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

PURPOSE: 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.

METHODS: 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.

RESULTS: 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.

CONCLUSIONS: 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.