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Comprehensive Metabolic Panel (CMP)

BUN/Creatinine Ratio

Optimal Result: 10 - 24 :1 ratio.

What is the BUN/Creatinine Ratio?

The Blood urea nitrogen (BUN): Creatinine Ratio [BUN/Creatinine Ratio] is a renal (related to kidneys) function indicator, commonly used to diagnose acute or chronic renal (kidney) disease or damage.

Blood Urea Nitrogen (BUN) and creatinine are both waste products of normal metabolism in the human body. 

BUN represents the amount of nitrogen produced from the metabolism of proteins. 

Creatinine is a normal waste product of muscle. 

Once a person performs both a BUN and Creatinine test, doctors can use the combined results to find the BUN-to-creatinine ratio. The BUN and creatinine tests alone are excellent methods of testing liver and kidney health, but together, doctors get a much better understanding of your kidney health and can provide a more accurate diagnosis to kidney-related issues.

Most clinicians agree that creatinine is a more specific indicator of glomerular function than BUN. However, the BUN to creatinine ratio may be used as an indirect estimate of renal function. 

Note: If results for BUN and Creatinine are both within the normal reference range, the BUN/Creatinine ratio will not be reported (not applicable).

What is a BUN test?

A Blood Urea Nitrogen (BUN) test measures the amount of nitrogen in your blood, and is used to provide doctors with information on how well your kidneys and liver are functioning. The nitrogen comes from the waste product urea, which is converted from nitrogen and produced in the liver when protein is metabolized after eating. Healthy kidneys then filter the urea and remove other waste products like urea through urination. It is normal to have some urea present in the blood since your liver is always producing it, but too much may indicate an issue with your kidneys.

What is a Creatinine test?

Creatinine is another type of molecule that is generated from muscle metabolism. Like urea, creatinine is transported through the bloodstream and to the kidneys, where it is also filtered and excreted through the urine. The rate of creatinine formation depends on the individual’s muscle mass. Because muscle mass remains fairly consistent, creatinine levels remain essentially unchanged throughout the day. When results of a creatinine test reveal excess amounts, it could signify impaired kidney function or kidney disease.

Why are we measuring the BUN/Creatinine ratio?

The BUN/Creatinine ratio blood test is used to diagnose acute or chronic renal (kidney) disease or damage. It may also be used to determine gastrointestinal bleeding or trauma. BUN (blood urea nitrogen) and creatinine are both filtered in the kidneys and excreted in urine. The two together are used to measure overall kidney function.

The BUN/Creatinine ratio provides specific clinical information about the kidney that can be used for multiple purposes. The BUN/Creatinine ratio is obtained to assess normal kidney function, help identify possible kidney diseases, to monitor the progression of kidney disease, or to monitor the effectiveness of medications in treating kidney disease.

The BUN/Creatinine ratio is useful in the differential diagnosis of acute or chronic renal disease. Reduced renal perfusion, e.g., congestive heart failure, or recent onset of urinary tract obstruction will result in an increase in BUN/Creatinine ratio. Increased urea formation also results in an increase in the ratio, e.g., gastrointestinal bleeding, trauma, etc. When there is decreased formation of urea as seen in liver disease, there is a decrease in the BUN/Creatinine ratio. In most cases of chronic renal disease the ratio remains relatively normal.

Basic on your kidneys:

You have two kidneys, each about the size of an adult fist, located on either side of the spine just below the rib cage. Although they are small, your kidneys perform many complex and vital functions that keep the rest of the body in balance. 

For example, kidneys:

- Help remove waste and excess fluid

- Filter the blood, keeping some compounds while removing others

- Control the production of red blood cells

- Make vitamins that control growth

- Release hormones that help regulate blood pressure

- Help regulate blood pressure, red blood cells, and the amount of certain nutrients in the body, such as calcium and potassium.

BUN and creatinine are two metabolites steadily produced in the body. 

The Blood Urea Nitrogen (BUN) level is a reflection of the amount of protein breakdown in the blood and it will accumulate with kidney malfunction. Creatinine is a breakdown product of muscle metabolism. It will also accumulate in the blood with kidney malfunction. 

BUN is filtered in the nephrons and then reabsorbed in the blood, whereas creatinine is filtered and then secreted by the tubular cells into the tubular lumen.

