High BUN Levels Explained: Symptoms, Causes, and Tests You Need to Know

The blood urea nitrogen (BUN) test is a widely used diagnostic tool that provides valuable insights into kidney function and overall metabolic health. With chronic kidney disease (CKD) affecting approximately 15% of the U.S. adult population, or roughly 36 million people, its prevalence is a significant public health concern. Alarmingly, nearly 90% of those with CKD are unaware they have it, underscoring the importance of routine screening and early detection. (30)

The asymptomatic nature of early-stage kidney disease makes regular monitoring of kidney function through tests like BUN crucial for identifying potential issues before they progress to more advanced stages.

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What Is BUN?

Blood urea nitrogen (BUN) is a clinical measurement that reflects the amount of nitrogen in the blood that comes from urea, a waste product formed in the liver when the body breaks down proteins. Urea is released into the bloodstream and transported to the kidneys, where it is filtered out of the blood and excreted in urine.

The BUN test is commonly used to assess kidney function, as the kidneys remove 85% of urea from the bloodstream. Elevated BUN levels may indicate impaired kidney function. Conversely, low BUN levels can be seen in conditions such as liver disease, malnutrition, or overhydration. (42)

Overall, BUN is a valuable marker in routine health assessments and diagnosing and monitoring conditions affecting the kidneys and related systems. It is commonly ordered as part of a comprehensive metabolic panel (CMP), which also assesses blood sugar, electrolyte balance, liver function, and other parameters of kidney health as a thorough metabolic assessment. Interpreting BUN levels with clinical context and other laboratory findings improves diagnostic accuracy.

Doctors will order a BUN test for various reasons, including (4, 5):

Suspicion of kidney disease or damage
To monitor kidney health if you are at a higher risk for developing kidney disease due to diabetes, high blood pressure, heart disease, or a family history of kidney disease
As part of a comprehensive evaluation for other health conditions, such as liver disease, urinary tract obstruction, or gastrointestinal bleeding

Understanding High BUN Levels

High BUN levels refer to an increase in the concentration of urea nitrogen in the blood above the normal reference range.

The normal range for BUN in adults typically falls between 6 and 20 milligrams per deciliter (mg/dL). However, this can vary slightly depending on laboratory and patient factors such as age, sex, and overall health.

For example, BUN levels in children are about two-thirds of those in healthy young adults, making the normal reference range 5 to 18 mg/dL.

An elevated BUN alone does not provide a definitive diagnosis. Elevated BUN can result from a variety of conditions, ranging from benign and easily reversible causes to more serious underlying diseases. Therefore, it is important to interpret BUN levels in the broader context of the patient's overall health, history, and additional laboratory results to arrive at an accurate diagnosis and appropriate treatment plan.

Symptoms Associated with High BUN Levels

If you have high BUN levels due to kidney disease, you may also experience the following symptoms:

Frequent urination (polyuria)
Frequent urination at night (nocturia)
Change in urine volume
Blood in urine
Foamy urine
Muscle cramps
Fatigue
Poor appetite
Swelling, especially in the hands, feet, and face

The severity of symptoms generally mirrors the severity of the underlying disease and the extent to which BUN levels are elevated. Mildly elevated BUN levels may not produce noticeable symptoms and are often discovered incidentally during routine blood tests. However, as BUN levels rise, particularly in cases of acute kidney injury (AKI) or advanced CKD, symptoms can become more pronounced and debilitating.

Causes of High BUN

High BUN levels can arise from both renal and non-renal causes.

Renal causes involve conditions that impair the kidneys' ability to filter blood effectively. CKD is a common renal cause, where long-term damage to the kidneys reduces their capacity to excrete urea, leading to its accumulation in the bloodstream. AKI is another renal cause, often resulting from sudden damage due to factors like severe infections, toxins, or ischemia (lack of blood flow). In both CKD and AKI, the kidneys' glomerular filtration rate (GFR) declines. GFR is the rate at which blood is filtered through the kidneys. Reductions in GFR cause a buildup of urea and other wastes in the blood, which can manifest as elevated BUN levels. (29)

Non-renal causes of high BUN levels stem from conditions that increase urea production or decrease its elimination without directly involving intrinsic kidney damage. Dehydration is a notable non-renal cause, where reduced blood volume leads to decreased kidney perfusion, causing a temporary reduction in GFR. This leads to less urea being filtered out of the blood, resulting in elevated BUN levels.

Protein is a macronutrient composed of nitrogen-containing amino acids that serves as the building block for life. The liver produces urea as a metabolic waste product of protein metabolism. A high-protein diet increases urea production, leading to higher BUN levels (31).

