The main aim is to study correlations between Peak inspiratory flow measured during a spirometry (PIFspiro) and Peak inspiratory flow measured over a pre-set resistance (PIFresist) in COPD patients and severe asthma patients. PIFresist will be measured using 5 different resistances, representing all DPI's relevant for the treatment of obstructive lung diseases. If this shows a distinct relationship between PIFspiro and PIFresist, PIFspiro cut-off points will be sought in an attempt to predict which patients are likely to be able to produce optimal flows for DPI use. If successful, this will make the actual measurement of PIFresist redundant in clinical practice. Also the relationship between PIFresist and device internal resistance in addition to PIFspiro (which corresponds to a very low resistance) will be examined.

The correct use of dry powder inhalers (DPI) crucially depends on the peak inspiratory flow (PIF) that the patient can generate when overcoming internal resistance of the inhaler device.

It has been shown that some patients cannot achieve sufficient peak inspiratory flows with some DPI's, especially in the elderly population. The In-check Dial device (Clement Clarke International Ltd, Harlow, UK) allows to measure these inspiratory flows for different pre-set resistances, representing different DPI's.

Study design: interventional, single center, single visit study

30 Severe asthmatics and 70 COPD patients will be recruited in the UZ Brussel respiratory outpatient clinic.

All participants will provide written informed consent.

Patients will perform:

• pre-bronchodilator spirometry for the measurement of PIFspiro as per European respiratory Society/American Thoracic Society (ERS/ATS) guidelines for standard lung function5 (part of routine clinical follow-up).
• 5 forced inspiratory manoeuvres using the In-check Dial device G16, set at 5 different resistances (performed in order of increasing resistance), yielding the PIFresist values for the different DPI's

Inclusion Criteria:

• Severe asthmatics (M/F; > 20 y) entering the Belgian Severe Asthma Registry (BSAR)
• COPD patients (GOLD stage II-IV; M/F; > 40 y)

Exclusion Criteria:

• Inability to perform spirometry