BUN creatinine ratio calculator counts the ratio of a patient's blood urea nitrogen to their creatinine levels. Read on to find out how to calculate BUN creatinine ratio, why we measure it, and what does it mean if the BUN creatinine ratio is high or low.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

Blood urea nitrogen (BUN) vs creatinine

Blood urea nitrogen and creatinine are both metabolites, so they're produced constantly by the body at a fairly steady pace. The difference lies in the way they behave in the nephron, the functional unit of the kidney. Creatinine is usually filtered, secreted directly into the lumen, and does not leave the lumen of the tubule. BUN is filtered and then reabsorbed back in the blood, so, naturally, we would have more BUN in the blood than creatinine.

If the BUN creatinine ratio is high, it's usually connected to the low glomerular filtration rate. In this case, both metabolites get filtered poorly due to small flow, but urea (that contains urea nitrogen) 'leaks out' back to the blood which increases the ratio. If the ratio is low, it means that substances just passed the kidney, didn't get filtered.

Picture of nephrons

Schematic picture of nephrons

BUN creatinine ratio is just one of the parameters to assess kidneys. If we measure creatinine in urine, we are able to calculate fractional excretions - e.g., of sodium or urea. If the potassium is our concern, transtubular potassium gradient is more than useful. And if all we've got is the serum creatinine, we can estimate the GFR with creatinine clearance parameter.

BUN creatinine ratio calculator - how to calculate BUN creatinine ratio?

The blood urea nitrogen (BUN) test is valuable itself for determining if there is a problem with kidneys or your nutrition. But to deepen the diagnosis and try to find the exact cause of the renal malfunction, we perform a test called BUN creatinine ratio.

It uses two values, both measured in serum - blood urea nitrogen and creatinine. BUN and creatinine levels depend on kidney function but bound together give you a clue about the localization of the damage - whether it's an intrinsic renal or prerenal problem. If you know how to calculate the BUN creatinine ratio, you're home.

As the name says, this parameter is a ratio:

BUN creatinine ratio = blood urea nitrogen (mg/dL) / serum creatinine (mg/dL)

In the formula, the units are precise, but in our calculator you can put the most convenient ones for you - it will recalculate them on their own. Just put in the known values, and you will have your result ready.

BUN creatinine ratio - high and low values

What does it mean a patient's BUN creatinine ratio is high or low? The answer is simple:

  • BUN/creatinine ratio >20 suggests a prerenal cause, while
  • BUN/creatinine ratio <10 suggests an intrinsic renal cause

The table shows the list of possible causes for both outcomes - all should be considered:

BUN/creatinine >20 BUN/creatinine <10
dehydration acute tubular necrosis
hypovolemia liver disease
shock, heart attack, severe burns malnutrition
congestive heart failure pregnancy
very high protein intake SIADH (syndrome of inappropriate antidiuretic hormone)
gastrointestinal bleeding rhabdomyolysis
advanced age
corticosteroids administration or Cushing's syndrome

Blood urea nitrogen - levels and definition

Blood urea nitrogen (or BUN for short) is a laboratory blood test that measures the amount of urea nitrogen. Urea is a waste product of protein breakdown, formed in the liver. As a parameter, it serves as a way of measuring the state of nutrition of a person, as well as the wellbeing of the kidneys and liver.

  • the normal range of blood urea nitrogen is 8 - 20 mg/dL (2.9 - 7.1 mmol/L)

  • high values (>20 mg/dL) can be caused by: a protein-rich diet (e.g., high meat intake), kidney malfunction, congestive heart failure, gastrointestinal hemorrhage, or states of increased catabolism (massive burns or cancer)

  • low values (<8 mg/dL) can be caused by: malnutrition, liver disease, SIADH (syndrome of inappropriate antidiuretic hormone)

Creatinine - levels and definition

Creatinine is a product of creatinine phosphate breakdown in muscles. In a normal, healthy state is depends on the amount of muscle mass in the body - that's why usually men will have higher creatinine levels than women. It also reflects the state of organs such as the kidneys and liver.

  • the normal range of serum creatinine levels is 0.7 - 1.3 mg/dL (62 - 115 µmol/L)

  • high values (>1.3 mg/dL) can be caused by: kidney disease, or high contribution of muscle mass to total body weight

  • low values (<0.7 mg/dL) can be caused by: malnourishment, muscle atrophy or severe liver disease

Aleksandra Zając, MD
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