THE CONTRIBUTION OF RESPIRATORY-FUNCTION TESTS TO CLINICAL-DIAGNOSIS

The purpose of this study was to investigate the amount of diagnostic information contained in a set of routine lung function studies and to attempt to determine which tests could be omitted without significant loss of discrimination.Cluster analysis was performed on a set of phy siological and questionnaire data, collected prospectively in 1,542 ma le patients, referred consecutively for measurement of forced expired volumes, static lung volumes and measurements of the transfer factor f or carbon monoxide. A respiratory questionnaire was completed for each patient. A physician assigned the patients to a rigorously defined di agnostic category, based on supporting clinical information, as well a s pulmonary function, apart from 241 patients with unusual diagnoses a nd those in whom the criteria did not apply satisfactorily. This diagn osis was never included as a classification variable. Basing the class ification on three independent measurements, total lung capacity, the ratio of forced expiratory volume in one second (FEV1) to vital capaci ty, and the transfer factor for carbon monoxide, the computer generate d six groups: one normal, one showing an isolated gas exchange defect, and four with varying degrees of restriction and obstruction. This cl assification performed well in separating the patients with the clinic al diagnoses of chronic airflow obstruction, bronchial asthma and inte rstitial lung disease from those with ischaemic and valvular heart dis ease and other miscellaneous disorders. Omitting total lung capacity r esulted in some loss of specificity, but valid information was still o btained. The inclusion of all the static and dynamic lung volumes and of carbon monoxide transfer coefficient made little difference. Inclus ion of information from the respiratory questionnaire about smoking, s putum and breathlessness was unhelpful, as the resulting groups were l ess well recognizable physiologically. We conclude that in addition to confirming the validity of historic classifications, disorders of ven tilation and gas exchange, these results show that standard respirator y function tests can contribute useful diagnostic information even whe n considered in isolation, and may be allowed an important role in the diagnostic process.