Mouthpieces serve at least two purposes:
-The first is to prevent cross-contamination among patients using PFT equipment. The ATS Statement on General Considerations for Lung Function Testing, (Eur Resp J, 2005; 26: 153-161) discusses this and it is clear that mouthpieces should be used whenever and wherever pulmonary function tests are performed.
-The second purpose however, is to prevent patients from leaking air during testing. Leaks are a chronic problem for all pulmonary function tests.