BUN/Creatinine Ratio
A calculated comparison of your BUN with your creatinine that clinicians use as one clue to why a kidney-related number is high or low.
AI-verified against cited sources. Awaiting expert reviewAlso called BUN:Cr, BUN/CrLOINC 3097-3
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The usual range for BUN/Cr Ratio depends on sex. Choose female or male above to see where you stand.
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What it is
The ratio is your BUN divided by your creatinine, with both measured in mg/dL. Many metabolic panels calculate it for you. BUN and creatinine are affected by different things. BUN moves with hydration and protein intake, and creatinine moves with muscle mass. So the ratio can offer a clue about why one of them is off. It is not a separate test and only means something next to both of its numbers.
Why it matters
A clinician may look at the ratio when BUN or creatinine is out of range. A higher ratio is seen with dehydration, bleeding in the digestive tract and a lot of protein breakdown. A lower ratio is seen with low protein intake, liver disease and some kidney problems. The ratio alone cannot show what is going on, and it does not tell you how well your kidneys work. It can sit in the usual span even when BUN and creatinine are both high.
If it’s low
A low ratio means BUN is low compared with creatinine. Low protein intake, extra fluid in the body, pregnancy, and a high creatinine from large muscle mass or creatine supplements are common reasons. Less often it reflects liver disease or injury to the kidney tissue itself. Clinicians read it alongside both values.
Common causes
- Low protein intake or malnutrition
- Large muscle mass or creatine supplements, which raise creatinine
- Pregnancy
- Extra fluid in the body, including IV fluids
- Severe liver disease
- Injury to the kidney's filtering tubules
What can help
- Ask your clinician whether your protein intake, muscle mass or supplements could explain a low ratio
- Ask whether your BUN and creatinine should be rechecked together
If it’s high
A high ratio means BUN is high compared with creatinine. That pattern is common with dehydration or reduced blood flow to the kidneys. It also shows up with a high-protein diet, some medicines and bleeding in the digestive tract. It is a clue, not a diagnosis, so clinicians look at BUN and creatinine themselves, plus other tests.
Common causes
- Dehydration or low blood volume
- A high-protein diet
- Low muscle mass, which lowers creatinine and so raises the ratio
- Corticosteroids and other medicines or illnesses that increase protein breakdown
- Bleeding in the digestive tract
- Heart failure or reduced blood flow to the kidneys
What can help
- Ask your clinician whether hydration, diet or medicines could explain the ratio
- Ask whether to recheck BUN and creatinine and look at your eGFR
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on sex and age. Choose women or men to see the one that applies.
See BUN/Cr Ratio by age and sex, with the U.S. survey numbers where they exist.
How common is your level?
NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.
Use the left and right arrow keys to read each age. Shift moves five years.
Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.
Explore BUN/Cr Ratio by age and sex: every percentile as a table, with a download.
What can affect your result
- Anything that changes BUN, such as protein intake, hydration, bleeding and corticosteroids
- Anything that changes creatinine, such as muscle mass, meat meals and creatine supplements
- Age and sex: usual spans differ for children, men and women
- Units: the US ratio uses mg/dL for both values and is not comparable with a ratio built from mmol/L and µmol/L
- Rounding: the ratio is built from two rounded numbers, so small differences between labs are expected
- Anything that changes BUN (protein meals, dehydration, corticosteroids, digestive tract bleeding) or creatinine (meat meals, creatine supplements, muscle mass, hard exercise) changes the ratio.
- The ratio only applies when both values are in mg/dL and come from the same blood draw.
Moves with BUN and creatinine, so it can change within a day or two.
When to talk to a clinician
The ratio alone rarely needs urgent action. Talk with your clinician soon if your BUN or creatinine is flagged high or is rising. Get prompt care if you are passing much less urine than usual, have swelling in your legs or face, are vomiting blood, or have black or bloody stools. Call 911 or your local emergency number for chest pain, trouble breathing, confusion or fainting.
Questions you could ask
- Should I pay attention to the ratio, or mainly to my eGFR and creatinine?
- Could dehydration, my diet or a medicine explain a high ratio?
- Should we recheck my BUN and creatinine once I am well hydrated?
- Could my protein intake, muscle mass or supplements explain a low ratio?
- Does this change how you read my kidney or liver results?
Questions people ask
What is a usual BUN/creatinine ratio?
It depends on your lab, age and sex. Labcorp prints 9–20 for men and 9–23 for women aged 18 to 59. Clinicians often use about 10 to 20 as a rule of thumb. The range printed on your own report is the best guide.
Can I have a high ratio and healthy kidneys?
Yes. Dehydration, a high-protein diet, some medicines and low muscle mass can all raise the ratio without any kidney problem. A clinician will usually look at BUN, creatinine and eGFR before drawing any conclusion.
Why does my report say 'Not Applicable' instead of a number?
Some labs print text instead of a number when they do not report a calculated ratio for that sample. The ratio is just BUN divided by creatinine, so you can work it out yourself when both values are in mg/dL. You can also ask the lab why it was left out.
Is the ratio the same in other countries?
No. Labs outside the US often report urea in mmol/L and creatinine in µmol/L. A ratio built from those numbers is on a different scale and cannot be compared with the US ratio.
Does the ratio show how well my kidneys work?
No. eGFR, which is calculated from creatinine, is the usual way to estimate that. The ratio is only a clue about why BUN or creatinine may be high or low.
Show the science and sources
For clinicians and researchers
Calculated as BUN (mg/dL) divided by serum creatinine (mg/dL). LOINC 3097-3 is a mass ratio. In prerenal states, greater tubular urea reabsorption raises BUN more than creatinine (Labcorp notes that with reduced renal blood flow creatinine rises less quickly than urea nitrogen). The Merck Manual lists a ratio above 20 for prerenal AKI and below 10 for tubular injury. These are rules of thumb, not diagnostic cut-offs, and Merck notes that BUN itself is often misleading because catabolism and bleeding raise it. A high ratio is also seen with gastrointestinal bleeding, a high protein load and catabolic states (glucocorticoids, surgery, trauma, burns, parenteral nutrition). A low ratio is seen with low protein intake, severe liver disease and tubular injury, and with high creatinine generation from muscle mass or creatine use. Labcorp reference intervals vary by sex and age, from about 9–20 in men 18 to 59 to much higher spans in infants. Some US reports print text such as 'Not Applicable' instead of a number. A ratio built from SI units (urea in mmol/L divided by creatinine in µmol/L) is on a different scale and must not be compared with the US value. The separate marker urea-creatinine-ratio covers that form.
LOINC 3097-3 (primary), 44734-2. Specimen: calculated.
Sources
- Pediatric Testing Reference Ranges (BUN/Creatinine Ratio, result 011577), Labcorp (2022)
- Basic Metabolic Panel (8), sample report (test 322758), Labcorp (2022)
- Comp. Metabolic Panel (14), sample report (test 322000), Labcorp (2022)
- Urea Nitrogen (BUN) (test 001040), Labcorp
- Acute Kidney Injury (AKI), Merck Manual Professional Edition, Merck Manual (2026)
- LOINC 3097-3: Urea nitrogen/Creatinine [Mass Ratio] in Serum or Plasma, LOINC via NLM Clinical Tables
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.