Optimum conditions for the 13C-phenylalanine breath test.

We have conducted optimization studies to develop a superior 13C-phenylalanine breath test for the diagnosis of liver disease. First, we examined the optimum 13C-labeling position in phenylalanine for use in a breath test based on infrared spectroscopic detection of 13CO2 in exhaled air. L-[1-13C]Phenylalanine gave the best result. Next, a suitable dosage to give a short peak time (the time expressed in minutes at which 13CO2 excretion is maximal) after administration was determined. The 13CO2/12CO2 ratio in exhaled air after administration of 100 mg/body of L-[1-13C]phenylalanine peaked sharply at 15 min. We also examined the effect of food on the hepatic metabolism of L-[1-13C]phenylalanine. We found that a fasting period of over 7 h before the test resulted in a higher 13CO2 peak excretion. The peak appeared sooner than that in the 13C-phenacetin breath test and, therefore, the 13C-phenylalanine breath test appears preferable for the rapid evaluation of hepatic function.