The author shows how the spirometric flow transducer resistance cooperates with the patients respiratory resistance during forced expiration. It depends on the relation existing between values of both resistances. The values of almost all expiratory lung parameters have been changed. The differences are the biggest particularly at the beginning part of the forced expiration, where airflow velocity is maximum. Then the respiratory system defect is diagnosed incorrectly and the error that appears here is classified as an error of method. Presenting a metrological analysis, the author explains theoretically in what manner the expiratory parameters can be changed by resistance of a used transducer. The measuring experiments of a group of patients confirm theoretical analysis.