Inclusion Criteria:

CF population

- Children known with CF were recruited from the outpatient clinics. CF disease was
defined as a combination of typical clinical features (e.g. persistent pulmonary
problems, meconium ileus, failure to thrive, steatorrhoe) and an abnormal sweat test
(Chloride > 60 mM). Uncontrolled CF was diagnosed by the paediatric pulmonologist
based on a change in the presence or severity of respiratory symptoms in association
with CF, and/or a decrease in lung function parameters compared to previous
measurements during the last four weeks.

Control population

- Control children without lung disease were recruited from the outpatient clinic of
the University Hospital Maastricht. The reasons of consultation were constipation and
enuresis nocturna. All children completed the ‘International Study of Asthma and
Allergies in Childhood (ISAAC) questionnaire to exclude children with any (history
of) airway or allergy complaints, in order to exclude asthmatic disease

Exclusion Criteria:

For both study populations:

- Diseases that may interfere with the results of the study (e.g. upper airway
infection, heart disease, anatomic abnormalities of the airways and other chronic
inflammatory diseases, such as Crohns disease and rheumatoid arthritis)

- Mental retardation

- Inability to perform the EBC collection procedure

- Active smoking

- Use of the following medication: papaverin, sodium nitroprusside,
angiotensin-converting enzyme (ACE) inhibitors, oxymetazoline, L-arginine, or nitric
oxide synthase (NOS) inhibitors.