BackgroundWe studied the differences in immunoreactive trypsin (IRT) in neonatal screening for cystic fibrosis (CF) associated individually with the age of the newborn, ethnicity and environmental temperature. In this study, we determine the overall influence of environmental temperature at birth, gender, feeding, gestational age, maternal age and ethnic origin on an abnormal IRT result.MethodsCross-sectional observational study. A sample was selected of newborns from Alicante (Spain) who underwent neonatal CF screening in 2012¿2013. Primary variable: abnormal IRT levels (¿65 ng/ml). Secondary variables: gender, maternal origin, maternal age (years) (<20, 20¿40, >40), gestational age (weeks) (<32, 32¿37, >37), type of feeding (natural, formula, mixed and special nutrition), >20 days from birth to blood collection, and average temperature during the month of birth (in°C). Using a multivariate logistic regression model the adjusted odds ratios (ORs) were estimated to analyze the association between atypical IRT levels and the study variables. The ¿ error was 5% and confidence intervals (CI) were calculated for the most relevant parameters.ResultsOf a total of 13,310 samples, 199 were abnormal (1.34%). Significant associated factors: feeding method (natural¿¿¿OR¿=¿1; mixed¿¿¿OR¿=¿0.53, 95% CI: 0.31-0.89; formula¿¿¿OR¿=¿0.72, 95% CI: 0.48-1.07; special¿¿¿OR¿=¿21.88, 95% CI: 6.92-69.14; p¿<¿0.001).ConclusionsNewborns receiving special nutrition have a 20-fold higher risk for abnormal IRT levels, and screening is advisable once normalized feeding is initiated. It is advisable to consider ethnic variability. Seasonality was not important.