Biomarker 101: Urea (BUN)

Urea is what your body does with leftover nitrogen from protein — and it reflects your kidneys, your liver, your diet and your hydration all at once.

What urea is

When protein is broken down, amino acids release nitrogen and ammonia, which are toxic if they accumulate. The liver converts them into urea, which is far less harmful, and releases it into the bloodstream. The kidneys then filter urea out and it leaves in urine.

Healthy kidneys remove more than 90 per cent of the urea produced each day. A small amount circulating is normal; when filtering slows, blood urea rises.

Why it's measured

Urea usually appears alongside creatinine and electrolytes in what laboratories call a urea and electrolytes panel, or as part of a general health screen.

It is worth understanding that urea is not the best single measure of kidney function — creatinine and eGFR are more reliable, precisely because urea responds to so many other things. Its value lies in that sensitivity: read alongside creatinine, a mismatch between the two can point toward dehydration or another cause rather than the kidneys themselves.

What the numbers mean

A typical adult range is roughly 3.0 to 10.0 mmol/L, varying by laboratory, age and sex. Levels tend to rise gently with age, and men generally run higher than women because of greater muscle mass and protein turnover.

What moves urea

Higher urea can reflect reduced kidney filtering, but there is a long list of alternatives: a high-protein diet, dehydration from any cause, serious illness or injury, bleeding into the gut, reduced blood flow to the kidneys, and several common medications. This is exactly why urea is not interpreted alone.

Lower urea is less common and rarely concerning by itself. It can occur in later pregnancy, with significant liver disease — since the liver is where urea is made — and with low protein intake. Rare inherited conditions affecting the urea cycle also lower it.

What urea won't tell you

• It isn't a kidney function test on its own. eGFR and creatinine do that job better.
• It's easily influenced. A high-protein meal or a day of poor hydration will move it.
• It doesn't identify a cause. Context from creatinine, electrolytes and your circumstances is what makes it interpretable.
• A raised result is often benign. Dehydration is a far more common explanation than kidney disease.

The bottom line

Urea sits at the junction of protein metabolism, liver function, hydration and kidney filtering. On its own it is noisy. Read next to creatinine and electrolytes, that same sensitivity becomes useful information.