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

The [¹³C]methacetin breath test ([¹³C]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 [¹³C]methacetin provide concordant [¹³C]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 [¹³C]MBT on separate days, taking in random order [¹³C]methacetin: a fixed 75 mg dose (FX75), or a 1 mg kg^?1 body mass-adjusted dose (BMAD). Samples of expiratory air for ¹³CO₂ measurement were collected over 3 h. The maximum momentary ¹³C 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 [¹³C]MBT. The dosage-related differences appear, however, to be insignificant if the result of the [¹³C]MBT is reported as a cumulative ¹³C recovery in breath air.