The next Pulmonary function test (PFTs) will come for sure.
To perform well at the next PFTs your child may need some training to learn the most required exhalation patterns.
Long exhale: put a straw in a glass with water and exhale through the straw as long as it is possible.
Strong exhale: blow into a simple whistle as long and as loud as possible.
Caution: make sure water and strawfie are fresh and clean, the whistle has dried from the last use or you take a new one way whistle.
Source: Medical University Children’s Clinic CF-Center at the AKH-Vienna