BUN/Creatinine Ratio Calculator

The BUN/creatinine ratio in brief

The BUN/creatinine ratio compares two waste products in the blood: blood urea nitrogen (BUN) and creatinine. Both are cleared by the kidneys, but they are handled differently. Creatinine is filtered and barely reabsorbed, whereas urea is partly reabsorbed in the kidney tubules – and that reabsorption increases when the body is short of fluid. Comparing the two can therefore give a clue about why kidney-related values are abnormal.

The calculator above divides BUN in mg/dL by creatinine in mg/dL. If you have urea in mmol/L (common in the UK, Europe and Australia) and creatinine in µmol/L, it converts both first: BUN (mg/dL) = urea (mmol/L) ÷ 0.357, and creatinine (mg/dL) = µmol/L ÷ 88.42.

A ratio of about 10 to 20 is generally considered typical. A high ratio (above 20) often points to reduced blood flow to the kidneys, such as dehydration, or to extra urea production, such as bleeding in the upper gut. A low ratio (below 10) can reflect low protein intake, liver disease or other conditions that reduce urea production.

The ratio is a supporting clue – never a diagnosis on its own – and it is only meaningful when read together with the individual BUN and creatinine values and the clinical situation.

Worked examples

• BUN 15 mg/dL, creatinine 1.0 mg/dL: ratio 15 – typical.
• BUN 40 mg/dL, creatinine 1.2 mg/dL: ratio 33.3 – high; both values are raised, with BUN disproportionately so, a pattern often seen with dehydration.
• BUN 8 mg/dL, creatinine 1.0 mg/dL: ratio 8 – low; BUN is at the lower end of normal.
• Urea 7.0 mmol/L, creatinine 90 µmol/L: BUN ≈ 19.6 mg/dL, creatinine ≈ 1.02 mg/dL, ratio ≈ 19.3 – upper end of typical.

Converting to the UK urea:creatinine ratio

Where results are reported in SI units, some clinicians calculate a urea:creatinine ratio directly by dividing urea (mmol/L) by creatinine (mmol/L, i.e. µmol/L ÷ 1,000). This gives a different number on a different scale: multiply a BUN/creatinine ratio by about 4.0 to get the equivalent SI ratio. A BUN/creatinine ratio of 20 therefore corresponds to a urea:creatinine ratio of about 80, and a ratio of 10 to about 40. In the last example above, 7.0 ÷ 0.090 = 78. Always check which ratio is being quoted before comparing with published cut-offs.

Where BUN and creatinine come from

Urea

Urea is made in the liver from ammonia, which is released when proteins and amino acids are broken down. Urea production therefore rises with protein intake, tissue breakdown (catabolism) and digestion of blood in the gut, and falls with low protein intake and severe liver disease. "BUN" is the laboratory measurement of the nitrogen contained in urea, in mg/dL, which is why the US unit differs from the urea measurement used elsewhere. Urea in mg/dL is about 2.14 times BUN in mg/dL.

Typical adult reference ranges are about 7–20 mg/dL for BUN, or about 2.5–7.8 mmol/L for urea, although ranges vary by laboratory and rise modestly with age.

Creatinine

Creatinine comes from the breakdown of creatine phosphate in muscle at a fairly steady daily rate that depends mainly on muscle mass. Typical adult ranges are about 0.7–1.3 mg/dL (62–115 µmol/L) for men and 0.6–1.1 mg/dL (53–97 µmol/L) for women.

Why they behave differently

Both substances are filtered freely by the glomeruli. Creatinine is not reabsorbed, and a small amount is secreted. Urea, by contrast, is partly reabsorbed in the tubules, alongside water. When the kidneys are underperfused – for example in dehydration or heart failure – the body conserves water and sodium, urea reabsorption increases, and BUN rises proportionally more than creatinine. This is the physiological basis for the high ratio in "prerenal" states.

Ranges by age and sex

Reference ranges differ between laboratories, but typical adult values look like this:

Because men usually have more muscle, their creatinine is higher, so at the same BUN their ratio tends to be slightly lower than women's. Older adults often have higher BUN and lower creatinine, which raises the ratio, and ratios above 20 are common in healthy older people. Children's creatinine is low, so their ratios are often higher than adults' and adult cut-offs should not be applied. In pregnancy, increased GFR lowers both values.

Interpreting the ratio

High BUN/creatinine ratio (above 20)

A high ratio is classically associated with prerenal causes – conditions that reduce blood flow to otherwise healthy kidneys:

• dehydration from vomiting, diarrhoea, fever, heat or not drinking enough
• blood loss or low blood pressure
• heart failure, in which reduced cardiac output lowers kidney perfusion
• diuretics and other medicines that reduce circulating volume

Other causes raise urea production rather than reabsorption:

• upper gastrointestinal bleeding – blood in the gut is digested as a protein meal, raising urea. Several studies have found that a high ratio can help distinguish upper from lower GI bleeding, though it is not reliable enough to decide on its own
• high protein intake, including high-protein diets and some tube feeds
• corticosteroids and catabolic states such as severe illness, burns or sepsis, which increase protein breakdown
• tetracyclines, which can increase protein catabolism

The low ratio (below 10)

A low ratio usually means BUN is low rather than creatinine high:

• low protein intake or malnutrition
• severe liver disease, which reduces urea synthesis
• overhydration or the syndrome of inappropriate antidiuretic hormone (SIADH)
• pregnancy, during which BUN falls as GFR rises
• rhabdomyolysis, where muscle breakdown releases creatinine disproportionately
• in some people with high muscle mass, whose creatinine is relatively high

The typical ratio with abnormal values

When both BUN and creatinine are raised in proportion, the ratio stays in the 10–20 range. This pattern is more consistent with intrinsic kidney disease, such as acute tubular injury or chronic kidney disease, in which reduced filtration affects both substances similarly.

