++
The mean Hb concentration progressively increases throughout gestation.
It is 6 to 9 g/dL at 10 weeks’ gestation, 14 g/dL
at 24 weeks, and 16.5 g/dL at term. The RBC count increases
simultaneously from about 0.5 to 1.5 × 1012/L
at 10 weeks’ gestation to 4.7 × 1012/L
at term. Embryonic and fetal RBCs are much larger than those from
newborns, children, and adults. The mean corpuscular volume (MCV)
decreases from 250 fL at 10 weeks’ gestation to 107 fL at
term. Thus, in neonates, the normal MCV is much higher than in older
individuals. Moreover, many nucleated red blood cells (NRBCs) and
reticulocytes are normally present in the peripheral blood of embryos
and fetuses.
++
At the time of delivery, cord blood has, on average, an Hb concentration
of 16.8 g/dL, an MCV of 107 fL, 5% reticulocytes,
and 500 NRBCs/mm3. A postnatal decrease in plasma volume
occurs between birth and 3 days of age that increases the Hb, hematocrit,
and number of RBCs. At 3 days of age, the Hb concentration averages
17.8 g/dL, the MCV 100 fL, the reticulocyte count 2%,
and 5 NRBCs/mm3. NRBCs disappear from the peripheral
circulation after 3 days of age, and their presence beyond then
should be considered abnormal. The Hb concentration then falls and returns
to the cord blood value by about 2 weeks of age. During the first
few days after birth, there are significant differences in the Hb
concentrations between venous and capillary blood. Capillary Hb
is about 2 g/dL higher than venous Hb because of loss of
plasma from the capillaries and relative hemoconcentration.
++
The mean Hb of the newborn is about 17 g/dL. This relative
polycythemia is the result of the low arterial oxygen tension in
utero and the increased oxygen affinity of Hb F. These factors stimulate erythropoietin
production and erythropoiesis in the fetus. The arterial oxygen
tension abruptly increases on birth in the oxygen-rich extrauterine
environment, so erythropoietin concentration and the rate of erythropoiesis
decrease. Consequently, erythroid progenitors in the bone marrow
decrease from an average of 35% to 15% of total
nucleated cells after 1 week, NRBCs disappear from the peripheral
circulation after 3 days of age, and the reticulocyte count falls
to near zero for about 2 months after birth. The RBC lifespan in
newborns is about 90 days, compared with 120 days in older children and
adults. So, due to decreased production and a slightly shorter RBC
lifespan, Hb values in term infants decrease steadily during the
first 8 to 10 weeks of life to reach a mean nadir of 11 g/dL. This
is called the physiologic anemia of infancy (Fig. 429-3). The lower limit of normal for
Hb concentration at the physiologic nadir is approximately 9.5 g/dL. Afterward,
RBC production resumes, as reflected in a rise in reticulocyte count
and an increase in Hb (Fig. 429-3). The rate
of production thereafter is sufficient to maintain a stable mean
Hb concentration of 11 to 13 g/dL, despite the threefold
increase in blood volume and weight that occurs during the first year of life.
++
++
In the preterm infant, the Hb concentration falls lower and more
quickly than the term infant, even with optimal nutrition and support. The
smallest infants have the greatest decline in Hb concentration.
By 6 to 8 weeks of age, the Hb falls to a mean of about 9.5 g/L
in preterm infants with birth weights between 1500 and 2000 g to
9 g/dL in those with birth weights between 1000 and 1500
g, and even lower in very-low-birth-weight infants (Fig.
429-3). This decline in Hb concentration is called the anemia
of prematurity. The lower limit of normal for Hb concentration
at the nadir of the anemia of prematurity is 7 to 7.5 g/dL.
This exaggerated physiologic anemia has several causes. The concentration
of erythropoietin in premature infants is inappropriately low for
the degree of anemia, perhaps because hepatic production of erythropoietin
is less sensitive to tissue oxygenation than the kidney. Other factors
include a large expansion in blood volume because of rapid growth,
a shorter RBC survival than term neonates, and phlebotomy for laboratory
tests, especially in sick infants.
+++
White Blood
Cells and Platelets
++
The normal leukocyte and neutrophil counts vary tremendously,
even by the hour of life, in the first few days after birth. Therefore,
it is necessary to consult appropriate nomograms in the immediate neonatal
period to properly interpret leukocyte counts. Outside the neonatal
period, the normal absolute neutrophil count has a lower limit of
normal of approximately 1000/mm3, except
in African American infants, where it may normally be quite lower.
The absolute neutrophil count is the product of the total leukocyte count and
the percentage of neutrophils (segmented neutrophils and band forms).
The normal platelet count is 150,000 to 500,000/mm3,
which is the same as children and adults. Thrombocytosis (platelet counts
> 500,000/mm3) may occur in thriving preterm
infants without adverse consequences.
+++
Children and Adolescents
++
At 1 year of age, the mean concentration for Hb is about 12.5
g/dL for both boys and girls, with a lower limit of normal
of about 11 g/dL (Table 429-1).2 Thereafter,
the mean Hb concentration increases steadily with age. RBC indices
such as the MCV also change with age. Values in boys and girls first
begin to diverge significantly with the onset of puberty, after
which boys have higher Hb concentrations and MCVs than girls (Fig. 429-4). The
erythroid-stimulating effects of androgenic hormones account for
this difference between sexes. Thus, it is always important to refer to
age- and sex-specific normal values when interpreting the blood counts
of children (Table 429-1). It is also necessary
to consider a child’s sexual development. For example,
a 14-year-old boy who is Tanner stage IV or V will have a higher
Hb concentration than one who is Tanner stage I or II.
++
++
+++
White Blood
Cells and Platelets
++
The lower limit of normal for the absolute neutrophil count is
approximately 1000/mm3 in the first 1
to 2 years of age and 1500/mm3 thereafter.
African Americans tend to have total leukocyte counts and absolute
neutrophil counts that are lower than that of Caucasians. The absolute
neutrophil count may be as low as 700 to 800/mm3 in
normal African Americans. This has been called “ethnic
pseudoneutropenia,” an arcane name unnecessarily ascribed
to a biologically normal state. The normal platelet count is 150,000
to 500,000/mm3 in children and adults.