- 01Read BUN and creatinine first
A ratio of 25 can come from a high BUN (fluid, protein, a gut bleed) or from a low creatinine (low muscle with healthy kidneys). The two individual numbers tell you which, and that decides the next step.
Most high BUN to creatinine ratios start outside the kidney: too little fluid, a lot of protein, or blood being digested in the gut. Causes inside the kidney are less common. You can usually tell which group your result falls into by reading BUN and creatinine on their own before you look at the ratio.
What counts as a high BUN to creatinine ratio
The lab divides blood urea nitrogen (BUN) by creatinine, with both in mg/dL. It usually appears on a metabolic panel. The usual adult range is about 10:1 to 20:1, and a result above 20 is the conventional cutoff for a high ratio 1. A BUN of 24 mg/dL divided by a creatinine of 1.0 mg/dL gives 24, just above range.
Labs in the UK, Europe and Australia report urea in mmol/L and creatinine in µmol/L, so the same healthy range reads roughly 40 to 81, and a "60" on a British report is normal. The lab units guide has the conversion factors, and the BUN/creatinine ratio marker page covers the whole range, including low results.
Where an example ratio of 25 falls:1 (mg/dL)
CalculatorBUN to creatinine ratio calculatorEnter BUN and creatinine in US or SI units to get the ratio on both scales.Work it out Why urea runs ahead of creatinine
BUN measures urea, the waste the liver makes when it breaks down protein. Creatinine is the waste from muscle, and the body makes it at a steady rate set by how much muscle you have. The kidneys filter both. They differ in what happens next: the kidney tubules take back part of the filtered urea, and they take back more when urine flow is low, while creatinine is not reabsorbed 1.
How a dry day turns into a high ratio
Two other routes lead to the same number. The liver can make more urea, after a high-protein day, after digesting blood, or while on steroids. The body can also make less creatinine, because a person with little muscle produces little creatinine, so the bottom of the fraction shrinks.
Read the two numbers before the ratio
The ratio does not show which of the two numbers moved. The same ratio of 25 can come from a high BUN over a normal creatinine, or from a normal BUN over a low creatinine, and those two results lead to different next steps.
| Pattern on your report | What usually explains it | What is usually checked next |
|---|---|---|
| BUN high, creatinine normal | Dehydration, a high-protein diet, an upper gut bleed, steroids, fever or another state where the body breaks down a lot of protein | Repeat after normal fluids; hemoglobin if a bleed is possible |
| BUN high, creatinine high, ratio above 20 | Reduced blood flow to the kidney (called prerenal injury), heart failure, a blocked urinary tract | eGFR, comparison with an earlier creatinine, urine tests; often same-day review |
| BUN normal, creatinine low | Older age, low muscle mass, an amputation, frailty; the kidneys are usually fine | Cystatin C |
Read these two beside the ratio
If BUN alone is raised, the high BUN page covers it in detail. If creatinine is raised as well, start with the high creatinine page.
If your ratio is just above 20, creatinine and eGFR are normal and you feel well, this is rarely urgent. Mention it to the clinician who ordered the test and ask whether a repeat BUN and creatinine, taken after a normal day of drinking and eating, is worth doing.
Why is my BUN creatinine ratio high? The common causes
- Not enough fluid. Vomiting, diarrhea, a hot day, a long run, or a fasted morning draw without water. This is the most common reason, and it usually resolves on a repeat test once you are drinking normally.
- High protein intake. A high-protein diet or a shake the night before raises urea production and BUN 1.
- Bleeding in the upper gut. Blood in the stomach is digested and absorbed like a protein meal, which lifts BUN 2. A stomach ulcer is a typical source.
- Heart failure. A weak pump lowers kidney blood flow, which triggers the same water and urea reabsorption that dehydration does 4.
- Medicines. Corticosteroids and tetracycline antibiotics raise BUN by increasing protein breakdown 1; diuretics shrink fluid volume; and starting an SGLT2 inhibitor for diabetes can nudge BUN up slightly.
- States where the body breaks down a lot of protein: fever, burns, major surgery, starvation, an overactive thyroid.
- Low muscle mass, which lowers creatinine rather than raising BUN.
- A blocked urinary tract, from an enlarged prostate or a kidney stone, which can push the ratio above 20 1.
CalculatorProtein calculatorSee how your usual protein intake compares with typical needs; well above that can lift BUN.Work it out Creatine supplements move the ratio the other way. They raise creatinine, which makes the ratio smaller, so creatine is not a cause of a high ratio.
