High urea in a blood test: what it means, common causes and next steps

Urea is made in the liver when protein is broken down, and it travels in the blood to the kidneys, which remove it in urine. Lab Tests Online UK describes the urea test as a blood test used to assess kidney function and to monitor kidney disease or dehydration, because abnormal levels may indicate impaired waste removal. It is also known as BUN (blood urea nitrogen).

In the NHS, urea is usually reported as part of a panel called urea and electrolytes, or U&E. The NHS blood tests page describes urea and electrolytes, or renal panel, as checking if the amount of certain substances in the blood may be causing your symptoms. No special preparation is usually needed for the test.

Lab Tests Online UK says high urea levels suggest poor kidney function, which may be due to acute or chronic kidney disease, but that there are many things besides kidney disease that can affect urea. It lists decreased blood flow to the kidneys, as in congestive heart failure or shock, stress, a recent heart attack, severe burns, bleeding from the gastrointestinal tract, conditions that obstruct urine flow and dehydration.

In everyday practice the commonest explanation for a mildly raised urea is that the body is short of fluid. The NHS says dehydration means the body loses more fluid than it takes in and lists causes such as being sick or having diarrhoea, being in a hot environment, drinking too much alcohol, sweating a lot after exercise, having a high temperature, having diabetes and taking medicines that make you pee more (diuretics).

• Dehydration or poor fluid intake.

• A high-protein diet.

• Increasing age; urea levels rise with age and are slightly higher in men than in women.

• Medicines that impair kidney function, which Lab Tests Online UK says may increase urea.

• Reduced blood flow to the kidneys, for example in heart failure or shock.

• Bleeding from the gut.

• Blockage to the flow of urine.

• Acute or chronic kidney disease.

Lab Tests Online UK says urea and creatinine are often the first tests used to check how well the kidneys filter waste from the blood and are often requested together to suspect kidney problems, monitor treatment or monitor kidney function while on certain drugs. Creatinine is a waste product made by the muscles each day that healthy kidneys filter out; a high level may be a sign that the kidneys are not working as they should.

If urea is raised but creatinine is normal, the cause is more often something other than kidney damage, such as dehydration, diet or bleeding. If both are raised, the doctor will look more closely at kidney function, usually by repeating the test and looking at the estimated glomerular filtration rate (eGFR) and at a urine test. This is general guidance; the interpretation in your case depends on the whole picture.

Lab Tests Online UK says low urea levels are not common and are not usually a cause for concern. They can be seen in severe liver disease or malnutrition but are not used to diagnose or monitor these conditions, and low levels are seen in normal pregnancy, when blood volume increases and the baby uses protein for growth. Levels are also lower in infants and small children and in very low-protein diets.

A single mildly high result in someone who was unwell, dehydrated or on new medicines is often simply repeated after the cause has settled. Your GP may ask about fluid intake, illness, diet and medicines, check your blood pressure and arrange a urine test. The NHS says chronic kidney disease can be diagnosed using blood and urine tests, that the results are used to give the stage of the disease, and that people at higher risk (for example with high blood pressure or diabetes) may be advised to have regular tests.

The NHS lists the common causes of chronic kidney disease as high blood pressure, diabetes, high cholesterol, kidney infections, kidney inflammation, polycystic kidney disease, blockages to the flow of urine and long-term regular use of certain medicines such as lithium and NSAIDs. If kidney disease is suspected, you may be referred to a kidney specialist. Medicines such as NSAIDs may need to be reviewed; do not stop prescribed medicines without advice.

A high result should be acted on promptly if it comes with symptoms. The NHS says to ask for an urgent GP appointment or get help from NHS 111 for dehydration if you feel unusually tired or dizzy on standing and it does not go away, have dark yellow pee or pee less than normal, or are breathing quickly or have a fast heart rate. Call 999 or go to A&E if you are confused, very sleepy or difficult to wake, have cold or blotchy skin, or have difficulty breathing.

Black tarry stools or vomiting blood alongside a raised urea can point to bleeding in the digestive system and need emergency assessment through 999 or A&E.

On the NHS, urea is part of routine blood tests ordered by a GP or hospital, with results reviewed by the clinician and repeated if needed. Private health checks also report urea; a result outside the lab's reference range is a prompt to look for the cause, not a diagnosis, and reference ranges differ between laboratories, so interpret the figure against the range printed on your report.