Value of Phenylalanine in Amniotic Fluid Detected by MS/MS for Screening of ma ternal PKU in the Second-Trimester

Background: Phenylketonuria is the most common inherited metabolism disease in China. This study aimed to identify a new, sensitive, simple, and easy method of screening maternal Phenylketonuria in the fetal stage.Methods: Samples of amniotic fluid, dried blood spots, and blood collected on the same day were obtained from women in the second-trimester (16–28 weeks). Women had to meet the following criteria: (i) isolated high risk of quadruple marker screening or cell-free DNA screening, (ii) women who gave birth to at least one child with intellectual disability of unknown etiology, and (iii) women who gave birth to a child who died young from unknown etiology. The exclusion criteria were as follows: clinically evident chorioamnionitis, multiple gestation, and laboratory signs of infection in the amniotic fluid sample. Phenylalanine levels were measured using tandem mass spectrometry. Women who met criterion (i) served as controls whose results were used to set reference values of Phenylalanine in amniotic fluid. Women who met criteria (ii) and (iii) served as the case group whose results were used to check for maternal Phenylketonuria. The Spearman rank correlation test was used to analyze the correlation of Phenylalanine in amniotic fluid and in venous blood. Results: We analyzed 365 samples of amniotic fluid. Among them, 345 were included in the control group, with reference values of Phenylalanine in amniotic fluid of 10.79–48.47 µmol/L. Twenty cases were included in the case group. One woman in the case group was diagnosed with hyperphenylalaninemia whose fetus was diagnosed with maternal Phenylketonuria by comprehensive analysis of Phenylalanine in dried blood spots and in amniotic fluid, head circumference in fetal ultrasound, and a previous history of adverse pregnancy. The correlation between Phenylalanine concentrations in venous blood and in amniotic fluid was weak.Conclusions: Phenylalanine can be readily measured in amniotic fluid. Relative to controls, amniotic fluid levels of Phenylalanine was increased in maternal Phenylketonuria. These results can support the use of Phenylalanine as a screening tool for maternal Phenylketonuria. The significance of these change requires further study.