A cumulative $^13$C breath test can be used to detect a pancreatic disorder, for example. The test is burdened by uncertain input data. Among them, the estimation of CO$_2$ production rate is a key issue. An established estimate based on the body surface area could be replaced by an estimate using the basal metabolic rate. Although the latter estimate might be considered more appropriate than the former, the differences between them are large in some sex and agegroups. Such disagreements pose a danger to the reliability of the cumulative breath tests and ask for further research.