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. Author manuscript; available in PMC: 2013 Jul 24.
Published in final edited form as: Pediatr Res. 2010 Oct;68(4):349–354. doi: 10.1203/PDR.0b013e3181ed15e5

Figure 1.

Figure 1

13 CO2 atoms percent excess (APE), of the 27 patients. The APE is an expression of stable isotope enrichment within the total CO2 pool and reflects 13C-methionine tracer oxidation in the liver. It can be defined as: APE = tracer/(tracer + tracee) × 100, where the tracer (in this case, 13CO2), is a compound that is chemically and functionally identical to the naturally occurring compound of interest, the tracee (12CO2) (40). As time from 13C-methionine administration increases, the appearance of 13CO2 increases, which is in accordance with expectations because as discussed in the introduction, metabolism of 13C-methionine is anticipated to result in production of 13CO2.