Each of your kidneys is made up of about a million filtering units called nephrons. Those are the functional units of the kidney. They perform the filtering of the blood. Each nephron includes a filter, called the glomerulus, and a tubule. The nephrons work through a two-step process: the glomerulus filters your blood, and the tubule returns needed substances to your blood and removes wastes.

What is acute renal failure (ARF)?

Acute renal failure (ARF), also known as acute kidney injury (AKI), is a sudden episode of kidney failure or kidney damage that happens within a few hours or a few days. AKI causes a build-up of waste products in your blood and makes it hard for your kidneys to keep the right balance of fluid in your body. ARF can also affect other organs such as the brain, heart, and lungs. Acute kidney injury is common in patients who are in the hospital, in intensive care units, and especially in older adults.

What are the signs and symptoms of acute kidney injury?

Signs and symptoms of acute kidney injury differ depending on the cause and may include:

- Too little urine leaving the body

- Swelling in legs, ankles, and around the eyes

- Fatigue or tiredness

- Shortness of breath

- Confusion

- Nausea

- Seizures or coma in severe cases

- Chest pain or pressure

In some cases, AKI causes no symptoms and is only found through other tests done by your healthcare provider.

What are normal levels:

The ratio of BUN to creatinine is usually between 10:1 and 20:1.

A healthy individual typically has serum BUN levels higher than serum creatinine levels. A measurable increase in the serum creatinine levels in hospitalized patients is indicative of Acute renal failure (ARF).

Differentiating Acute renal failure (ARF) into 3 categories (prerenal, intrinsic renal, and postrenal failure) is of significant clinical utility.

→ BUN/Creatinine Ratio between 10 and 20 is considered a normal value or may be suggestive of a post-renal cause of ARF.

→ High BUN/Creatinine Ratio (>20) may be suggestive of a pre-renal cause of AFR. 

→ Low BCR (<10) may be suggestive of an intrinsic renal cause of AFR.

What is the BUN/Creatinine ratio used for?

Looking at the BUN/Creatinine ratio can further aid in distinguishing which disease state may be causing abnormal lab values. The BUN/Creatinine ratio usually remains normal in chronic kidney disease. A BUN:creatinine ratio that is >20 suggests dehydration or another state that causes decreased renal perfusion. 

A BUN:creatinine ratio >30 can suggest a GI bleed.

One may look at the BUN / Creatinine ratio to help determine the cause of renal failure. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as CHF (=Congestive Heart Failure) or dehydration. It may also be seen with increased protein, from BI bleed, or increased protein in the diet. The ratio may be decreased with liver disease (due to a decrease in the formation of urea) and malnutrition.

Reference Range(s):

Bun/Creatinine Ratio 6-22 (calc)  
Urea Nitrogen (BUN)    
Age Male (mg/dL) Female (mg/dL)
<1 Month 4-12 3-17
1-11 Months 2-13 4-14
1-3 Years 3-12 3-14
4-19 Years 7-20 7-20
≥20 Years 7-25 7-25
Creatinine    
Age Male (mg/dL) Female (mg/dL)
≤2 days 0.79-1.58 0.79-1.58
3-27 days 0.35-1.23 0.35-1.23
1 month-9 years 0.20-0.73 0.20-0.73
10-12 years 0.30-0.78 0.30-0.78
13-15 years 0.40-1.05 0.40-1.00
16-17 years 0.60-1.20 0.50-1.00
18-19 years 0.60-1.26 0.50-1.00
20-49 years 0.60-1.35 0.50-1.10
50-59 years 0.70-1.33 0.50-1.05
60-69 years 0.70-1.25 0.50-0.99
70-79 years 0.70-1.18 0.60-0.93
≥80 years 0.70-1.11 0.60-0.88

For patients >49 years of age, the upper reference limit for creatinine is approximately 13% higher for people identified as African-American.

Linked Sources:

https://www.kidney.org/atoz/content/AcuteKidneyInjury

https://www.webmd.com/a-to-z-guides/blood-urea-nitrogen-test

References:

Uchino S, Bellomo R, Goldsmith D. The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clin Kidney J. 2012 Apr;5(2):187-191. doi: 10.1093/ckj/sfs013. PMID: 29497527; PMCID: PMC5783213.

