L-[1-13C] phenylalanine oxidation as a measure of hepatocyte functional capacity in end-stage liver disease**

Background Liver disease is associated with impaired metabolism of these amino acids phenylalanine and tyrosine. Decreased metabolism of these amino acids leads to abnormal plasma elevations and impaired clearance rates. We have developed a noninvasive breath test that measures hepatic cytosolic enzyme activity. Methods The rate of hepatic phenylalanine metabolism was quantitatively calculated from the appearance of 13 CO 2 in the breath using the non-radioactive tracer L-[1- 13 C]phenylalanine. Results Normal controls (n = 47) oxidized phenylalanine more than twice that of end-stage liver disease patients (n = 117). Significant differences in the percent of phenylalanine oxidized per hour (mean ± SEM) were found between controls (7.08% ± 0.33%, 95% Cl: 6.42%–7.74%) and Child Pugh classification patients, class A (4.96% ± 0.69%, 95% Cl: 3.50%–6.42%), class B (2.88% ± 0.13, 95% Cl: 2.39%–3.38%) and class C (1.75% ± 0.13, 95% Cl: 1.50%–2.01%). The phenylalanine breath test score significantly correlated with albumin levels, prothrombin time and total bilirubin. Conclusion We have demonstrated that phenylalanine oxidation is significantly decreased with end-stage liver disease and is correlated with the best clinical measures of liver disease.