BUN Creatinine Ratio Calculator

Created by Aleksandra Zając, MD
Reviewed by Dominik Czernia, PhD and Jack Bowater
Based on research by
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. Renal Failure (February 2022)See 1 more source
Walker HK, Hall WD, Hurst JW Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Butterworths (1990)
Last updated: Dec 09, 2022

BUN creatinine ratio calculator counts the ratio of a patient's blood urea nitrogen to their creatinine levels. Read on to find out how to calculate the BUN creatinine ratio, why we measure it, and what it means if the BUN creatinine ratio is high or low.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

Blood urea nitrogen (BUN) vs creatinine

Blood urea nitrogen and creatinine are both metabolites, so they're produced constantly by the body at a fairly steady pace. The difference lies in the way they behave in the nephron, the functional unit of the kidney. Creatinine is usually filtered, secreted directly into the lumen, and does not leave the lumen of the tubule. BUN is filtered and then reabsorbed back in the blood, so, naturally, we would have more BUN in the blood than creatinine.

If the BUN creatinine ratio is high, it's usually connected to the low glomerular filtration rate (you can count it in the GFR calculator). In this case, both metabolites get filtered poorly due to small flow, but urea (that contains urea nitrogen) 'leaks out' back to the blood, which increases the ratio. If the ratio is low, it means that substances just passed the kidney and didn't get filtered.

Picture of nephrons.
Schematic picture of nephrons.

BUN creatinine ratio is just one of the parameters to assess kidneys. If we measure creatinine in the urine, we are able to count fractional excretions – e.g., calculate FENa or FEUrea. If potassium is our concern, transtubular potassium gradient calculator is more than useful. And if all we've got is the serum creatinine, we can estimate the GFR with the creatinine clearance parameter.

BUN creatinine ratio calculator – how to calculate BUN creatinine ratio?

The blood urea nitrogen (BUN) test is valuable for determining if there is a problem with your kidneys or your nutrition. But to deepen the diagnosis and try to find the exact cause of the renal malfunction, we perform a test called BUN creatinine ratio.

It uses two values, both measured in serum – blood urea nitrogen and creatinine. BUN and creatinine levels depend on kidney function but bound together give you a clue about the localization of the damage - whether it's an intrinsic renal or prerenal problem. If you know how to calculate the BUN creatinine ratio, you're home.

As the name says, this parameter is a ratio:

BUN creatinine ratio = blood urea nitrogen (mg/dL) / serum creatinine (mg/dL)

In the formula, the units are precise, but in our calculator, you can put the most convenient ones for you – it will recalculate them on its own. Just put in the known values, and you will have your result ready.

BUN creatinine ratio - high and low values

What does it mean if a patient's BUN creatinine ratio is high or low? The answer is simple:

  • BUN/creatinine ratio >20 suggests a prerenal cause; while
  • BUN/creatinine ratio <10 suggests an intrinsic renal cause.

The table shows the list of possible causes for both outcomes – all should be considered:

BUN/creatinine >20

BUN/creatinine <10

Dehydration

Acute tubular necrosis

Hypovolemia

Liver disease

Shock, heart attack, severe burns

Malnutrition

Congestive heart failure

Pregnancy

Very high protein intake

SIADH (syndrome of inappropriate antidiuretic hormone)

Gastrointestinal bleeding

Rhabdomyolysis

Advanced age

Corticosteroids administration or Cushing's syndrome

Blood urea nitrogen - levels and definition

Blood urea nitrogen (or BUN for short) is a laboratory blood test that measures the amount of urea nitrogen. Urea is a waste product of protein breakdown formed in the liver. As a parameter, it serves as a way of measuring the state of nutrition of a person, as well as the well-being of the kidneys and liver.

  • The normal range of blood urea nitrogen is 8-20 mg/dL (2.9-7.1 mmol/L).

  • High values (>20 mg/dL) can be caused by: a diet rich in high protein foods (e.g., high meat intake), kidney malfunction, congestive heart failure, gastrointestinal hemorrhage, or states of increased catabolism (massive burns or cancer).

  • Low values (<8 mg/dL) can be caused by: malnutrition, liver disease, and SIADH (syndrome of inappropriate antidiuretic hormone).

Creatinine – levels and definition

Creatinine is a product of creatinine phosphate breakdown in muscles. In a normal, healthy state, it depends on the amount of muscle mass in the body – that's why men will usually have higher creatinine levels than women. It also reflects the state of organs such as the kidneys and liver.

  • The normal range of serum creatinine levels is 0.7-1.3 mg/dL (62-115 µmol/L)

  • High values (>1.3 mg/dL) can be caused by: kidney disease or high contribution of muscle mass to total body weight.

  • Low values (<0.7 mg/dL) can be caused by: malnourishment, muscle atrophy, or severe liver disease.

FAQ

How do I calculate the BUN-to-creatinine ratio?

To calculate the bun creatinine ratio:

  1. Determine the patient's blood urea nitrogen (BUN) and serum creatinine levels.

  2. Make sure the units are the same – usually mg/dL.

  3. Divide BUN by serum creatinine to obtain the BUN creatinine ratio.

What is the ratio of 26 BUN and 0.87 creatinine?

In this case, the BUN-to-creatinine ratio is 29.9. We find this result by dividing the blood urea nitrogen by serum creatinine levels:

BUN/creatinine = (26 mg/dL) / (0.87 mg/dL) = 29.9.

What is a normal BUN creatinine ratio?

The normal BUN creatinine ratio is between 10 and 20. Values over 20 typically suggest prerenal causes, whereas those below 10 may imply an intrinsic renal cause.

What does a BUN/creatinine ratio of 21 mean?

A BUN creatinine ratio of 21 is outside the normal range and may suggest a prerenal cause. Potential causes include (but aren't limited to) hypoperfusion of kidneys (e.g., due to dehydration), gastrointestinal bleeding, or very high dietary protein content.

Aleksandra Zając, MD
BUN - blood urea nitrogen
mg/dL
Serum creatinine
mg/dL
BUN Creatinine Ratio
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