Comparison Between the Oral and Intravenous L-[1-13C]Phenylalanine Breath Test for the Assessment of Liver Function

To simplify the L-[1- 13 C]phenylalanine breath test which is used to assess liver function the tracer is usually given orally, and CO 2 production rate is estimated. In 12 healthy volunteers and 10 liver cirrhotics we compared the oral approach with i.v. tracer administration combined with measurement of individual CO 2 production rate. The 13 CO 2 / 12 CO 2 enrichment was assessed by isotope-ratio mass spectrometry. After i.v. [1- 13 C]phenylalanine application exhaled 13 C recovery per minute peaked within 10 minutes (controls: 0.17 ± 0.06%; cirrhotics: 0.05 ± 0.02%, p < 0.01). The oral approach yielded comparable separation between 30-60 minutes, with average peak values being 0.18 ± 0.03% and 0.06 ± 0.03% (p < 0.01), respectively. Variable gastrointestinal resorption kinetics after oral application probably causes this difference.