BUN/creatinine ratio: how it is calculated and what it suggests

The BUN/creatinine ratio divides your blood urea nitrogen (BUN) by your creatinine. The kidneys clear both, but urea also moves with fluids, protein and the liver, while creatinine comes from muscle. Setting one against the other gives a clue about where a raised result is coming from, and a clue is all it is.

How it is calculated

In conventional units the ratio is a simple division:

BUN/creatinine ratio = BUN (mg/dL) ÷ creatinine (mg/dL)

A BUN of 14 mg/dL with a creatinine of 1.0 mg/dL gives a ratio of 14. Many laboratories report urea in mmol/L and creatinine in µmol/L, and dividing those two directly gives a very different number. To reach the conventional ratio, convert both: urea in mmol/L divided by 0.357 gives BUN in mg/dL, and creatinine in µmol/L divided by 88.4 gives creatinine in mg/dL. A urea of 5.0 mmol/L with a creatinine of 88 µmol/L works out to a ratio of about 14. In SI units, the cut-off of 20 discussed below corresponds to about 0.081 when urea in mmol/L is divided by creatinine in µmol/L.

How it is read

Laboratories that print the ratio set their own reference band, so read your result against the one on your report. The figure quoted most often is 20.

• A ratio above about 20 has traditionally been taken to suggest a cause before the kidneys, such as dehydration or reduced blood flow to them, rather than damage within the kidneys. Urea can also rise out of step with creatinine with a high protein intake, bleeding in the stomach or intestines, or heart failure.
• A low ratio follows from a BUN that is low relative to creatinine. MedlinePlus lists a low-protein diet, malnutrition, liver disease and over-hydration as causes of a low BUN.

Comparing BUN with creatinine from the same sample can help find the cause of a kidney problem, which is what the ratio is for. A doctor typically looks at the two numbers behind it, at eGFR, and at fluids, recent meals, medicines and any sign of bleeding. Talk to a doctor about an unusual ratio, with the result in front of you.

Where it falls short

The 20 threshold is widely used but has little evidence behind it. In a study of more than 20,000 hospital patients, about half of those who developed acute kidney injury had a ratio above 20, the ratios formed no clean split between groups, and the authors concluded their findings did not support the ratio as a marker of the dehydration-type cause it is used to flag. So the ratio is read with the results behind it, never by itself.

What moves it

Anything that moves urea or creatinine separately moves the ratio: fluids, protein intake including supplements, bleeding in the gut, liver function, muscle mass, and medicines that affect either result.

Preparing for the test

There is no separate preparation. Prepare as for the kidney results it comes from: drink water as you normally would, avoid meat in the 12 hours before the draw as NICE advises for creatinine-based tests, and mention supplements and medicines.

How Clare's panel handles it

The BUN/creatinine ratio is a derived biomarker in Clare's panel. The lab measures urea and creatinine; Clare converts both to mg/dL and divides BUN by creatinine, so the result sits on the conventional scale whichever units the lab reports in. When Clare opens, members will see the ratio beside the two results it comes from.

Price

The ratio is calculated rather than sold separately, and its two inputs come in a kidney function test. When we read public prices on 11 October 2026, a kidney function test cost from 89 to 900 at six UAE providers. Five of the six were 250 or less, and the top of the range was a specialised package. VAT was rarely stated, some figures were offer or "from" prices, and home collection can add a fee.

Related tests

The ratio is read with its two inputs, urea (BUN) and creatinine, and with eGFR, the estimate of kidney filtering calculated from creatinine. For preparing for a full panel, see Comprehensive blood test in Dubai: what to expect.

Why the number means more over time

The ratio swings with a day's fluids and meals, so one high figure after a hot day or a high-protein week says less than a ratio that stays high across tests taken under similar conditions. Over time its value is as a cross-check: when creatinine and eGFR shift, the ratio shows whether urea moved with them.

Clare is opening with a waitlist at clare.health.