Expired ¹³CO₂ recovery from an oral L-[1-¹³C]Phenylalanine (¹³C-PHE) dose has been used to quantify liver function. This parameter however does not solely depend on liver function but on total CO₂ production, PHE turnover and initial tracer distribution. Therefore, we evaluated the impact of these factors on breath test values. 9 ethyl-toxic cirrhotics and 9 controls received 2 mg/kg ¹³C-PHE i.v., breath and blood samples were collected over 4 hours. CO₂ production was measured by indirect calorimetry. The exhaled ¹³CO₂ enrichments were analysed by isotope ratio mass spectrometry and the ¹³C-PHE and L-[1-¹³C]Tyrosine enrichments by gas chromatography mass spectrometry. The cumulative ¹³CO₂ recovery was significantly lower in cirrhotics (7% vs. 12%; p<0.01), in part due to lower total CO₂ production rates. PHE turnover in cirrhotics was significantly lower (33 vs. 44 µmol/kg/h; p<0.05). Considering these extra-hepatic factors in the calculation of the PHE oxidation rate, the inter-group differences were even more pronounced (3 vs. 7 µmol/kg/h) than those for ¹³CO₂ recovery data. Also, the PHE to TYR conversion rate, another indicator of PHE oxidation, was significantly reduced (0.7 vs. 3.0 µmol/kg/hr).