BUN (blood urea nitrogen): the UK urea test explained

Urea is made in the liver as the body breaks down protein. It travels in the blood to the kidneys and is passed out in urine. Lab Tests Online UK describes the urea test as measuring this waste product in the bloodstream to assess kidney function and monitor kidney disease or dehydration. In the United States the same measurement is expressed as the nitrogen content of urea and is called blood urea nitrogen. UK reports normally give urea itself in millimoles per litre.

Because of the different unit and naming, a BUN figure quoted online cannot be compared directly with a UK urea result, and you should use the interval printed on your own report.

Urea and creatinine are used to evaluate how well the kidneys work and to monitor people with kidney disease or on dialysis. Urea is routinely measured as part of a U&E screen, requested for non-specific illness, as a routine group of tests, and to check kidney function before certain drugs. Symptoms that may prompt it include tiredness, poor concentration, swelling around the eyes, wrists, ankles or tummy, foamy or blood-stained urine, passing less urine, burning on urination, mid-back pain below the ribs and high blood pressure, as Lab Tests Online UK lists.

No preparation is required; blood is taken from a vein in the arm. Several non-kidney factors move urea. Levels rise with age and with the amount of protein in the diet, so very high-protein diets can cause abnormally high levels and very low-protein diets abnormally low ones. Levels are lower in infants and small children, about two thirds of young adult levels in very young babies, and are slightly higher in men. Drugs that impair kidney function can raise urea.

Urea is normally slightly lower in pregnancy, especially in the last months, because the foetus uses protein for growth and the mother's blood volume increases and dilutes urea.

High urea suggests poor kidney function from acute or chronic kidney disease, but Lab Tests Online UK stresses there are many other causes: reduced blood flow to the kidneys as in heart failure, shock, stress, recent heart attack or severe burns, bleeding from the gastrointestinal tract, conditions that obstruct urine flow, and dehydration.

Bleeding in the gut raises urea because blood is digested as protein. For that reason a raised urea out of proportion to creatinine in someone with black stools or vomiting blood is a signal for urgent assessment; call 999 or attend A&E.

Low urea is uncommon and not usually a cause for concern. It can be seen in severe liver disease or malnutrition, but is not used to diagnose or monitor those conditions, and it is a normal finding in pregnancy.

A raised urea leads to a review of fluid intake, medicines, symptoms and the creatinine and eGFR. Repeat tests after rehydration are common. Persistent abnormality may lead to a urine albumin-to-creatinine ratio, kidney ultrasound and referral. Drink fluids as advised, avoid anti-inflammatory painkillers if your kidneys are a concern unless a clinician says otherwise, and seek help for very low urine output.