Hepatic phenylalanine metabolism measured by the [13C]phenylalanine breath test

Background The amino acid clearance test including phenylalanine is known to reflect liver functional reserve, which correlates with surgical outcome; however, the procedure is not clinically useful because of its laborious and time-consuming nature. This study evaluates whether phenylalanine oxidation capacity measured by a breath test could reflect liver functional reserve. Design We determined phenylalanine oxidation capacity in 42 subjects using; the L-[1-C-13]phenylalanine breath test (PBT). The (CO2)-C-13 breath enrichment was measured at 10-min intervals for 120 min after oral administration of 100 mg of L-[1-C-13]phenylalanine. Subjects were divided into the following three groups according to their plasma retention rate of indocyanine green at 15 min (ICG R15): Group I (ICG R15 < 10%), Group II (ICG R15 10-20%), and Group III (ICG R15 > 20%). First, we determined the parameters of the phenylalanine oxidation capacity that differentiated these groups and then, using these parameters, we compared the PET with the ICG clearance test, Child-Pugh classification score and standard liver blood tests. Results The %C-13 dose h(-1) at 30 min and cumulative excretion at 80 min were significantly different among the three groups (P < 0.05). These two parameters significantly correlated with the ICG R15, Child-Pugh classification score (P < 0.0001) and results of standard liver blood rests (P < 0.05). Conclusions Phenylalanine oxidation capacity measured by the PET was reduced according to the severity of liver injury assessed by the ICG clearance test, Child-Pugh classification, and standard liver blood tests. These results indicate that the PET can be used as a noninvasive method to determine liver functional reserve.