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.