This relates to ‘screening’, for example with respect to COPD. Respiratory symptoms correlate poorly with the severity of airway obstruction; hence lung function tests are indispensable in establishing obstructive lung disease. However, one should take into account the sensitivity of spirometric test as well as the prevalence of obstructive lung disease in the population to be investigated. A case-finding approach whereby smokers and ex-smokers older than 40 years with at least one symptom are tested may be more cost effective.