Comparison of two dosage regimens of the substrate for the [13C]methacetin breath test

The [(13)C]methacetin breath test ([(13)C]MBT)--a valuable non-invasive tool dedicated to the assessment of the liver metabolic capacity--still needs standardisation. The aim of this study was to check whether currently used dosage regimens of [(13)C]methacetin provide concordant [(13)C]MBT results in subjects with an atypical body constitution. Healthy volunteers: low body mass<55 kg (eight women), and high body mass>95 kg (eight large body frame men) were recruited. They underwent [(13)C]MBT on separate days, taking in random order [(13)C]methacetin: a fixed 75 mg dose (FX75), or a 1 mg kg(-1) body mass-adjusted dose (BMAD). Samples of expiratory air for (13)CO(2) measurement were collected over 3 h. The maximum momentary (13)C elimination in breath air occurred earlier and was higher following BMAD than with FX75 in the low body mass females (T (max) 14.6 ± 1.0 min vs. 22.1 ± 2.4 min, p = 0.019; D (max) 41.9 ± 2.9 % dose h(-1) vs. 36.6 ± 3.6 % dose h(-1), p = 0.071). In the high body mass men, T (max) remained unchanged, whereas D (max) was slightly higher with BMAD compared to FX75 (21.5 ± 3.2 min vs. 23.0 ± 3.0 min; 38.5 ± 2.9 % dose h(-1) vs. 32.3 ± 2.5 % dose h(-1)). It is concluded that in subjects with a body constitution outside the general population average, the dosage of the substrate may affect some results of the [(13)C]MBT. The dosage-related differences appear, however, to be insignificant if the result of the [(13)C]MBT is reported as a cumulative (13)C recovery in breath air.