(2) The initial amniocentesis is usually done at about 26 weeks, but may be
performed earlier if there is a past history of a baby with HDN. Amniocentesis should
not be performed without clear indications since the procedure may cause infection,
provoke more severe sensitization if the needle allows the escape of fetal blood into the
mother's circulation, or may precipitate labor or fetal death. Amniocentesis is never
indicated in ABO-HDN.
(3) Normal amniotic fluid is colorless. The fluid becomes increasingly yellow
with increasing severity of fetal hemolytic disease. When the amniotic fluid is obtained,
it can be centrifuged and the small number of RBCs present can be typed for ABO and
Rh. If a KIeihauer blood smear is performed on these RBCs, it may be possible to tell
whether the cells are adult or fetal, and thus the infant's blood type may be predicted.
(4) Interpretation of the amniotic fluid analysis depends on sequential
studies repeated at 2-week intervals, or closer if indicated. A spectral absorbance curve
from 350 to 750 nm is made of the amniotic fluid. A line is projected from 375 to 525
nm and the change in absorbance seen at 450 nm (Δ0D450) is plotted on the Liley
graph according to the appropriate week of gestation. Trends are much more valuable
than isolated points. If the Δ0D450 is in the low zone or falls into the low zone as the
pregnancy progresses, the baby will be unaffected or have mild disease. A middle zone
pattern should suggest that the baby will probably be moderately affected, and one
should prepare for an exchange transfusion after an early delivery. With Δ0D450
values above 0.3 in the upper zone or a trend from the middle zone to the upper zone,
fetal death is imminent. If the gestation is at least 32 to 34 weeks, the pregnancy is
usually interrupted to deliver the baby. Before 32 to 34 weeks, the risk to neonatal
death resulting from prematurity is very high, so a intrauterine transfusion may be
indicated. A determination of the Iecithin/sphingomyelin ratio on the amniotic fluid may
be valuable in predicting fetal lung maturity, and thus the ability of the fetus to survive
2-18. LABORATORY INVESTIGATION OF HEMOLYTIC DISEASE OF NEWBORNS
DURING THE NEONATAL PERIOD
a. Unsuspected Hemolytic Disease of The Newborn.
(1) A clotted sample of cord blood (preferably collected by needle and
aspiration) should be obtained on every newborn. This tube should be labeled in the
delivery suite with the mother's name, baby's identification (for example, Baby Boy
Smith or baby's name), hospital number, and date. Such samples should be sent to the
blood bank, and stored in the refrigerator for at least 7 days.
(2) This blood is then readily available for testing if the newborn develops
signs and symptoms suggestive of HDN. This blood sample also can be used, if it is
desired, to test certain special groups of patients, such as all babies of group O
mothers, Rh-negative mothers, or mothers who have delivered a baby affected with
HDN. If these patients are to be studied, the following tests are recommended:
(1) ABO group, (2) RhO(D) type, and DU, if Rh-negative, and (3) direct antiglobulin test.