34 Urea

Urea: end product of protein metabolism; kidney, pre-renal and dietary causes of high urea, and low urea.

Urea is the main end product of protein breakdown. The liver converts toxic ammonia, released when amino acids are broken down, into urea, which is excreted by the kidneys. Urea is the largest component of the blood's "residual nitrogen". Its blood level depends on kidney function, but also on protein intake, protein breakdown, liver function and hydration. It is therefore less specific than creatinine, but useful together with it.

Kidney functionProtein metabolismHydration

Adults≈ 2.5–7.8 mmol/L (15–47 mg/dL)

As BUN≈ 6–20 mg/dL

Conversionurea mmol/L ≈ BUN mg/dL × 0.357

Interpret withcreatinine and eGFR

Figure 1. Serum urea zones. Reference ranges vary by laboratory and increase slightly with age.

High urea

Reduced kidney function — chronic kidney disease, acute kidney injury, urinary obstruction
Pre-renal causes — dehydration, heart failure, shock: the kidneys reabsorb more urea when blood flow is low
More protein to break down — high-protein diet, upper gastrointestinal bleeding (digested blood), corticosteroids, fever, burns, catabolic states

A high urea-to-creatinine ratio suggests dehydration, reduced kidney blood flow or gastrointestinal bleeding. A proportional rise of both suggests intrinsic kidney disease. Very high urea causes uraemic symptoms: tiredness, nausea, itching, loss of appetite, confusion and, in severe cases, pericarditis and bleeding.

Low urea

Low urea is seen with a low-protein diet or malnutrition, severe liver disease (reduced urea synthesis), pregnancy and overhydration, including the syndrome of inappropriate antidiuresis (SIAD).

Seek urgent care

if high urea comes with black or bloody stools, vomiting blood, very little urine, severe weakness, confusion or breathlessness.

Planned evaluation

Urea should always be interpreted with creatinine, eGFR, hydration and diet. A mildly raised value after a high-protein meal or with poor fluid intake should be rechecked under normal conditions.

References

Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int. 2024;105(4S):S117–S314. doi:10.1016/j.kint.2023.10.018
MedlinePlus. BUN — blood test. U.S. National Library of Medicine. medlineplus.gov/ency/article/003474.htm

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 12 May 2024