Phenylalanine and tyrosine measurements across gestation by tandem mass spectrometer on dried blood spot cards from normal pregnant women

Maternal phenylketonuria (MPKU) requires strict control of phenylalanine (Phe) and supplemental tyrosine (Tyr). Monitoring during pregnancy using dried blood spot (DBS) cards by tandem mass spectrometry (MS/MS) is now standard practice, however there are no Phe and Tyr reference ranges for DBS MS/MS method in healthy pregnant women. DBS cards (63–1364 days in storage) from healthy women with singleton pregnancies were analyzed by MS/MS. Three hundred ninety DBS cards from 170 pregnancies (5/1–39/6 weeks’ gestation), were tested. Both Phe and Tyr levels declined from the first trimester (Phe: 36.2 +/− 10.6; Tyr 25.7 +/− 9.7 µmol/L) to the second trimester (Phe 33.4+/−9.3; Tyr 21.7+/− 6.7 µmol/L) and remained stable in the third trimester (Phe 32.3 +/− 8.7; Tyr 21.0 +/− 6.6 µmol/L). Phe and Tyr levels declined over time since collection (Phe: 0.004 µmol/L per day; Tyr 0.002 µmol/L). Nomograms by gestational age were created using raw data and data adjusted for time from sample collection. Reference ranges by trimester are provided. Both Phe and Tyr decline quickly during the first trimester and remain relatively constant over the second and third trimesters. These nomograms will provide a valuable resource for care of MPKU.