Mouthpiece, dentures, tongue, nose clip
The mouthpiece used in spirometric testing may vary from instrument to instrument. It is essential that the patient puts it between the teeth with the lips around it, so that the air can pass freely and the mouthpiece is sealed firmly by the lips. Dentures are best left in place because they provide essential support to cheeks and lips; however, if they fit poorly and obstruct airflow during the maneuvers it is better to remove them from the mouth.
Some subjects use the tongue to obstruct the mouth or mouthpiece for an explosive start of the FVC maneuver. Instruct the patient not to do this: it causes the peak flow to be artificially high and affects the shape of the flow-volume curve, so that it can be less reliably interpreted.
A noseclip is not mandatory for forced expiratory maneuvers. However, occasionally subjects blow out or inhale through the nose at the end of the maneuver, so that is safer to always apply a noseclip.