The goal of this study is to compare the rate of Quantity Not Sufficient (QNS) during sweat collection in infants under 6 months of age, using the Macroduct Advanced device for sweat testing. The main question it aims to answer is:

Does sweat collection from the thigh (lower limb) reduce the QNS rate compared to the forearm (upper limb) in infants? Do chloride concentration levels differ between sweat collected from the forearm and thigh in the same infants?

This is a prospective, observational study aimed at evaluating the effectiveness of the Macroduct Advanced device for sweat testing in infants under 6 months of age. The primary goal is to compare the rate of Quantity Not Sufficient (QNS) between two sweat collection sites: the forearm (upper limb) and the thigh (lower limb). The study seeks to determine whether using the thigh as an alternative collection site can reduce the incidence of QNS, a common challenge in sweat testing in infants, where at least 15 microliters of sweat are required for an accurate result.

Study Objectives:

To assess if sweat collection from the thigh results in a lower QNS rate compared to the forearm in infants.

To compare the chloride concentration levels of sweat collected from both the forearm and thigh, in order to evaluate if the collection site affects the chloride measurements.

Study Design:

Participants will undergo sweat collection from both the forearm and thigh, within the same study session, using the Macroduct Advanced device.

The QNS rate will be recorded for each collection site. Chloride levels from the sweat samples will be measured for comparison between the two sites.

The study will involve healthy infants under 6 months of age, and data will be collected to evaluate the feasibility and effectiveness of using the thigh as a collection site for sweat testing.

Primary and Secondary Outcomes:

Primary Outcome: Reduction in the QNS rate when sweat is collected from the thigh compared to the forearm.

Secondary Outcome: Comparison of chloride concentration levels from the forearm and thigh to determine if there is a significant difference in chloride levels depending on the collection site.

This study will provide valuable insights into improving the accuracy and feasibility of sweat testing in infants, especially in cases where QNS is common and may require repeated tests. If successful, using the thigh as a collection site could enhance the overall success rate of sweat tests and reduce the need for retesting, improving clinical care for infants with conditions such as cystic fibrosis.

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)