the present invention discloses a method to determine the lung clearance index or other indices of ventilation inhomogeneity of the lungs by combining two pulmonary gas exchange techniques ; inert gas rebreathing is used for rapid wash - in of the inert tracer gas and for accurate determination of the functional residual capacity by gas analysis alone . this is followed by multiple - breath wash - out for determination of the cumulative expired volume required to clear the inert tracer gas from the lungs , and calculation of the lci as the ratio between v ce and frc . the advantages of the method are i ) significant reduction of required test time , ii ) significant reduction of consumed gas mixture for wash - in of tracer gas , iii ) potential for further reduction of the use of tracer gas , and iv ) more accurate determination of the frc by gas dilution alone . furthermore the present invention relates to a corresponding system and computer - readable medium .