The BUN-to-creatinine ratio compares two blood measurements after expressing them on a common mass-concentration basis. BUN stands for blood urea nitrogen. Creatinine is a different waste product measured in the same blood-testing context. Dividing BUN in mg/dL by creatinine in mg/dL produces the conventional numerical ratio used on this page.
A ratio is a comparison, not a diagnosis. It hides the size of the individual results, so always keep the original BUN and creatinine values with it. A clinician may need other measurements and the reason the blood test was taken to interpret the pair.
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How to Use This Calculator
Start with BUN and serum creatinine from a laboratory report, preferably the same sample. Confirm that the first measurement is actually BUN rather than a differently labeled urea result. Select the unit matching each value, then calculate.
For a straightforward check of this implementation, use mg/dL for both inputs. The current form’s micromole-per-liter option has a character-encoding mismatch with the server’s expected option, so do not rely on that selection to convert creatinine. If your report uses µmol/L, verify the conversion separately and enter the converted mg/dL value. Divide creatinine in µmol/L by 88.4 for that conversion.
Read the converted measurements and ratio together. A zero creatinine cannot be used as a valid denominator. This page returns an error-style message in that situation, and its zero ratio should not be interpreted as a physiological measurement.
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Formula Used
BUN/creatinine ratio = BUN in mg/dL ÷ serum creatinine in mg/dL. This implementation converts its BUN mmol/L option by multiplying by 2.8, and intends to convert creatinine in µmol/L by dividing by 88.4.
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Variables Explained
Input
Unit or choice
Meaning
BUN
mg/dL or stated mmol/L option
Blood urea nitrogen numerator; verify substance and unit.
Serum creatinine
mg/dL
Blood creatinine denominator; current µmol selector is unreliable.
BUN is the numerator. Read the substance name as carefully as the unit: urea concentration and urea nitrogen concentration do not represent identical mass quantities. A label such as “urea, mmol/L” should not be copied into a BUN mg/dL field without checking the appropriate conversion.
Serum creatinine is the denominator. Because division is involved, an incorrectly scaled creatinine changes the ratio dramatically. Entering 88.4 as mg/dL when the report actually states 88.4 µmol/L gives a denominator roughly eighty-eight times larger than intended.
The ratio output is normally written as a number or as a value to one, such as 20 or 20:1. It is not a percentage. A ratio of twenty does not mean that twenty percent of the blood is BUN or that kidney function is twenty percent.
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Worked Example
Suppose BUN is 18 mg/dL and creatinine is 0.9 mg/dL. Divide 18 by 0.9 to obtain a ratio of 20. The arithmetic is exact for those entered values, although the laboratory measurements themselves have their own precision.
For a unit-conversion example, creatinine of 88.4 µmol/L corresponds to 1 mg/dL using the page’s factor. With BUN of 20 mg/dL, the intended ratio is 20 ÷ 1 = 20. Enter the converted creatinine in the mg/dL field while the form’s encoded unit option remains unreliable.
A ratio alone cannot distinguish 10 mg/dL BUN with 0.5 mg/dL creatinine from 40 mg/dL BUN with 2 mg/dL creatinine. Both give twenty, but the individual concentrations differ fourfold. That is why a copied ratio without the original results can omit important information.
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Why Use This Calculator?
The page labels ratios below ten as low, from ten through twenty as normal, and above twenty as high. These are the implementation’s comparison bands. Their labels and short cause suggestions are not enough to establish the meaning of an individual result.
BUN can vary with factors beyond kidney filtration, including hydration and protein-related circumstances. Creatinine can vary with muscle-related factors. Those influences help explain why a single ratio cannot identify a unique cause. Discuss the complete report with the clinician who knows the clinical setting.
This calculation is different from eGFR, creatinine clearance and urine albumin-to-creatinine ratio. Their names share some words, but they compare different quantities or use different equations. Do not substitute this number into a decision that asks for one of those measures.
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Common Mistakes
Avoid mixing blood and urine measurements. Avoid copying values taken on different dates without stating that limitation. Do not silently replace an undetectable or less-than laboratory result with zero or an invented exact value. A value reported as “less than” describes a limit, not a measured number equal to that limit.
Check the individual values whenever an answer changes sharply. A misplaced decimal in 0.9 versus 9 alters the ratio tenfold. Do not stop prescribed medicines or change fluid intake solely to make this ratio fall within the page’s displayed band.
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Frequently Asked Questions
Is a ratio of twenty always reassuring?
No. The same ratio can come from very different concentrations. Review the BUN and creatinine values, the laboratory’s ranges and the clinical context rather than using the ratio’s category as a complete kidney assessment.
Why should I use measurements from the same sample?
The ratio is intended to compare two values under the same circumstances. Measurements from different dates may reflect changes in health, hydration or testing conditions, so their quotient can be harder to interpret.
Can I enter creatinine in µmol/L?
The current visible option and server comparison do not match correctly. Until that is repaired, convert separately using the stated factor and select mg/dL. Check the source report carefully before entering the converted number.
Does a high result prove dehydration?
No. The page’s short message lists possibilities rather than a confirmed cause. A clinician needs the wider history, examination and laboratory findings to decide what an abnormal result means.
Why is this not an eGFR calculator?
It divides two concentrations and has no age or other inputs used by kidney-filtration equations. eGFR estimates filtration using a different method. A ratio value cannot be relabeled as an eGFR result.
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