The invention relates to an analysis method for determining a functional parameter of an organ of a human or animal individual by measuring the 13CO2 content in the air exhaled by the individual to which a substrate has been administered the reaction of which in the body of the individual enriches the air exhaled by the individual with 13CO2. The method is characterized by using a measuring device, the maximum reaction rate of the substrate in the body of the individual being determined via a change of the measured 13CO2 content in the air exhaled by the individual using zero-order enzyme kinetics. The invention also relates to an aqueous methacetin solution for use in said analysis method, the pH of the solution being greater 7.0. The invention also relates to a face mask (1) for use in the inventive method for separating the exhaled air from the air inhaled by an individual. Said face mask comprises a face mask body (2) and an air cushion (3) encircling the face mask body. Said air cushion is filled with a gas and establishes a substantially gas-tight contact between the face of the individual and the face mask (1) placed on the face of the individual so that the air required by the individual for breathing and the air exhaled by the individual has to flow essentially completely through the face mask (1). At least one exhalation valve (5) and at least one inhalation valve (4) are integrated into the face mask and allow a flow of inhaled and exhaled air through the face mask. The invention finally relates to a diagnostic method for determining the functional parameters of an organ of a human or animal individual.