When there is bleeding in the gastrointestinal (GI) tract, blood proteins are digested and metabolized into urea. This process leads to a higher concentration of urea in the bloodstream, reflected as elevated BUN levels. (36)

In heart failure, reduced cardiac output decreases blood flow to the kidneys, impairing their function and leading to elevated BUN levels. BUN can actually be used as a prognostic indicator for heart failure. The mortality rate of heart failure patients increased by 21% for every 10 mg/dL increase in BUN. (22)

Many over-the-counter and prescription medications can also increase BUN levels, including:

Antibiotics
Allopurinol
Aspirin
Carbamazepine
Diuretics
Methotrexate
Methyldopa
Propranolol

Diagnostic Tests and Biomarkers

The BUN blood test is a simple and inexpensive routine procedure used to measure the amount of urea nitrogen in the blood. A healthcare professional collects a blood sample from a vein, typically in the arm, during the test. This sample is then analyzed in a laboratory to determine the concentration of urea nitrogen. (27)

Measure BUN through Rupa Health with one of the following tests:

BUN by Access Med Labs
Basic Metabolic Panel by Access Labcorp Draw
Comprehensive Metabolic Panel by Boston Heart Diagnostics

BUN is rarely measured alone. Most times, doctors will interpret BUN results by comparing them with other complementary tests to provide a more comprehensive assessment of kidney function and help identify underlying health conditions.

Creatinine is a waste product of muscle metabolism and is also eliminated from the body by the kidneys. With BUN, these two markers are used to calculate the BUN-to-creatinine ratio. This ratio helps differentiate the cause of high BUN. A high BUN-to-creatinine ratio (> 20:1) indicates prerenal disease, such as gastrointestinal bleeding or reduced blood flow to the kidneys due to dehydration. On the other hand, a ratio closer to 10:1 indicates elevated BUN caused by kidney disease or injury. (44)

GFR is a test that estimates how well the kidneys filter blood. It is calculated using creatinine levels, age, sex, and body size. A low GFR indicates reduced kidney function and is one of the parameters used to stage CKD. (16)

Electrolyte levels, such as sodium, potassium, and bicarbonate, are also important in assessing kidney function, as maintaining electrolyte balance is one of the kidneys' responsibilities. Abnormal levels can indicate impaired kidney function or complications such as acidosis or hyperkalemia (high potassium), which are common in advanced kidney disease.

In cases where elevated BUN levels suggest an underlying condition beyond routine kidney dysfunction, specialized tests and imaging may be required. For instance, renal ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) can help identify structural abnormalities like kidney stones, tumors, or blockages (34). A kidney biopsy may be performed to diagnose specific types of kidney disease, such as glomerulonephritis or interstitial nephritis, by examining a small tissue sample under a microscope. In suspected cases of autoimmune diseases affecting the kidneys, tests for specific antibodies, such as anti-nuclear antibodies (ANA) or anti-glomerular basement membrane (anti-GBM) antibodies, may be conducted.

What a Dangerous High BUN Level Indicates

BUN levels exceeding 50 mg/dL are concerning and suggest serious kidney dysfunction. BUN levels above 100 mg/dL are flagged as critical and typically indicate a medical emergency requiring immediate intervention to stabilize the patient, identify the underlying cause through follow-up tests, and prevent serious complications.

Severe BUN elevation is associated with increased risks of uremia, a condition where accumulated waste products in the blood can affect multiple organ systems, leading to life-threatening complications if not promptly addressed. Signs and symptoms of uremia include:

Nausea
Vomiting
Itching
Hyperpigmented skin
Confusion
Seizure
Pancreatitis
Pulmonary edema (fluid buildup in the lungs)
Pericarditis (inflammation of the sac that surrounds the heart)
Coma

Management and Treatment Options

The following steps can help protect your kidneys and prevent the development of kidney disease:

Maintain a blood pressure of less than 120/80 mmHg
Control your blood sugar to prevent and manage insulin resistance, prediabetes, and diabetes
Eat a healthy diet rich in fruits and vegetables
Limit dietary intake of salt and saturated fat
Exercise a minimum of 150 minutes every week
Maintain a healthy weight
Do not smoke

An integrative treatment approach can help manage kidney disease, which causes elevated BUN levels. This could include:

Subscribing to the Mediterranean or DASH diets, both of which have evidence to support their ability to improve cardiovascular and kidney function
Limiting sodium intake to less than 2,300 mg daily
Supplementing with omega-3 fatty acids 1-6 grams daily
Taking Coenzyme Q10 (CoQ10) 200 mg daily
Avoiding medications and supplements that can impair kidney function

In severe cases of AKI or advanced CKD where BUN levels are critically high and kidney function is significantly diminished, patients may require dialysis or a kidney transplant (29).

When elevated BUN levels are caused by overeating protein, patients can limit their daily protein intake by moderating the amount of protein-rich foods they eat, such as meat, fish, eggs, beans, nuts, and dairy (12). They can replace these foods with fruits and vegetables (23). Additionally, patients should focus on staying well hydrated. In general, drinking half of your body weight in ounces of water daily is recommended to prevent dehydration.

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Key Takeaways

High BUN levels are an important indicator of kidney function and overall metabolic health.
Elevated BUN can arise from various causes, including renal conditions like chronic kidney disease and acute kidney injury and non-renal factors such as dehydration, high protein intake, and heart failure. The severity of BUN elevation and the associated symptoms can vary, with dangerously high levels indicating a potential medical emergency that requires immediate intervention.
Management of high BUN levels involves a combination of medical interventions and lifestyle modifications. Lifestyle changes, particularly dietary adjustments and proper hydration, are foundational in supporting overall kidney health. In severe cases, dialysis may be necessary to remove waste products from the blood.
Routine BUN testing, often conducted as part of standard blood panels, allows healthcare providers to track trends in kidney function over time. Deviations from the normal range can signal early kidney dysfunction, prompting further investigation and timely intervention.