BUN Creatinine Ratio Calculator — Azotemia Workup
The BUN-to-creatinine ratio is a key lab-interpretation tool. Enter BUN and serum creatinine (both in mg/dL) to get the ratio and its clinical category — normal, pre-renal, or low-protein / liver-disease pattern.
mg/dL
mg/dL
Normal — Normal renal perfusion and protein metabolism
How does this calculator work?
BUN:Creatinine ratio = BUN ÷ serum creatinine (both mg/dL). Normal: 10–20. > 20 suggests pre-renal azotemia (dehydration, GI bleed); < 10 suggests low protein intake, liver disease, or haemodilution; > 35 strongly favours a pre-renal or upper GI bleeding cause. Clinical context is essential.
Formula
How this is calculated
Blood urea nitrogen (BUN) is a waste product of protein catabolism, filtered by the kidneys and partly reabsorbed in the tubules. Serum creatinine is a muscle-metabolism byproduct filtered more consistently. Because tubular reabsorption of urea increases when urine flow is low (e.g. dehydration), the BUN rises disproportionately to creatinine in pre-renal states, pushing the ratio above 20.
Interpreting the ratio: a value between 10 and 20 is generally considered normal and suggests intact renal perfusion. A ratio above 20 — and especially above 35 — points strongly to a pre-renal cause (dehydration, heart failure, shock) or an upper gastrointestinal bleed (blood is a large protein load that raises BUN). A ratio below 10 may indicate low dietary protein, hepatic insufficiency (the liver synthesises urea), SIADH-related haemodilution, or rhabdomyolysis with disproportionately elevated creatinine.
The ratio must be interpreted in clinical context: the absolute values of BUN and creatinine, the patient's muscle mass, diet, medications (steroids raise BUN; trimethoprim raises creatinine), and hydration status all affect interpretation. A high ratio with low absolute creatinine in an elderly cachectic patient carries different implications than the same ratio with a creatinine of 4 mg/dL. This tool is for educational and informational use; always consult a healthcare professional for diagnosis.
Frequently asked questions
The conventionally accepted normal range is 10:1 to 20:1. Values outside this range are a signal to investigate, not a diagnosis in themselves. Some sources use 10–15 as the tighter normal range; others allow up to 25 in older adults with lower muscle mass.
It supports the diagnosis in the right clinical context — dry mucous membranes, tachycardia, concentrated urine, and low urine output — but many other conditions also elevate the ratio. Urinary fractional excretion of sodium (FeNa) and urine osmolality are additional markers used alongside the ratio.
Digested blood in the gut is a massive protein load. Bacteria in the intestine break down haemoglobin into amino acids, which are metabolised to urea. This raises BUN while creatinine (reflecting muscle mass and GFR) stays relatively constant, pushing the ratio above 20–35 and suggesting an upper GI source rather than a lower one.
TG we-Calculate Editorial Team. (2026). BUN Creatinine Ratio Calculator — Azotemia Workup [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/bun-creatinine-ratio-calculator
TG we-Calculate Editorial Team. "BUN Creatinine Ratio Calculator — Azotemia Workup." TG we-Calculate. 2026. https://we-calculate.com/calculator/bun-creatinine-ratio-calculator.
TG we-Calculate Editorial Team, "BUN Creatinine Ratio Calculator — Azotemia Workup," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/bun-creatinine-ratio-calculator
@misc{wecalculate_bun_creatinine_ratio_calculator, title = {BUN Creatinine Ratio Calculator — Azotemia Workup}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/bun-creatinine-ratio-calculator}}, year = {2026}, note = {TG we-Calculate} }
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