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Urea: Function, Lab Values and Medical Use

Urea is a natural waste product formed during protein metabolism and excreted by the kidneys. It is a key laboratory marker for assessing kidney function.

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Things worth knowing about "Urea"

Urea is a natural waste product formed during protein metabolism and excreted by the kidneys. It is a key laboratory marker for assessing kidney function.

What is Urea?

Urea (chemical name: carbamide, molecular formula: CH&sub4;N&sub2;O) is a nitrogen-containing organic compound produced in the human body as the primary end product of protein and amino acid metabolism. It is synthesized mainly in the liver through the urea cycle (also known as the ornithine cycle) and is then transported via the bloodstream to the kidneys, where it is filtered and excreted in the urine.

Urea is the most important vehicle by which the body safely eliminates excess nitrogen. It is highly water-soluble, making it easy to transport through the blood and excrete efficiently via the kidneys.

Biological Function and Formation

When proteins are broken down, amino acids are released and further metabolized, producing ammonia (NH&sub3;) as a byproduct. Since ammonia is highly toxic to the body, the liver converts it into the far less harmful urea through the urea cycle. This process involves several enzymatic steps and is essential for the detoxification of the body.

Urea is readily soluble in water and is transported efficiently in the blood.
The kidneys filter urea from the blood and excrete it through the urine.
A small proportion is also excreted through sweat and the intestines.

Urea as a Laboratory Value

Measuring the blood urea level (often expressed as blood urea nitrogen, BUN) is an important part of clinical laboratory diagnostics, particularly for evaluating kidney function and protein metabolism.

Normal Values

Reference ranges may vary slightly between laboratories. Typical normal values in blood serum for adults are:

Blood urea: approximately 10–50 mg/dL (1.7–8.3 mmol/L)
Values may differ slightly in children and elderly individuals.

Elevated Urea Levels (Azotemia)

An elevated blood urea level can point to various causes:

Kidney insufficiency: The most common cause; the kidneys are unable to excrete urea adequately.
High protein intake: A very protein-rich diet can raise urea levels.
Dehydration: Reduced blood flow to the kidneys leads to urea accumulation.
Heart failure or shock: Decreased renal perfusion results in urea retention.
Gastrointestinal bleeding: Blood in the gut is broken down, increasing nitrogen load.
Catabolism: Increased protein breakdown, for example during fever, severe infection, or after surgery.

Low Urea Levels

Abnormally low urea levels are less common and may indicate:

Liver disease: Severe liver damage impairs urea synthesis.
Malnutrition: Very low protein intake results in reduced urea production.
Pregnancy: Physiologically lower levels may occur due to increased renal blood flow.

Urea in Medicine and Cosmetics

Beyond its role as a laboratory marker, urea is also used therapeutically in various fields of medicine.

Dermatological Use

In dermatology and skin care, synthetic urea is used in concentrations ranging from 5% to 40% in creams, lotions, and ointments. Its effects include:

Keratolytic action: At concentrations above approximately 10%, urea softens and dissolves hardened skin layers.
Moisturizing (humectant): Urea attracts and retains water in the skin, keeping it hydrated.
Anti-itch effect: At lower concentrations, it may help relieve itching.

Urea-containing products are commonly used for very dry skin, psoriasis, atopic dermatitis (eczema), ichthyosis, and thickened toenails.

Diagnostic Use: Urea Breath Test

The urea breath test (UBT) is a non-invasive diagnostic procedure used to detect infection with the stomach bacterium Helicobacter pylori. The patient ingests a solution containing labeled urea (C-13 urea). If Helicobacter pylori is present, the bacterium breaks down the urea, and the released labeled CO&sub2; can be detected in the exhaled breath.

Urea and Dialysis

In patients with severe kidney insufficiency who can no longer adequately excrete waste products, urea accumulates dangerously in the blood. Dialysis (renal replacement therapy) artificially takes over the function of the kidneys, removing urea and other metabolic waste products from the blood. The Kt/V value is a key parameter for assessing the adequacy of dialysis and is based on urea elimination from the blood.

References

Herold, G. et al.: Internal Medicine. Self-published, Cologne, 2023.
KDIGO Work Group: KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International Supplements, 2013.
Kaplan, L.A. & Pesce, A.J.: Clinical Chemistry: Theory, Analysis, Correlation. 5th ed. Mosby Elsevier, 2010.

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