Although at least one series reported no significant difference in spirometry with or without noseclips, there are legitimate unpublished accounts of large differences with and without noseclips in the occasional patient.

The 1994 American Thoracic Society statement on standardization of spirometry (Am j Respir Crit Care Med Vol 152. pp 1107-1136) encourages the use of noseclips.

Noseclips should be used during the performance of spirometry maneuvers.