Kidney function
BUN/creatinine ratio calculator
Enter the BUN (blood urea nitrogen) and creatinine from your blood test to get the BUN to creatinine ratio, see where it sits against the usual 10 to 20 range, and read what a high or low ratio can and cannot tell you. The ratio carries most meaning when one or both values are abnormal; when both are normal, it says little.
Enter your values above to see your result and what it means.
BUN/creatinine ratio normal range
A BUN to creatinine ratio between 10 and 20 is generally considered typical. Above 20 is the traditional threshold for a prerenal pattern, and below 10 is the pattern seen with injury to the kidney tubules. These cutoffs come from the setting of acute kidney injury. When BUN and creatinine are both normal, the ratio has little to add, so read the individual values first.
| Ratio | Band | What it can point to |
|---|---|---|
| Below 10 | Low | Tubular injury in acute kidney injury; also low protein intake, liver disease, pregnancy |
| 10 to 20 | Typical | The usual range; on its own it is no measure of filtering capacity |
| Above 20 | High | Prerenal causes such as dehydration; also gut bleeding, high protein intake, corticosteroids, low muscle mass |
What a high BUN/creatinine ratio means
A ratio above 20 has long been read as a sign of dehydration or other prerenal causes. BUN also rises when the body has more protein to process: after bleeding into the gut, with a high protein intake, with corticosteroid treatment, or after surgery or trauma. A low muscle mass lowers creatinine and pushes the ratio up from the other side. Uchino and colleagues found that about half of 3,641 patients with acute kidney injury had a ratio above 20, and concluded that the ratio did not work as a marker of prerenal injury. A high ratio is a reason to look at the full picture, and on its own it identifies no single cause.
What a low BUN/creatinine ratio means
A ratio below 10 means BUN is low relative to creatinine. In acute kidney injury this pattern points towards damage within the kidney tubules. More often it reflects a low BUN for ordinary reasons: a low protein intake, liver disease (the liver makes urea), or pregnancy, which lowers BUN.
How the ratio is calculated
The ratio is BUN divided by creatinine, both in mg/dL. A BUN of 14 with a creatinine of 1.0 gives 14.0, written 14:1. Many labs outside the US report urea instead of BUN. Urea and BUN are different numbers for the same substance: BUN counts only the nitrogen, which makes up 28/60 of the urea molecule by weight, so BUN is about 0.446 times the urea value. A urea of 30 mg/dL is a BUN of 14.0. Dividing urea directly by creatinine would give a figure about twice as large, which the 10 and 20 cutoffs do not apply to, so this calculator converts urea to BUN first. In SI units the threshold of 20 corresponds to 0.081 (urea in mmol/L divided by creatinine in µmol/L).
Frequently asked questions
What is the BUN/creatinine ratio?
It is your blood urea nitrogen (BUN) divided by your creatinine, two waste products that the kidneys clear from the blood. Some lab reports print it automatically. Clinicians use it as one clue when kidney function is abnormal, mainly to think about whether reduced blood flow to the kidneys is involved.
What is a normal BUN/creatinine ratio?
Roughly 10 to 20. Your own lab may print a slightly different range, and that range is the one to compare against. The ratio matters most when BUN or creatinine is abnormal; when both are normal, a ratio outside 10 to 20 usually means little.
What does a high BUN/creatinine ratio mean?
A ratio above 20 means BUN is high compared with creatinine. Dehydration is the classic cause. Bleeding in the stomach or gut, a high protein intake, corticosteroids and a low muscle mass can all do the same. Research in acute kidney injury shows the ratio is an unreliable guide to the cause, so it needs to be read alongside the individual values and how you feel.
What should I do if my BUN/creatinine ratio is high?
Look at BUN and creatinine separately. If both are normal and you feel well, a high ratio rarely needs action. If you have been vomiting, had diarrhoea or drunk little, rehydrating and repeating the test is a common approach, guided by a clinician. Black or tarry stools, vomiting blood, or passing very little urine call for prompt medical review.
What does a low BUN/creatinine ratio mean?
A ratio below 10 means BUN is low compared with creatinine. Common reasons include a low protein diet, liver disease and pregnancy. In someone with acute kidney injury, a low ratio fits damage within the kidney itself. With normal BUN and creatinine, a low ratio is rarely significant.
Sources
- Uchino S, Bellomo R, Goldsmith D. The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clin Kidney J. 2012;5(2):187-191.
- Malkina A. Acute Kidney Injury (AKI). MSD Manual Professional Version.
- Hosten AO. BUN and Creatinine. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods. 3rd ed. Butterworths; 1990. Chapter 193.