Plasma Osmolality Calculator
Calculate plasma osmolality, effective osmolality (tonicity), osmol gap, corrected sodium for hyperglycemia, and a simple free-water estimate.
BUN/Creatinine Ratio Calculator
Calculate the BUN/Creatinine ratio to differentiate prerenal, intrinsic renal, and postrenal causes of kidney dysfunction. Includes eGFR and differential diagnosis.
⚠️ Medical Disclaimer: This calculator is for educational purposes. Lab results must be interpreted in clinical context by a healthcare provider.
Planning notes, formulas, and examples
About the BUN/Creatinine Ratio Calculator
The BUN/Creatinine ratio is a common laboratory pattern used to add context when kidney-related labs are abnormal. Blood urea nitrogen (BUN) and serum creatinine are both waste products filtered by the kidneys, but they are influenced by different physiologic processes, which is why looking at them together can be informative.
BUN is produced from protein metabolism in the liver and is freely filtered by the kidneys but partially reabsorbed in the tubules—particularly when urine flow is slow (as in dehydration). Creatinine is produced at a relatively constant rate from muscle metabolism and is freely filtered with minimal reabsorption. This difference is the key to the ratio's diagnostic utility.
A ratio greater than 20:1 can support a prerenal pattern such as dehydration, reduced kidney perfusion, or GI bleeding, because slower tubular flow increases urea reabsorption more than creatinine. A ratio of 10–20:1 is often considered a typical range, while a ratio below 10:1 can be seen when urea production is low or creatinine rises out of proportion. None of those patterns are diagnostic by themselves. This calculator computes the ratio, organizes the common interpretation patterns, and estimates GFR using the CKD-EPI race-free equation.
When This Page Helps
The BUN/Creatinine ratio is a quick way to review whether the lab pattern leans toward prerenal, intrinsic, or low-urea-production explanations before more specific testing is considered. This calculator keeps the ratio, the absolute values, and the eGFR context together so the interpretation is not based on the ratio alone.
How to Use the Inputs
- Enter the BUN (blood urea nitrogen) value in mg/dL.
- Enter the serum creatinine value in mg/dL.
- Optionally enter age and sex for estimated GFR calculation.
- Use presets to see classic patterns for different clinical scenarios.
- Review the ratio, interpretation, and differential diagnosis table.
- Check the clinical pearls for additional interpretation guidance.
Formula used
BUN/Creatinine Ratio = BUN (mg/dL) ÷ Serum Creatinine (mg/dL). Normal ratio: 10–20:1. eGFR (CKD-EPI race-free equation): 142 × min(Cr/κ, 1)^α × max(Cr/κ, 1)^−1.200 × 0.9938^age × (1.012 if female).Example Calculation
Result: Ratio = 33.3:1 — Elevated ratio suggesting prerenal azotemia
BUN 40 / Creatinine 1.2 = 33.3:1. This elevated ratio with relatively preserved creatinine suggests a prerenal cause such as dehydration, heart failure, or GI bleeding. The disproportionate BUN rise occurs because urea reabsorption increases when kidney perfusion drops.
Tips & Best Practices
- Always interpret the ratio alongside absolute BUN and creatinine values — a ratio of 20:1 means different things at BUN 20/Cr 1.0 vs BUN 60/Cr 3.0.
- In elderly patients, baseline creatinine may be misleadingly low due to decreased muscle mass.
- Track BUN and creatinine trends over time — a rapidly rising ratio suggests worsening prerenal perfusion.
- FENa <1% can support a prerenal pattern; FENa >2% can support impaired tubular sodium handling.
- In patients on diuretics, use FEUrea instead of FENa (FEUrea <35% = prerenal).
- Cystatin C-based GFR is more reliable than creatinine-based in patients with extreme body composition.
Why the Ratio Changes
A high BUN/Creatinine ratio usually reflects increased urea reabsorption from low renal perfusion, but the same pattern can also appear with GI bleeding, catabolic states, or steroid use. A low ratio is less common and points toward reduced urea production or disproportionate creatinine elevation.
How to Read Borderline Results
Borderline values are easiest to interpret when you look at the absolute BUN, creatinine, urine studies, and the patient's fluid status together. A ratio that is only mildly elevated may mean something different in a dehydrated young adult than in an older patient with chronic kidney disease.
Why eGFR Is Included
The ratio explains pattern, while eGFR helps quantify the degree of kidney impairment. Putting those two pieces together makes the calculator more useful for AKI triage and for checking whether the entered numbers fit a prerenal or intrinsic picture.
Sources & Methodology
Last updated:
Methodology
This page divides blood urea nitrogen by serum creatinine to produce the BUN/creatinine ratio, then places the result into the usual broad interpretation bands that support a prerenal, intrinsic, or low-urea-production pattern. It also shows eGFR separately so the ratio can be read together with the absolute level of kidney impairment rather than in isolation.
The ratio is presented as a supportive laboratory clue, not a stand-alone diagnosis of acute kidney injury or chronic kidney disease. GI bleeding, catabolic state, liver disease, body composition, medications, and volume status can all change the ratio without proving one specific cause.
Sources
- Brenner and Rector’s The Kidney (Elsevier) — Standard nephrology reference for interpreting BUN, creatinine, and prerenal versus intrinsic patterns.
- KDIGO Clinical Practice Guideline for Acute Kidney Injury (Kidney Disease: Improving Global Outcomes) — Guideline context for using laboratory patterns alongside broader AKI evaluation.
Frequently Asked Questions
Normal is 10:1 to 20:1. Values above 20:1 suggest prerenal causes, while values below 10:1 suggest intrinsic renal disease or low BUN production.
Digested blood in the GI tract is broken down into amino acids, which are metabolized to urea in the liver. This protein load raises BUN disproportionately while creatinine remains stable, producing ratios often exceeding 30:1.
Yes. Corticosteroids increase protein catabolism (raising BUN). Trimethoprim and cimetidine inhibit creatinine secretion (raising creatinine). High-dose diuretics can cause prerenal azotemia.
In AKI, a high ratio (>20:1) can support a prerenal pattern, especially when volume depletion or reduced kidney perfusion is also suspected. A normal ratio with elevated absolute values can fit intrinsic renal injury, but the distinction still depends on the broader clinical picture, urine studies, and the trend over time.
Fractional excretion of sodium (FENa), urine osmolality, urine sodium, and urine microscopy complement the BUN/Cr ratio in differentiating prerenal from intrinsic renal failure. They are most useful when the ratio is borderline or when volume status is unclear.
Yes. Very muscular patients have higher baseline creatinine, which lowers the ratio. Elderly or cachectic patients have low creatinine, which can falsely elevate the ratio.
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