The role of postnatal age and magnesium on parathyroid hormone responses during "exchange" blood transfusion in the newborn period.

One hundred and eight "exchange" blood transfusions were done on 61 newborn infants. Baseline serum PTH concentrations and the PTH rise in response to citrate-induced hypocalcemia were studied. Baseline PTH values increased with postnatal age, particularly after the first three days of life. The acute response of PTH to citrate-induced hypocalcemia appears within ten minutes following the initiation of exchange transfusion and was shortlived in spite of further decline of serum ionized calcium. The dominant effect of postnatal age over gestational age was demonstrated: postnatally older but gestationally less mature infants exhibited greater responsiveness than postnatally younger, but gestationally more mature, infants. The PTH response during exchange transfusion was blunted in hypomagnesemic infants. Since lower serum magnesium concentrations were also present during the first three days of life, a separate effect of serum magnesium concentrations on parathyroid responsiveness cannot be ruled out in this study.