How reliable is the ratio?

Textbooks often teach that a ratio above 20 suggests prerenal acute kidney injury and a ratio of 10–15 suggests intrinsic kidney damage. In practice, the ratio has limited accuracy. Studies in hospital patients have found considerable overlap between groups, and it is influenced by diet, liver function, muscle mass, medicines and bleeding as much as by kidney perfusion. In older adults, a high ratio is common and does not always indicate dehydration.

Clinicians therefore use it as one piece of evidence, alongside:

• history and examination – fluid intake, losses, blood pressure, heart rate, skin turgor
• urine tests, such as urine sodium and the fractional excretion of sodium (FENa) or urea (FEUrea)
• trends in creatinine over hours to days
• imaging, such as a kidney ultrasound to exclude obstruction

Urine tests that complement the ratio

Two urine-based calculations are often more informative than the blood ratio when clinicians need to distinguish prerenal from intrinsic kidney injury:

• Fractional excretion of sodium (FENa) = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. A value below 1% suggests the kidneys are avidly retaining sodium, as in prerenal states; above 2% suggests tubular injury. Diuretics make FENa unreliable.
• Fractional excretion of urea (FEUrea) uses urea instead of sodium. A value below about 35% suggests a prerenal state and is less affected by diuretics than FENa.

Neither test is perfect, and both require paired blood and urine samples taken at about the same time.

The ratio in specific situations

Older adults

Older people are more prone to dehydration because thirst sensation declines and kidney concentrating ability falls with age. A high BUN/creatinine ratio in an older person can be a useful prompt to assess hydration. However, lower muscle mass in older people reduces creatinine, which itself raises the ratio. Some research has linked a high ratio in older hospital patients with poorer outcomes, but this reflects illness severity rather than a specific treatable cause.

Heart failure

In heart failure, a rising BUN – and a rising ratio – can reflect reduced kidney perfusion, the activity of hormones that conserve water and sodium, or overly aggressive diuretic treatment. A raised BUN is associated with worse outcomes in heart failure studies.

Gastrointestinal bleeding

Blood in the upper gut (stomach and duodenum) is broken down and absorbed, increasing urea production. A ratio above about 30 makes an upper source more likely, but endoscopy rather than a ratio determines the source. Anyone with black, tarry stools or vomiting blood needs urgent medical care regardless of the ratio.

Chronic kidney disease

In stable CKD, both values are raised and the ratio is usually typical. A rising ratio in someone with CKD can indicate dehydration, bleeding, increased protein intake or an intercurrent illness on top of the kidney disease.

Common result patterns

Looking at the two values together with the ratio is more useful than the ratio alone. These patterns are typical, although real results often fall between them:

If your results fit one of the abnormal patterns, the next step is not to treat the number but to find the cause – usually with a clinical assessment, repeat blood tests and sometimes urine tests or imaging.

Factors that affect BUN and creatinine

• Hydration: dehydration raises BUN more than creatinine.
• Protein intake: high intake raises BUN; low intake lowers it.
• Muscle mass: affects creatinine, and hence the ratio, in the opposite direction.
• Liver function: severe liver disease lowers BUN.
• Medicines: corticosteroids and tetracyclines raise BUN; trimethoprim and cimetidine raise creatinine.
• Recent meals: a large cooked-meat meal can raise creatinine temporarily.
• Laboratory method: older creatinine methods could be affected by ketones, some antibiotics and bilirubin.

Calculating the ratio by hand

To calculate the ratio from SI units without the calculator:

1. Convert urea to BUN: urea (mmol/L) ÷ 0.357. For a urea of 10.0 mmol/L, BUN = 28.0 mg/dL.
2. Convert creatinine to mg/dL: µmol/L ÷ 88.42. For a creatinine of 110 µmol/L, creatinine = 1.24 mg/dL.
3. Divide: 28.0 ÷ 1.24 = 22.5.

This result is slightly above 20. Whether it matters depends on the clinical picture: in a well person who had a high-protein meal it may mean little, whereas in someone with vomiting and dizziness it supports dehydration.

Why the ratio depends on units

Because the ratio divides one concentration by another, the number depends entirely on the units used:

Mixing units – for example, dividing urea in mmol/L by creatinine in mg/dL – produces meaningless numbers. The calculator avoids this by converting everything to mg/dL before dividing.

When to see a doctor

The ratio itself does not need treatment; the underlying cause does. Seek medical advice if:

• your BUN or creatinine is above the reference range, especially if it has risen since a previous test
• you have symptoms of dehydration – dizziness on standing, very dark urine, passing little urine, dry mouth, confusion – particularly if you are older, unwell or taking diuretics
• you have long-term conditions such as heart failure, CKD, liver disease or diabetes and your results have changed

Seek urgent care if you have black or tarry stools, vomit blood or material like coffee grounds, have very little urine output, or feel faint or confused.