Salvador LG, Carolina GF, Jesús RD, Virgilia SAM, Susana RA, Jonathan CÍ, Luis SPJ, Claudio R. A low BUN/creatinine ratio predicts histologically confirmed acute interstitial nephritis. BMC Nephrol. 2023 Mar 27;24(1):75. doi: 10.1186/s12882-023-03118-0. PMID: 36967386; PMCID: PMC10041724.

Sujino Y, Nakano S, Tanno J, Shiraishi Y, Goda A, Mizuno A, Nagatomo Y, Kohno T, Muramatsu T, Nishimura S, Kohsaka S, Yoshikawa T; West Tokyo Heart Failure Registry Investigators. Clinical implications of the blood urea nitrogen/creatinine ratio in heart failure and their association with haemoconcentration. ESC Heart Fail. 2019 Dec;6(6):1274-1282. doi: 10.1002/ehf2.12531. Epub 2019 Dec 9. PMID: 31814319; PMCID: PMC6989280.

Shen S, Yan X, Xu B. The blood urea nitrogen/creatinine (BUN/cre) ratio was U-shaped associated with all-cause mortality in general population. Ren Fail. 2022 Dec;44(1):184-190. doi: 10.1080/0886022X.2022.2030359. PMID: 35166184; PMCID: PMC8856064.

Ok F, Erdogan O, Durmus E, Carkci S, Canik A. Predictive values of blood urea nitrogen/creatinine ratio and other routine blood parameters on disease severity and survival of COVID-19 patients. J Med Virol. 2021 Feb;93(2):786-793. doi: 10.1002/jmv.26300. Epub 2020 Jul 22. PMID: 32662893; PMCID: PMC7405288.

Hosten AO. BUN and Creatinine. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 193. PMID: 21250147.

Matsue Y, van der Meer P, Damman K, Metra M, O'Connor CM, Ponikowski P, Teerlink JR, Cotter G, Davison B, Cleland JG, Givertz MM, Bloomfield DM, Dittrich HC, Gansevoort RT, Bakker SJ, van der Harst P, Hillege HL, van Veldhuisen DJ, Voors AA. Blood urea nitrogen-to-creatinine ratio in the general population and in patients with acute heart failure. Heart. 2017 Mar;103(6):407-413. doi: 10.1136/heartjnl-2016-310112. Epub 2016 Sep 22. PMID: 27658757.

What does it mean if your BUN/Creatinine Ratio result is too high?

The BUN (Blood Urea Nitrogen) to Creatinine ratio is a valuable tool in diagnosing acute or chronic renal disease. It helps differentiate between various underlying conditions affecting kidney function.

Causes of Increased BUN/Creatinine Ratio

  • Dehydration: Dehydration is a common cause of elevated BUN/Creatinine ratios. When the body lacks sufficient fluids, blood flow to the kidneys decreases, causing waste products like urea to concentrate in the blood. This leads to a disproportionate increase in BUN compared to creatinine, as the kidneys try to conserve water by reabsorbing urea. Proper hydration is crucial for maintaining normal kidney function and BUN/Creatinine ratios.
  • Congestive Heart Failure: In congestive heart failure, the heart's reduced pumping efficiency leads to decreased blood flow to the kidneys. This reduced renal perfusion causes the kidneys to increase urea reabsorption, resulting in a higher BUN level while creatinine remains relatively stable. The combination of reduced kidney blood flow and increased urea reabsorption contributes to an elevated BUN/Creatinine ratio in heart failure patients.
  • Urinary Tract Obstruction: When there's a blockage in the urinary tract, such as from kidney stones or tumors, it can lead to a buildup of waste products in the blood. This obstruction prevents the normal excretion of urea and creatinine, causing their levels to rise in the bloodstream. However, BUN typically increases more rapidly than creatinine, resulting in an elevated BUN/Creatinine ratio. Prompt treatment of the obstruction is essential to prevent kidney damage.
  • High Protein Intake: A diet high in protein can cause an increase in the BUN/Creatinine ratio. When the body metabolizes protein, it produces urea as a waste product. Excessive protein consumption leads to more urea production, which can elevate BUN levels more significantly than creatinine levels. This dietary factor highlights the importance of considering a your diet when interpreting BUN/Creatinine ratios.
  • Gastrointestinal Bleeding: Gastrointestinal (GI) bleeding can increase the BUN/Creatinine ratio for several reasons: 
    • Protein absorption: When bleeding occurs in the upper GI tract (stomach or small intestine), blood proteins are broken down into amino acids and absorbed in the small intestine. These amino acids are then metabolized by the liver, producing urea, which increases blood urea nitrogen (BUN) levels.   
    • Renal hypoperfusion: Significant blood loss from GI bleeding can lead to reduced blood flow to the kidneys (renal hypoperfusion). This triggers increased reabsorption of urea in the kidneys, further elevating BUN levels.
    • Disproportionate increase: While both BUN and creatinine may rise due to reduced kidney perfusion, BUN typically increases more rapidly and disproportionately compared to creatinine.
    • Upper vs. lower GI bleeding: The BUN/Creatinine ratio is often higher in upper GI bleeding compared to lower GI bleeding. This is because blood in the upper GI tract has more time for absorption and protein breakdown before being expelled.
    • Diagnostic value: A BUN/Creatinine ratio of 30 or greater has been shown to have high specificity (98%) for upper GI bleeding, especially in children.
  • Medications: Certain medications can affect the BUN/Creatinine ratio. For example, corticosteroids can increase protein breakdown, leading to higher urea production and elevated BUN levels. Some antibiotics may also impact kidney function or urea metabolism. It's crucial for healthcare providers to consider a patient's medication regimen when interpreting BUN/Creatinine ratios, as drug-induced changes can sometimes mimic other medical conditions.

