Cystic Fibrosis Screening Efficacy and Seasonal Variation in California: 15-Year Comparison of IRT Cutoffs Versus Daily Percentile for First-Tier Testing

The California Genetic Disease Screening Program (GDSP) employs a fixed immunoreactive trypsinogen (IRT) cutoff followed by molecular testing to screen newborns for cystic fibrosis (CF). The cutoffs approximate a 1.6% yearly IRT screen-positive rate; however, seasonal variation in IRT population means has led us to develop a model to establish fixed IRT cutoffs that anticipate seasonal variation and minimize missed cases below cutoff. We utilized an ARIMA model to fit monthly IRT screen-positive percentiles and estimated regular seasonal expectations. We established a retrospective cohort followed for at least 1.5 years to capture missed false-negative CF cases. We compared missed CF cases identified by seasonal cutoffs vs. floating cutoffs. GDSP screened 7,410,003 newborns, from July 2007 to December 2022, and missed 36 CF cases below the fixed cutoff; five of the 36 were within 3 ng/mL below the cutoff. There was a regular, seasonal cycle that varied from 1.4% in summer to 1.8% in winter. We would have missed 59 CF cases using a 1.6% daily floating cutoff. California would need to use a 4% daily floating cutoff to improve our current detection rate, which would double the number of specimens sent for costly molecular analysis.