Contribution of four common pulmonary function tests to diagnosis of patients with respiratory symptoms : a prospective cohort study

Background: Few studies of the diagnostic value of pulmonary function testing are available. We assessed the diagnostic contribution of four basic pulmonary function tests: spirometry, lung volume, airway resistance, and diff using capacity. Methods: In this prospective cohort study, we enrolled patients presenting to a pulmonologist with respiratory symptoms but no clear diagnosis from 33 hospitals in Belgium. Each patient had spirometry, lung volume, airway resistance, and diff using capacity testing and all other tests necessary for a defi nitive diagnosis. Clinical history and pulmonary function data were presented to local focus groups, who established diff erential diagnoses and a preferred diagnosis after each test—the focus groups were masked to additional investigations. The final diagnosis was established by the attending physician on the basis of all the investigations done, and validated as the gold standard diagnosis by the local focus group. The primary outcome was a score calculated by 1/number of diff erential diagnoses, corrected for the accuracy of the diagnosis. Secondary outcomes were the number of diff erential diagnoses for patients with a correct preferred diagnosis and the proportion of preferred diagnoses that were correct. The study is registered at ClinicalTrials.gov, number NCT01297881. Findings: We screened 1285 people, of whom 1023 were enrolled and 979 analysed. The primary outcome score was 0·226 after spirometry, increasing to 0·296 after measurement of lung volume, 0·373 after airway resistance test, and 0·540 after measurement of diffusing capacity (p<0·0001 for each step). The number of differential diagnoses decreased after each step (4·2, 3·4, 3·0, and 2·4; p<0·0001 for each step) and the proportion of correct preferred diagnoses increased (61%, 65%, 70%, and 77%; p