What a ratio of 25, 30 or above 35 usually means
A ratio of 21 to 25 with creatinine in range is the most common high result. It usually comes from a dry morning or a high-protein day and often sits just above normal: a BUN of 21, 22 or 23 mg/dL over a creatinine of 0.9 lands right in this band. The BUN value pages for 21, 22 and 23 explain each of those results.
In a JAMA review of patients who were bleeding from the gut but not vomiting blood, a ratio above 30 raised the odds of an upper source (stomach or duodenum) about 7.5-fold 2. That figure applies only to people who are known to be bleeding. A ratio of 32 in a healthy person after a long hike more likely reflects marked dehydration.
At 35 or above, the question is whether creatinine is climbing too. A high ratio with a rising creatinine can mean the kidney is short of blood flow (prerenal acute kidney injury), and the cause needs finding the same day 5. In hospital, the ratio on its own does not settle whether the kidney tissue itself is damaged.
A high ratio narrows the likely causes. Repeat bloods, eGFR, hemoglobin and urine tests confirm which one applies.
When a high ratio is about the gut, the heart or intensive care
This section is mostly for people who are unwell or in hospital. If you feel fine, skip to what is usually checked next.
In a suspected gut bleed, hemoglobin helps gauge blood loss, but it can still be normal in the first hours of a bleed. Dark stools or vomiting blood need same-day care even when hemoglobin is normal. A high ratio with falling hemoglobin and dark stools points toward a bleed in the upper gut. A low hemoglobin is covered on the anemia page.
In heart failure that has suddenly worsened (doctors call this decompensated), Brisco and colleagues found that reduced kidney function together with a high BUN to creatinine ratio carried a higher risk of death than reduced kidney function with a normal ratio 4. Because the high ratio reflects poor blood flow rather than fixed damage, the authors described this form of kidney dysfunction as potentially reversible when that blood flow is restored.
In intensive care the ratio is a weak guide. In a large ICU study by Rachoin and colleagues, a ratio above 20 was linked with higher mortality, which argues against reading it as a harmless prerenal sign 3. For a routine blood test outside hospital, this is one more reason to read BUN, creatinine and eGFR together rather than leaning on the ratio alone.
What is usually checked next
- 1Repeat BUN and creatinineAfter one to two days of normal fluids and usual protein. A ratio that falls back below 20 points to fluid or diet. If you have chronic kidney disease, diabetes or high blood pressure, BUN and creatinine are often rechecked every few months anyway.
- 2Look at eGFR and earlier resultseGFR estimates how well the kidneys filter. Under KDIGO criteria, a creatinine rise of 0.3 mg/dL or more within 48 hours, or to 1.5 times the baseline within 7 days, counts as acute kidney injury 5. One reading matters less than the direction it moves; see why trends beat single readings.
- 3Check fluid statusSodium and serum osmolality show whether the blood is concentrated. The electrolytes guide explains what sodium tells you.
- 4Check hemoglobin if a bleed is possibleEspecially with dark stools, indigestion, or use of aspirin or ibuprofen-type painkillers.
- 5Use cystatin C if muscle mass skews creatinineCystatin C does not depend on muscle; the cystatin C vs creatinine comparison sets out when each is used.
- 6In hospital: urine tests of salt and urea handlingThese urine tests show whether the kidney is holding on to salt and urea because blood flow is low. A fractional excretion of sodium (FeNa) below 1% points to low blood flow rather than kidney damage. In people on water tablets (diuretics), a fractional excretion of urea of 35% or less is the more reliable sign of low blood flow 6.
Markers that complete the picture
If eGFR or urine albumin is abnormal on repeat testing, see the chronic kidney disease page and how kidney function is measured.
How a high ratio comes back down
The ratio falls once the cause is dealt with. Dehydration corrects with fluids, and a redraw a day or two later is usually normal. For a routine test, drink water before a fasted draw, and skip the protein shake and hard training session the day before; the blood test preparation guide covers the rest. If you take steroids or diuretics, discuss the result with the prescriber before changing anything. A bleed or heart failure needs medical treatment. No supplement corrects a high ratio.
Citations
- Hosten AO. BUN and Creatinine. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods, 3rd ed. Butterworths, 1990.
- Srygley FD, Gerardo CJ, Tran T, Fisher DA. Does this patient have a severe upper gastrointestinal bleed? JAMA 2012;307(10):1072-9.
- Rachoin JS et al. The fallacy of the BUN:creatinine ratio in critically ill patients. Nephrol Dial Transplant 2012;27(6):2248-54.
- Brisco MA et al. Blood urea nitrogen/creatinine ratio identifies a high-risk but potentially reversible form of renal dysfunction in decompensated heart failure. Circ Heart Fail 2013;6(2):233-9.
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl 2012;2:1-138.
- Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int 2002;62(6):2223-9.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
