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.

Sources:

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

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