An analysis method determines a functional parameter of an organ by measuring 13CO2 content in exhaled air of an individual to which a substrate has been administered. The reaction of the substrate in the body of the individual enriches the exhaled air with 13CO2. The maximum reaction rate of the substrate in the body is determined via a change of the measured 13CO2 content in the exhaled air using zero-order enzyme kinetics. An aqueous methacetin solution having a pH greater than 7.0 is used in the analysis method. A face mask separates the exhaled air from inhaled air. The face mask is configured such that inhaled and exhaled air flows essentially completely through the face mask. Exhalation and inhalation valves in the face mask allow a flow of inhaled and exhaled air through the face mask. A diagnostic method is used to determine the functional parameters of the organ.