Rate and predictors of insufficient sweat volume in very young infants after pilocarpine gel iontophoresis: prospective, population-based study.

OBJECTIVE: To verify the rate and predictors of 'quantity not sufficient' (QNS) among Brazilian infants younger than 3 months with positive newborn screening (NBS) for cystic fibrosis (CF). DESIGN: Prospective, population-based study. SETTING: Public Statewide Newborn Screening Programme where the incidence rate of CF is ≈1:11 000. PATIENTS: Subjects with positive two-tiered immunoreactive trypsinogen. INTERVENTIONS: Sweat induction and collection were performed in the same facility; one sweat sample was obtained per individual. MAIN OUTCOME MEASURES: The QNS rate and its predictors; analysis corresponded to the day of sweat collection. RESULTS: Among the 975 participants, QNS rates for 10 and 15 µL were 3.6% (95% CI 2.5% to 4.9%) and 8.3% (95% CI 6.6% to 10.2%). Infants weighing >3056 and >3845 g and with gestational age higher than 37 weeks had a greater likelihood (5.5 and 6.7, and 2.7 and 5.8 times more, respectively) of avoiding QNS than their peers. CONCLUSION: QNS rates fulfilled the requirements, but predictors differed from those recommended by the Cystic Fibrosis Foundations guidelines.

Sweat conductivity for diagnosing cystic fibrosis after positive newborn screening: prospective, diagnostic test accuracy study.

OBJECTIVE: To assess the accuracy of sweat conductivity among newborns and very young infants. DESIGN: Prospective, population-based, diagnostic test accuracy study. SETTING: Public Statewide Newborn Screening Programme where the incidence rate of cystic fibrosis (CF) is ≈1:11 000. PATIENTS: Newborns and very young infants with positive two-tiered immunoreactive trypsinogen. INTERVENTIONS: Sweat conductivity and sweat chloride were performed simultaneously, on the same day and facility by independent technicians, with the cut-off values of 80 mmol/L and 60 mmol/L, respectively. MAIN OUTCOME MEASURES: Sensitivity, specificity, positive and negative predictive values (PPV and NPV), overall accuracy, positive and negative likelihood ratios (+LR, -LR) and post (sweat conductivity (SC)) test probability were calculated to assess SC performance. RESULTS: 1193 participants were included, 68 with and 1108 without CF, and 17 with intermediate values. The mean (SD) age was 48 (19.2) days, ranging from 15 to 90 days. SC yielded sensitivity of 98.5% (95% CI 95.7 to 100), specificity of 99.9% (95% CI 99.7 to 100), PPV of 98.5% (95% CI 95.7 to 100) and NPV of 99.9% (95% CI 99.7 to 100), overall accuracy of 99.8% (95% CI 99.6 to 100), +LR of 1091.7 (95% CI 153.8 to 7744.9) and -LR of 0.01 (95% CI 0.00 to 0.10). After a positive and negative sweat conductivity result, the patient's probability of CF increases around 350 times and drops to virtually zero, respectively. CONCLUSION: Sweat conductivity had excellent accuracy in ruling in or ruling out CF after positive two-tiered immunoreactive trypsinogen among newborns and very young infants.