Blood Urea Nitrogen

BUN (Blood Urea Nitrogen) measures exactly the same substance as urea, it is simply a different way of reporting the same molecule. The urea molecule contains nitrogen, and BUN quantifies just that nitrogen fraction rather than the whole molecule. US laboratories and North American medical literature tend to report BUN (in mg/dL), while UK and European labs report total urea (in mmol/L). Seeing BUN on your results, or hearing it used by an American source, does not mean a new test has been done, it is the same measurement in a different notation. To convert, BUN in mg/dL roughly equals urea in mmol/L multiplied by 2.8.

Why do two conventions exist at all? Historically, early analytical methods measured the nitrogen content of blood protein products directly, which is where the BUN nomenclature comes from. Over time, newer methods allowed direct measurement of the whole urea molecule, and UK labs moved to reporting it that way. The underlying biology is identical: your liver converts ammonia, produced during protein breakdown, into urea, and your kidneys filter urea out of the blood and excrete it in urine. Whether called BUN or urea, this marker reflects how well your liver produces it and how efficiently your kidneys clear it.

Everything that raises or lowers urea raises or lowers BUN in the same way. A high BUN is commonly caused by dehydration (which concentrates waste products), a high-protein diet, intense exercise, gastrointestinal bleeding, heart failure, or reduced kidney filtration. A low BUN can reflect a low-protein or plant-based diet, overhydration, pregnancy, or, uncommonly, significant liver disease where the liver is no longer producing urea efficiently. Certain medications, including corticosteroids and some diuretics, can also shift the result.

If your BUN is outside the reference range, your GP will interpret it alongside creatinine and eGFR to decide whether the pattern points to a simple cause (dehydration, diet) or warrants a closer look at kidney function. For most people, a raised BUN in isolation is straightforward to address with better hydration and, if relevant, a review of protein intake.