Urea test (BUN): what the result means (UREA)

Urea forms in the liver as proteins and amino acids are broken down — it is the way the body gets rid of nitrogen it cannot use itself. From the liver, urea passes into the blood, and from there the kidneys filter it out and remove it in the urine. So the amount of urea in the blood depends on two things: how much protein is being broken down in the body, and how well the kidneys clear it. When kidney function is impaired, urea builds up in the blood and its concentration rises — which is why the test is often used together with creatinine to assess how the kidneys are working. In US laboratories this marker is most often called BUN (blood urea nitrogen) and is measured as the amount of nitrogen, while Lithuanian laboratories usually report it as the urea concentration in mmol/l. The marker is sensitive not only to the kidneys but also to diet, hydration and the state of the liver, so it is assessed together with the clinical picture and other kidney markers.

• As a preventive check of kidney function, or when kidney disease is suspected
• To monitor the course of an already known kidney disease and the effect of treatment
• When there is fatigue, weakness, nausea, swelling or a change in how often you pass urine
• In heart failure, urinary tract obstruction, or when bleeding from the digestive tract is suspected
• Before certain procedures or when adjusting medication doses, where it is important to know kidney function
• For risk groups — with diabetes, high blood pressure, obesity, or kidney disease in the family

A raised urea is most often taken as a sign of impaired kidney function, but dehydration, a large amount of protein in the diet or bleeding from the digestive tract can also raise it. Per MedlinePlus, the causes also include heart failure, urinary tract obstruction, shock and a heart attack. So the result needs to be assessed together with creatinine and the clinical condition.

• Kidney disease or kidney failure
• Dehydration (a lack of fluid in the body)
• A large amount of protein in the diet
• Bleeding from the digestive tract
• Heart failure or a recent heart attack
• Shock, or urinary tract obstruction

Usually looked at alongside a raised result: creatinine and the calculated GFR (eGFR) — together they show kidney function more accurately; hydration status — whether there was a lack of fluids at the time of the test; diet — whether it had recently been especially high in protein; and whether there are any signs pointing to bleeding from the digestive tract.

A low urea is less common and is most often linked to liver disease, when the liver can no longer produce enough urea from the products of protein breakdown, or to too little protein in the diet. Per MedlinePlus, the causes also include too much fluid in the body (overhydration) and poor nutrition (a low-protein diet, starvation).

• Liver failure or severe liver disease
• Too little protein in the diet, or starvation
• Poor nutrition (malnutrition)
• Excess fluid in the body (overhydration)

Usually looked at alongside a low result: liver function markers (ALT, AST, bilirubin), if liver disease is suspected; a nutrition assessment — whether protein intake is sufficient; and fluid balance and overall hydration status.