Urea

Urea is a waste product that your liver produces every time you break down protein from food. Once made, it travels in your bloodstream to the kidneys, where it is filtered out and excreted in your urine. Measuring urea in the blood therefore gives a useful window into two things at once: how your liver is processing protein and how well your kidneys are clearing waste. It is a long-established marker on almost every routine blood panel and is usually interpreted alongside creatinine and eGFR when assessing kidney function.

Think of urea as part of a relay race. Proteins from your diet are broken down into amino acids, which produce ammonia as a by-product. Ammonia is toxic to the brain and nervous system, so your liver converts it into urea through a series of reactions called the urea cycle. Urea is much less harmful and dissolves easily in blood, which makes it easy for your kidneys to filter and remove. A healthy urea level suggests that both handoffs, liver production and kidney clearance, are running smoothly. In the UK urea is reported directly in mmol/L, while in the US the same measurement appears as BUN (Blood Urea Nitrogen) in mg/dL.

Your urea result is more sensitive to lifestyle than most kidney markers. A high-protein diet (lots of meat, fish, eggs, or protein shakes) naturally pushes urea higher, as does dehydration, which concentrates everything in the blood. It can also rise after a gastrointestinal bleed (as the blood itself acts as a protein load), during periods of illness or rapid weight loss, and with certain steroid medications. A low urea usually points the other way: a lower-protein or plant-based diet, overhydration, pregnancy, or, less commonly, significant liver disease where the liver cannot produce urea efficiently.

If your urea is out of range, the most common explanations are dietary or hydration-related rather than kidney disease. Your GP will look at urea alongside your creatinine, eGFR, and often your sodium and haematocrit to decide whether the picture points to dehydration, diet, or a genuine kidney or liver concern. They may suggest a repeat test after improving fluid intake.