Managing Elevated BUN/Creatinine Ratios

  • Hydration: Proper hydration is crucial for maintaining healthy kidney function and managing elevated BUN/Creatinine ratios. Drinking adequate water helps flush out toxins and waste products from the body, reducing the workload on the kidneys. Aim for light yellow urine as an indicator of good hydration. However, patients with advanced kidney disease or on dialysis may need to limit fluid intake and should consult their healthcare provider for personalized recommendations.
  • Dietary Modifications: Adopting a kidney-friendly diet is essential in managing elevated BUN/Creatinine ratios. This typically involves limiting sodium, potassium, and phosphorus intake while ensuring moderate, high-quality protein consumption. Reducing protein intake, especially from animal sources, can help lower BUN levels. Incorporating more fiber-rich foods like fruits, vegetables, and whole grains can also support kidney health and potentially help reduce creatinine levels.
  • Lifestyle Changes: Regular exercise and stress management play significant roles in supporting overall kidney health. Moderate physical activity can improve kidney function and help lower creatinine levels. Additionally, managing stress through techniques like meditation, yoga, or deep breathing exercises can positively impact kidney health. Adequate sleep is also crucial, as it allows the body to repair and regenerate, potentially benefiting kidney function.
  • Medication Review: Regularly reviewing medications with a healthcare provider is crucial for managing elevated BUN/Creatinine ratios. Some medications can affect kidney function or interact with other treatments. A healthcare professional can assess the necessity of current medications, adjust dosages if needed, and potentially recommend alternatives that are less taxing on the kidneys. This is particularly important for patients with existing kidney issues or those at risk of developing kidney problems.

By understanding the factors affecting the BUN/Creatinine ratio and implementing these management strategies, individuals can take proactive steps to support kidney health and function.

What does it mean if your BUN/Creatinine Ratio result is too low?

A low BUN/Creatinine ratio can be indicative of several underlying conditions.

Some potential causes include:

→ Low protein intake: This could be due to malnutrition or starvation, which results in lower production of BUN.

→ Advanced liver disease: Impaired liver function can lead to decreased urea production, lowering BUN levels.

→ Sickle cell anemia: This condition can lead to decreased reabsorption of urea by the kidneys.

→ Hypothyroidism: Low thyroid hormone production can lead to increased creatinine levels.

→ Rhabdomyolysis: Muscle breakdown leads to elevated creatinine levels.

→ Kidney damage or failure: Impaired kidney function can cause an increase in blood creatinine levels.

→ Use of certain drugs: For instance, acetazolamide can affect BUN/Creatinine ratio.

It's important to work with a healthcare professional for an accurate diagnosis as this ratio is only one indicator and should be interpreted in the context of overall health and other test results.

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