The present invention relates to a method for determining the liverperformance of aliving organism, in particular a human, comprising administering at least one13C labelledsubstrate, which is converted by the liver by releasing at least one 13Clabelledmetabolization product, and determining the amount of the at least one 13Clabelledmetabolization product in the exhalation air over a definite time interval bythe means ofat least one measuring device with at least one evaluation unit. Using thismethod, it ispossible to describe the measured initial increase of the amount of the atleast one 13Clabelled metabolization product in the exhalation air using a differentialequation of firstorder and to determine a value A max (DOB max) and a time constant tau of theincrease ofthe amount of 13C labelled metabolization product from the solution of thedifferentialequation of first order.
