13C-alpha-Ketoisocaproic acid breath test revisited: an in-depth reproducibility study advocates an extended breath sampling period.

In this study we sought for a quantitative parameter which would offer the best reproducibility of a standard (13)C-alpha-ketoisocaproic acid ((13)C-KICA) breath test. On 3 separate days 14 healthy volunteers (7 females, 7 males) took orally 1 mg/kg body mass (13)C-KICA and 20 mg/kg body mass L-leucine. Samples of expiratory air for (13)CO(2) measurement with the use of isotope ratio mass spectrometry were collected every 5 min during the first hour and then at 70, 80, 90, 105, 120, 150, 180, 240, 300, 360, 540, 720, and 1440 min. Short-term reproducibility assessment involved paired examinations taken 3.5 days apart; paired examinations separated by 22 days (median) served for the medium-term reproducibility assessment. T(max), the time to reach the maximum momentary (13)C recovery (D(max)), showed an unsatisfactory reproducibility, and the D(max) displayed a fairly good reproducibility, whereas the cumulative (13)C recovery calculated for a time span within the range 0-90 and 0-120 min exhibited the best reproducibility. In no case did the medium-term reproducibility prove any worse than the short-term one. Taking into account its reproducibility, the cumulative (13)C recovery appears to be the most reliable parameter of the (13)C-KICA breath test.