As a nurse, you may be tasked with performing lab draws. But more importantly, you will be required to review and interpret lab results to recognize when a value requires immediate attention and to anticipate the appropriate actions. One common laboratory test you will encounter is BUN, or blood urea nitrogen.
In this article:
- What is Blood Urea Nitrogen?
- Why is BUN Important in Nursing Care?
- Conditions That Influence BUN Levels
- Understanding BUN: Creatinine Ratio
What is Blood Urea Nitrogen?
In short, urea is a waste product produced by the liver during the breakdown of proteins and excreted by the kidneys.
A condensed version of the urea cycle looks like this: When the body breaks down dietary protein or muscle tissue for energy (catabolism), the resulting amino acids release nitrogen. This nitrogen is initially converted into ammonia, which is toxic to the body. The liver then converts ammonia into urea, which is less toxic and readily excreted by the kidneys.
What is the Normal BUN Range?
While the exact range varies by laboratory and facility, a normal BUN value in the U.S. is 7-20 mg/dL. BUN levels are a typical part of a BMP (basic metabolic panel) and CMP (comprehensive metabolic panel) ordered by the physician.
Why is BUN Important in Nursing Care?
BUN levels can provide a picture of the patient’s kidney function. Be aware, though, that other factors can also affect BUN (more on this later). BUN levels are not assessed independently; they are combined with creatinine levels and, often, other, more direct tests to assess kidney health.
Conditions That Influence BUN Levels
Impaired kidney function is the most common cause of abnormal BUN levels, but other common factors that affect BUN include:
- Hydration status
- Diet
- Age
- Certain medications
- Pregnancy
- Medical conditions
What Causes High BUN Levels?
As a nursing student, consider the following when assessing your patient with an elevated BUN result:
- Dehydration: Fluid loss causes the kidneys to reabsorb water, including urea, leading to an increase in BUN.
- Protein Intake: A high-protein diet may increase BUN since there is more protein to break down and convert into urea.
- Older Adults: As kidney function declines with age, BUN levels may increase.
- Medications: Diuretics, chronic NSAID use, and some antibiotics may increase BUN.
- Gastrointestinal (GI) Bleeding: A GI bleed may cause a rise in BUN because digested blood acts as a large protein load in the digestive system, leading to a surge in urea production by the liver.
- Congestive Heart Failure (CHF): CHF alters blood flow to the kidneys, activates the renin-angiotensin-aldosterone system, and causes the renal tubules to reabsorb urea.
Nursing Interventions for High BUN
Treatment is dependent on the underlying cause. As a nurse, common interventions may include administering IV fluids, encouraging dietary modifications, reconciling medications, and managing chronic conditions.
What Causes Low BUN Levels?
The following are examples of causes that result in decreased BUN levels:
- Overhydration: Excessive fluid intake dilutes urea levels.
- Malnutrition: The opposite of excessive protein intake, if the patient is deficient in protein, BUN levels may be lower.
- Liver Disease: In liver diseases, BUN levels may be low since a damaged liver is unable to convert ammonia into urea.
- Pregnancy: Low BUN levels are normal in the second and third trimesters.
Nursing Interventions for Low BUN
Low BUN results do not always require intervention, such as during pregnancy. Nurses may focus on restoring fluid and electrolyte balance, nutrition support, and disease education.
Understanding BUN: Creatinine Ratio
Creatinine is another waste product formed during the breakdown of muscle that is excreted by the kidneys. Since BUN levels are significantly influenced by hydration status and health conditions, the BUN: Creatinine ratio provides a more complete picture of renal function.
The normal BUN to creatinine ratio (BUN: Cr) is typically 10:1 to 20:1.
Creatinine Ratio and Kidney Function
The BUN: Creatinine ratio offers more information about the exact issue with the kidneys. The presence of high levels of nitrogen and other waste products in the blood results in elevated BUN and creatinine levels, known as azotemia. There are three types of azotemia:
Pre-Renal Azotemia (The Most Common Cause)
This is the cause you will encounter most frequently in clinical practice. The kidneys are structurally intact, but decreased blood flow (perfusion) leads to sodium and water conservation, resulting in excessive urea reabsorption. The result is a BUN: Cr > 20:1.
Common Causes: Dehydration (e.g., from vomiting, diarrhea, insufficient intake), septic or hemorrhagic shock, and reduced cardiac output (e.g., congestive heart failure).
Intrinsic (Intra-Renal) Azotemia
Direct damage to the kidney impairs the nephron’s ability to filter and excrete waste normally. BUN and creatinine often rise proportionately, so BUN: Cr remains within a normal range. In some instances, urea is not adequately reabsorbed, while creatinine continues to rise, resulting in a lower BUN: Cr ratio (< 10:1).
Common Causes: Acute tubular necrosis (ATN) from ischemia or toxins, glomerulonephritis, and exposure to nephrotoxic drugs (such as chemotherapy, aminoglycoside antibiotics, or contrast dye).
Post-Renal Azotemia
Obstruction of urine flow after the kidney creates pressure that damages the nephrons and impairs filtration. The BUN: Cr ratio will be elevated initially ( > 20:1), but will drop ( < 10:1) if left untreated.
Common Causes: Kidney stones, urinary tract tumors, and benign prostatic hyperplasia (BPH).
Nursing Assessments With Acute Kidney Injury
In your nursing role, it won’t be within your scope of practice to identify a specific type of azotemia. But, it’s crucial for you to recognize abnormal findings that correspond with acute kidney injury (AKI), such as:
- Tachycardia
- Hypotension
- Dry mucous membranes
- Poor skin turgor
- Reduced urine output
- Swelling in the lower extremities
- Dyspnea
- Confusion
- A palpable bladder (obstruction)
Integrating BUN into Patient Care
Blood urea nitrogen is not a standalone indicator, but a collaborative test that reflects kidney function. By knowing the reference range and understanding the BUN/Creatinine ratio, you elevate your practice from simply reporting labs to critically thinking about the underlying causes of azotemia to advocate for appropriate interventions and prevent declining renal function.
References
- Alka Tyagi; Narothama R. Aeddula. Azotemia. Accessed October 2025. https://www.ncbi.nlm.nih.gov/books/NBK538145/
- Frank O’Brien. Acute Tubular Necrosis (ATN). Accessed October 2025. https://www.msdmanuals.com/professional/genitourinary-disorders/tubulointerstitial-diseases/acute-tubular-necrosis-atn
- MedlinePlus Editorial Staff. BUN (Blood Urea Nitrogen): MedlinePlus Medical Test. Accessed October 2025. https://medlineplus.gov/lab-tests/bun-blood-urea-nitrogen/
- Verena Gounden; Harshil Bhatt; Ishwarlal Jialal. Renal Function Tests. Accessed October 2025. https://www.ncbi.nlm.nih.gov/books/NBK507821/
- Walker Barmore; Farhan Azad; William L. Stone. Physiology, Urea Cycle. Accessed October 2025. https://www.ncbi.nlm.nih.gov/books/NBK513323/
