BUN creatinine ratio: how the BUN/Cr ratio is calculated

The BUN creatinine ratio is blood urea nitrogen divided by serum creatinine, both in mg/dL, rounded to 1 decimal place. In a 1990 study of gastrointestinal bleeding, Richards and colleagues reported that a ratio of 36 or more suggests an upper GI source. Magent's calculator returns the ratio and whether it reaches 36.

Not a medical device. Not a diagnosis or dosing recommendation.

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GI calculators
Richards RJ et al. · 1990
The BUN/Cr ratio formula
Worked examples
Frequently asked questions

What is the BUN creatinine ratio?

Blood urea nitrogen (BUN) and creatinine are two waste products measured on a routine chemistry panel. Both are cleared by the kidneys, but urea also rises when protein is absorbed from the gut. Blood that has leaked into the upper digestive tract is digested like a protein meal, so the urea level can climb while creatinine stays put. Dividing one by the other gives a single number that reflects that difference.

The ratio is used in several settings. This page follows the version Magent's calculator is built on: the Richards 1990 paper, which asked whether the BUN/Cr ratio could tell upper from lower gastrointestinal bleeding. The calculator reports the ratio and one yes/no flag tied to the 36 cut-off from that paper. It does not report any other ranges or labels.

This page explains the arithmetic. It doesn't say what a ratio means for a particular person.

The BUN/Cr ratio formula

BUN/creatinine ratio = BUN (mg/dL) ÷ creatinine (mg/dL)     rounded to 1 decimal
creatinine (mg/dL)   = creatinine (µmol/L) ÷ 88.42           if entered in µmol/L
flag                 = true when the rounded ratio is 36 or more

Exact expression the calculator uses:

bun_creatinine_ratio = bun_mg_dl / creatinine_mg_dl

Inputs

Input	Meaning	Range accepted
`bun_mg_dl`	Blood urea nitrogen, mg/dL	1 to 200
`creatinine_mg_dl`	Serum creatinine, mg/dL	0.1 to 20
`creatinine_umol_l`	Serum creatinine, µmol/L (use instead of mg/dL)	9 to 1768

Enter creatinine in one unit, not both. If neither is given, the calculator returns an invalid_creatinine error.

How the calculation works

Take creatinine in mg/dL. If it was entered in µmol/L, divide by 88.42 first.
Divide BUN by creatinine.
Round to 1 decimal place.
Compare the rounded ratio with 36. At 36 or above, the flag suggests_upper_gi_bleeding is true. Below 36 it is false.

The comparison uses the rounded ratio, so a value that rounds up to 36.0 counts as reaching the cut-off.

Units

The ratio is built from mg/dL values. Labs outside the United States often report urea rather than BUN, and creatinine in µmol/L. The calculator converts creatinine from µmol/L, but it takes BUN in mg/dL and doesn't convert urea. Dividing a urea value in mmol/L by creatinine in µmol/L gives a different number on a different scale, so the 36 cut-off can't be applied to it.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: just under the cut-off

BUN 35 mg/dL, creatinine 1 mg/dL.
Ratio 35.0. Below 36, so the flag is false.

Example 2: exactly at the cut-off

BUN 36 mg/dL, creatinine 1 mg/dL.
Ratio 36.0. The flag is true.

Example 3: above the cut-off

BUN 40 mg/dL, creatinine 1 mg/dL.
Ratio 40.0. The flag is true.

Example 4: creatinine entered in µmol/L

BUN 36 mg/dL, creatinine 88.42 µmol/L, which converts to 1.0 mg/dL.
Ratio 36.0. The flag is true, the same result as Example 2.

History of the BUN/Cr ratio cut-off

The calculator cites Richards and colleagues, Journal of Clinical Gastroenterology, 1990. The paper asked whether the BUN/creatinine ratio could separate upper from lower gastrointestinal bleeding. In their patients, no one with lower GI bleeding had a ratio of 36 or more, while 38% of those with upper GI bleeding did. A ratio of 36 or more therefore pointed toward an upper source. A ratio below 36 was common in both groups and didn't help locate the bleeding.

When it's used

In the setting the calculator follows, the ratio is worked out for a patient who is already known to be bleeding from the gut, as one piece of information about where the blood may be coming from.

Where it's used

Both values come from a standard blood chemistry panel, so the ratio can be worked out wherever those results are available, from an emergency department to a ward.

Who uses it

The person using the calculator enters BUN and creatinine from the lab report. The calculator converts creatinine if needed, divides, rounds and checks the cut-off.

Limits of the calculation

A ratio of 36 or more is a reported association from one study, not proof of where bleeding is coming from.
A ratio below 36 doesn't mean the source is in the lower GI tract. The calculator never returns a lower-GI label.
The calculator doesn't measure BUN or creatinine. It works with the values it is given.
It isn't the Glasgow-Blatchford score and doesn't score urea bands.
It doesn't order endoscopy or any other test, and this page doesn't say what to do with a result.

Clinical decisions belong to a qualified clinician who knows the patient.

Glasgow-Blatchford score adds points from urea, haemoglobin, blood pressure, heart rate and clinical findings in upper GI bleeding. The BUN/Cr ratio is a single division.
Oakland score is built for acute lower GI bleeding and adds weighted items. The BUN/Cr ratio doesn't classify lower GI bleeding.
Rockall score is scored after endoscopy for upper GI haemorrhage. The BUN/Cr ratio uses two lab values.

Glossary

BUN: blood urea nitrogen, the nitrogen carried in urea in the blood, reported in mg/dL.
Creatinine: a breakdown product of muscle, cleared by the kidneys.
Upper GI tract: the oesophagus, stomach and first part of the small intestine.
mg/dL and µmol/L: two units for creatinine. 88.42 µmol/L equals 1 mg/dL.

Common mistakes

Reading a ratio below 36 as lower GI bleeding. Richards found that values below 36 didn't help locate the source.
Using urea instead of BUN. The calculator expects BUN in mg/dL.
Mixing units. Creatinine in µmol/L must be converted before dividing, which the calculator does when creatinine_umol_l is used.
Entering creatinine twice. Give creatinine in mg/dL or µmol/L, not both.

Source

Richards RJ, et al. Can the blood urea nitrogen/creatinine ratio distinguish upper from lower gastrointestinal bleeding?. J Clin Gastroenterol. 1990;12(5):500-504. DOI: 10.1097/00004836-199010000-00004. PMID: 2229992

Frequently asked questions

How do you calculate the BUN creatinine ratio?

Divide BUN in mg/dL by creatinine in mg/dL. The calculator rounds the result to 1 decimal place.

What BUN/Cr ratio did Richards 1990 use?

A ratio of 36 or more was associated with an upper GI source in that study.

Does a ratio below 36 mean the bleeding is from the lower GI tract?

No. In Richards 1990, ratios below 36 were seen with both upper and lower bleeding, so they didn't help locate the source.

Can I enter creatinine in µmol/L?

Yes. The calculator divides µmol/L by 88.42 to get mg/dL before working out the ratio.

Does the calculator accept urea instead of BUN?

No. It expects BUN in mg/dL.