Neonatal birth parameters of positive newborns at PKU screening as predictors of false-positive and positive results at recall-testing

Objectives: To investigate the predictive role that neonatal birth parameters of positive newborns at phenylketonuria (PKU) screening have on false-positive and positive results. We reviewed 195 newborns (115 males and 80 females) that had false-positive results between 1998 and 2001. A total of 4386 randomly selected neonates (2191 males and 2195 females) who tested normal at the first investigation in the same period, were used as negative-controls. A total of 38 PKU neonates (17 males and 21 females) diagnosed between 1990 and 2001 were used as positive-controls. Methods: Phenylalanine concentration was measured with a fluorometric multitask plate counter Wallac 1420 VICTOR F (Perkin Elmer, Finland) and the fluorescent ninhydrine method (EG&G Wallac neonatal phenylalanine kit) using a recall cut-off level >120 mol/l (2 mg/dl) of phenylalanine on dried blood spots. A multivariate logistic regression analysis was performed to evaluate the predictive role that body parameters (sex, gestational age, parity, weight, length and head circumference) of positive newborns at PKU screening had on false-positive and positive results at recall PKU tests. Results: The risk of false-positive results is higher (~48%) in females than in males. Moreover, for each 100g of body weight reduction, the risk of false positive is around 4.2% higher. The risk of confirmation increased by 39% per week of gestational age. Conclusions: In conclusion, our results suggest that preterm or low-birth-weight neonates recalled at the first investigation are more likely to be due to false-positives, whereas the risk of confirmation is higher in at-term neonates. By implication, the phenylalanine cut-off value for premature or low-body-weight infants could be higher.