge for Stage 3 and above CKD patients. Make sure to coordinate with your doctor and dietitian so that they can give you fluid recommendations. 

2) Manage your Protein Intake

plant-based protein foods

As mentioned earlier, urea nitrogen is a waste product of the protein sources in the diet. This is why a high-protein diet can easily alter BUN levels. Protein may be obtained from fish, egg, chicken, red meat, nuts, seeds, and tofu. Seek advice from your doctor or dietitian for your protein portion sizes.

3) Aim for a normal blood pressure

Studies suggest that normal blood pressure may help lower BUN levels. This can be done through diet changes, cutting back sodium, and adding fiber to your diet. Your dietitian can help determine which food items can be added to and minimized from your diet. 

Exercise has also been proven to improve blood pressure. You can check our blog on exercise for tips. 

The Bottom line

Blood Urea Nitrogen (BUN) is a helpful blood chemistry test to check kidney function and condition and other health-related conditions. Nevertheless, you still need other Kidney-related lab tests to determine the actual condition of your kidneys, such as Serum Creatinine and Glomerular Filtration Rate (GFR). 

It is best to remember these three tips to help lower your Blood Urea Nitrogen (BUN):

  • Staying hydrated
  • Controlling your protein intake 
  • Aiming for normal blood pressure

Sources:

Blood Urea Nitrogen (BUN) Test; Mayo Clinic - https://www.mayoclinic.org/tests-procedures/blood-urea-nitrogen/about/pac-20384821

Blood Urea Nitrogen (BUN) Test; ClevelandClinic - https://my.clevelandclinic.org/health/diagnostics/17684-blood-urea-nitrogen-bun-test

Why are BUN and Creatinine important in Kidney Patients; St. Clair Nephrology - https://www.scsp.net/articles/why-are-bun-and-creatinine-important-to-monitor-in-kidney-patients/

What is the Difference between sCr, eGFR, ACR, and BUN; National Kidney Foundation - https://www.kidney.org/newsletter/what-difference-between-scr-egfr-acr-and-bun

Overview of Urea and Creatinine; Salazar J. - https://academic.oup.com/labmed/article/45/1/e19/2657879

The Meaning of Blood Urea Nitrogen/Creatinine Ratio in Acute Kidney Injury; Uchino, S., Bellomo, R., & Goldsmith, D. - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5783213/

Urea and Creatinine Concentration, The Urea: Creatinine Ratio; Higgins, C. - https://acutecaretesting.org/en/articles/urea-and-creatinine-concentration-the-urea-creatinine-ratio

Blood Urea Nitrogen (BUN); Lab Tests Online - https://labtestsonline.org/tests/blood-urea-nitrogen-bun

Blood Urea Nitrogen (BUN) Test; University of Michigan Health - https://www.uofmhealth.org/health-library/aa36271#aa36277


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