How often should I change the filter during a multi-test session for one patient?

You only need to use one filter per patient for an entire multi-test session. You don’t need to change it between spirometry and DLCO trials unless it becomes visibly soiled or saturated with moisture. Excessive moisture can increase resistance by 0.5 cmH2O/L/s or more during the test. We suggest monitoring the filter’s appearance during long sessions to ensure the 100% accuracy of every breath your patient takes.