Reticulocyte counts and blood smears are commonly recommended to evalu
ate jaundice in newborns. To investigate the results and diagnostic yi
eld of these tests, we reviewed a computerized database and medical re
cords of term newborns who had reticulocyte counts (n = 799) or blood
smears (n = 781) within the first week after birth at two hospitals. N
early a threefold difference was noted in reticulocyte counts between
the two hospitals (median 8.0% vs 2.8%; P<.0001), apparently due to di
fferences in laboratory methods. Among the patients with ''abnormal''
reticulocyte counts or blood smears (n = 192), isoimmunization was dia
gnosed in 54, presumed hemolysis of unknown etiology in two, G6PD defi
ciency in one, and pyropoikilocytosis in one. We conclude that better
standardization of reticulocyte counts is needed. When ordered as scre
ening tests for hemolysis in jaundiced infants, reticulocyte counts an
d blood smears seldom lead to diagnoses of hemolysis other than isoimm
unization.