Article — BUN/Creatinine Ratio Calculator
BUN/Creatinine Ratio Calculator: How to Read Your Number
The BUN/creatinine ratio is the blood urea nitrogen value divided by serum creatinine, both in mg/dL. A ratio between 10 and 20 is considered normal in adults; above 20 typically points to a pre-renal cause such as dehydration, and below 10 suggests intrinsic kidney injury or low urea production.
Doctors order BUN and creatinine together because the two values tell different parts of one story. Creatinine reflects glomerular filtration. BUN reflects filtration plus how much urea the body makes and reabsorbs. The ratio between them captures the gap, and that gap is often the most useful single number on the metabolic panel.
What the BUN/creatinine ratio shows
Urea is the main nitrogen waste from protein metabolism. The liver produces it, the kidneys filter it, and a portion is reabsorbed in the proximal tubule. Creatinine, by contrast, is a steady byproduct of muscle metabolism that the kidneys filter without much reabsorption. When kidney blood flow falls (pre-renal), more sodium and water are reabsorbed, and urea piggybacks along, so BUN climbs faster than creatinine and the ratio rises. When the kidney tissue itself is damaged (intrinsic injury), creatinine accumulates relatively faster than urea, and the ratio falls.
That is the entire idea behind the BUN/creatinine ratio: it separates "the kidneys are not getting enough blood" from "the kidneys themselves are sick."
BUN/creatinine ratio formula
The math is one line: divide BUN by creatinine, with both in milligrams per deciliter.
Ratio = BUN ÷ CreatinineNormal 10 to 20Pre-renal > 20 with near-normal CrIntrinsic < 10If your lab reports urea instead of BUN, convert first: urea (mg/dL) × 0.467 = BUN (mg/dL). For SI units, urea in mmol/L × 2.8 ≈ BUN in mg/dL, and creatinine in µmol/L ÷ 88.4 ≈ creatinine in mg/dL.
Normal BUN/creatinine ratio range
Most adult laboratories consider 10:1 to 20:1 the normal band. Some sources tighten it to 12 to 16. The width of the range reflects how many things influence each component. A 25-year-old vegetarian woman with low muscle mass and a 65-year-old male weightlifter on a high-protein diet can both have "normal" ratios that look quite different.
BUN was once measured in milligrams of nitrogen per 100 mL of blood. The American convention kept the older "mg/dL" while much of the world switched to reporting urea directly in mmol/L. Old habits stuck because nephrologists found the ratio more useful with BUN than with urea.
High BUN/creatinine ratio
A ratio above 20 usually means BUN has risen faster than creatinine. The classic causes are volume depletion (dehydration, vomiting, diarrhea), heart failure with poor renal perfusion, and gastrointestinal bleeding. An upper GI bleed delivers a protein load to the gut, which the body breaks down and absorbs as urea — ratios above 30 in the right clinical setting raise that flag. High-protein diets, corticosteroids and catabolic states (severe infection, burns) also push the ratio up.
If poor perfusion is not corrected quickly, sustained ischemia damages tubular cells and converts pre-renal azotemia into acute tubular necrosis. At that point the ratio falls back toward and below 10 even though the original problem was upstream. Following the trend over hours and days matters more than any single ratio.
Low BUN/creatinine ratio
A ratio below 10 means creatinine is rising faster than BUN, or that urea production has dropped. Acute tubular necrosis from sustained ischemia, nephrotoxins or sepsis is the most common cause of the first pattern. Advanced liver disease lowers BUN production because the liver cannot make urea efficiently, and the patient may have a ratio under 8 with a near-normal creatinine. Pregnancy and very low-protein diets work the same way.
Rhabdomyolysis is a special case. Released creatine from injured muscle is rapidly converted to creatinine, raising the creatinine value far faster than BUN, and the ratio can drop below 5 in severe cases.
What changes BUN and creatinine
Both values are sensitive to a handful of factors that have nothing to do with kidney disease, which is why the ratio is most useful in context.
- BUN up: dehydration, high-protein diet, GI bleed, steroids, catabolic illness
- BUN down: liver disease, low-protein diet, pregnancy, overhydration, SIADH
- Creatinine up: kidney injury, large muscle mass, creatine supplementation, certain drugs (trimethoprim, cimetidine)
- Creatinine down: low muscle mass, elderly frail patients, advanced liver disease
- Both up together: chronic kidney disease, severe dehydration on top of CKD
- Both down: overhydration, low protein intake
BUN/creatinine ratio vs eGFR
Estimated glomerular filtration rate (eGFR) is the modern way to grade kidney function. It is calculated from creatinine, age, and sex (with race removed from current US formulas), and it returns a number in mL/min/1.73 m². The BUN/creatinine ratio answers a different question: not "how well are the kidneys filtering," but "if filtering has dropped, why?" The two are complementary. A patient with eGFR 35 and a ratio of 12 looks like stable chronic kidney disease; the same eGFR with a ratio of 28 raises the suspicion of dehydration on top of chronic disease.
Look at the ratio together with urine output, blood pressure, weight changes and the trend of prior labs. A "high" ratio in a runner the morning after a hot race usually means nothing; the same number in a confused elderly patient with low urine output usually means a lot.
BUN/creatinine ratio pitfalls
The most common mistake is treating the ratio as a diagnosis. It is a clue, not a verdict. A second pitfall is unit confusion — a ratio of 16 in mg/dL is normal, but the same numerator and denominator in mmol/L give a different number because urea is reported as the whole molecule. A third pitfall is recent meals: a steak dinner the night before a fasting blood draw can push BUN up noticeably and shift the ratio out of normal even though nothing is wrong.
Drug effects are another quiet source of error. Trimethoprim and cimetidine block tubular secretion of creatinine, so they raise creatinine without changing real filtration and they push the BUN/creatinine ratio down. Steroids and tetracyclines do the opposite to BUN. Ask the clinician whether any current medication might be contributing before reading too much into the number.
Finally, the ratio is not symmetric in its meaning. A ratio just under 10 in someone with low muscle mass usually means little, but a ratio of 35 in a previously healthy adult almost always points to a real fluid or bleeding problem that needs prompt attention. Always interpret the BUN/creatinine ratio with the clinician who ordered the test, and never adjust medications or fluids based on a calculator alone.