[Respiratory function test, a key in the etiologic diagnosis of dyspnea or a simple quantitative test?].

When confronted with a case of dyspnoea, three questions must be asked: is the dyspnoea due to a pulmonary organic disease? How severe is it? What is its nature or its origin? In the majority of cases these questions are answered by elementary lung function tests (spirometry and measurement of the residual volume), but for these answers to be valid it must be borne in mind that respiratory function test (RFT) is primarily a physical exercise: no measurement is valid unless the operator is fully involved and the subject explored participates to the best of his possibilities. RFT also is a mental exercise: the data obtained can be well understood only if the operator is conversant with the data described in this article. RFT has multiple applications, and yet this examination is notoriously underestimated and underused by the medical profession. Good practice is its best publicity.