Sweat Testing in Infants: Comparing Sweat Collection on Upper Vs Lower Limbs

Inclusion Criteria: * minimal 3,000 kg * after a positive newborn screening (NBS) test. During this test, newborns are screened for rare diseases including CF. In case of a positive NBS test for CF, a sweat test is routinely performed to confirm or withdraw the diagnosis. * after clinical assessment for CF, with ST as diagnostic step * siblings of patients with CF in order to exclude CF * CF patients who are willing to participate * Healthy infants, born at the maternity unit of UZ Brussel, of which parents are willing to participate Exclusion Criteria: * chromosome abnormalities * metabolic abnormalities * cardiopathies * eczema * important skin lesions on the limbs * use of systemic corticosteroids * critically ill patients (for example hemodynamically unstable)