The BUN to creatinine ratio is blood urea nitrogen divided by serum creatinine, both measured by a laboratory in the same venous sample. No home device produces it, though nothing exotic needs ordering — both sit on the basic metabolic panel. The ratio is read when kidney numbers look off and the question is where the problem sits: before the kidney, inside it, or downstream. Our entry on Creatinine covers what that marker is; this page is about getting the ratio.
What you need
One blood draw, with both values from it:
- Blood urea nitrogen (BUN), or the urea result if your laboratory reports urea instead.
- Serum creatinine, from the same tube.
- eGFR, which most laboratories print alongside creatinine.
- The units your report uses, and its reference ranges.
- Context — what you had drunk that morning, your protein intake, any vomiting, diarrhea or black stools, and a medication list: diuretics, corticosteroids, trimethoprim and cimetidine each move one side of the ratio by themselves.
Units matter more here than on most panels. United States reports give both in milligrams per deciliter, the pairing the familiar ratio was built on; many other laboratories report Urea in millimoles per liter and creatinine in micromoles per liter. Urea is also not the same quantity as BUN, only the nitrogen part of it, so mixing conventions gives a ratio that looks alarming and means nothing.
How the measurement works
The ratio works because its two parts leave the body differently. Urea is made in the liver from the nitrogen left when protein is broken down; the kidney filters it, then reabsorbs part of it — more when the body is short of water. Creatinine comes from muscle at a steady rate and is barely reabsorbed at all. So creatinine tracks filtration; urea tracks filtration plus hydration plus protein turnover.
Dividing one by the other leaves the difference. If both rise together and the ratio holds, the filter itself is the likely problem; if urea climbs while creatinine barely moves, the kidney is probably under-perfused or handling an unusual protein load. That division, across two unit systems, is what the BUN/Creatinine ratio calculator is for: enter the values as your report prints them and it returns the ratio and the direction it points.
How to read the result
Read the two raw values first — a ratio only matters once one of its parts is abnormal. Textbooks conventionally place the usual band near 10:1 to 20:1 with both values in milligrams per deciliter, but sources differ on the anchors and laboratories on their ranges, so treat them as directions of travel, not lines.
| Direction | What is happening | Commonly behind it |
|---|---|---|
| Above the usual band | Urea disproportionately high | Dehydration, gut blood loss, a high protein load, corticosteroids, heart failure, obstruction |
| Within the usual band | Both markers moving together | Normal, or damage inside the kidney if both values are raised |
| Below the usual band | Urea low, or creatinine high | Low protein intake, liver disease, pregnancy, large muscle mass, muscle injury, some drugs |
Always ask which half moved. A high ratio driven by rising urea and one driven by falling creatinine are different situations wearing the same number.
What it does not tell you
It is not a kidney function test. It says nothing about how much filtering capacity you have, since both values can rise in proportion and leave it looking ordinary. Filtration is estimated separately, from creatinine and increasingly cystatin C — that is what Glomerular filtration rate (eGFR) reports — and early damage is caught by protein in the urine, not here.
It does not name a cause. A raised ratio narrows the list but will not choose between not drinking enough, a bleed in the stomach, a failing heart and a blocked outflow; history and further tests do that.
And it is distorted at both ends. Little muscle mass, common in older and frailer people, lowers creatinine and lifts the ratio with no kidney change; liver disease lowers urea production and pushes it down. Creatine supplements, heavy training and a high protein diet each shift a component. It is also a snapshot: what you drank that morning moves it, so one odd ratio is a prompt to repeat the test, not a finding. It has never been validated as a marker of aging.
Related reading
- Which Blood Tests Reflect Aging and How to Read Them
- Creatinine
- Creatine for Muscle, Brain, and Healthy Aging
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.