In general, it should not be difficult to collect a sufficient sample for sweat testing if the manufacturer's recommendations are followed. The Clinical and Laboratory Standards Institute (CLSI) has also published a guideline for sweat testing, which provides additional recommendations for successful sample collection and testing.*
QNS test results can be stressful for the patient and the family, and potentially delay the diagnosis of CF. According to the CF Foundation guidelines on sweat testing, the percentage of insufficient samples should not exceed 5% for patients older than 3 months of age and not more than 10% for patients less than or equal to 3 months of age.** If the laboratory collects sweat from two sites (ie, bilateral testing), the test is considered QNS only when both sites are inadequate. Insufficient sweat samples can be the result of several factors, including age, race, skin condition, and hydration status. Collecting an adequate amount of sweat can be more challenging in patients younger than one month of age, such as those referred following a positive newborn screening result. For this reason, it is recommended that sweat testing in asymptomatic individuals be performed when the infant is at least two weeks of age, is more than 36 weeks' gestation at birth, and weighs more than 2 kg.*** If an adequate sweat sample is not obtained, the test can be repeated as soon as is practical.
*Clinical and Laboratory Standards Institute (CLSI). Sweat Testing: Sample Collection and Quantitative Chloride Analysis; Approved Guideline. CLSI document C34-A3. Wayne , PA: CLSI; 2009.
***Farrell PM, Rosenstein BJ, White TB, Accurso FJ, Castellani C, Cutting GR et al. Guidelines for diagnosis of cystic fibrosis in newborns through older adults. Cystic Fibrosis Foundation consensus report. J Pediatr. 2008 153(2): S4-14.