Urea ¹³C Other names: Urea, labelled Urea 13C

After oral ingestion the labelled urea will rapidly disintegrate on reaching the stomach. In the case of infection with H. pylori, ¹³C-urea is metabolised by the enzyme urease of H. pylori.

The carbon dioxide which is liberated diffuses into the blood vessels and is transported as bicarbonate to the lungs where it is then liberated as ¹³CO₂ in exhaled air. Infection with H. pylori will significantly change the ¹³C/¹²C – carbon isotope ratio. The proportion of ¹³CO₂ in the breath samples is determined by isotope-ratio-mass spectrometry (IRMS) or by another suitably-validated method carried out by any qualified laboratory and stated as an absolute difference (excess) in the value between a pre-urea and post-urea breath sample.

The cut off point for discriminating between H. pylori positive and negative patients is based on a study with 885 patients. Values below 1.5‰, i.e. ≤1.5‰ are diagnosed as negative and values above i.e. >1.5‰ are diagnosed as positive.

Absorption

Urea is rapidly absorbed from the gastro-intestinal tract.

The orally applied ¹³C-urea is metabolised to carbon dioxide and ammonia or is integrated into the body’s own urea cycle. Any increase in ¹³CO₂ will be measured by isotopic analysis.

Absorption and distribution of ¹³CO₂ is faster than the urease reaction. Therefore, the rate-limiting step in the whole process is the cleavage of ¹³C-urea by Helicobacter’s urease.

Distribution

Urea is distributed into extracellular and intracellular fluids including lymph, bile, cerebrospinal fluid and blood. It is reported to cross the placenta and penetrate the eye.

Elimination

Urea is excreted unchanged in the urine.

Only in Helicobacter pylori-positive patients does the administration of 75 mg labelled urea lead to a significant increase of ¹³CO₂ in the breath sample within the first 30 minutes.

No concerns in relation to the clinical use of ¹³C-urea.