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PEDIATRICS Vol. 106 No. 2 August 2000, pp. 306-310
From the Division of Neonatology and Department of Pediatrics, MetroHealth Medical Center, Case Western Reserve University, Cleveland, Ohio.
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ABSTRACT |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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Background. Normative data for hematologic values in the very low birth weight infants are limited and inconsistent, with the reported mean hematocrit (HCT) in these infants ranging from 43.5% to 60%. No data are available on the effect of race.
Objectives. To establish normative data for hemoglobin
(Hb) and HCT by arterial sampling obtained during the first 3 hours
after birth in black and white premature infants 
31 weeks of
gestation.
Methods. Retrospective chart review of all infants 31 weeks of gestation born between June 1994 and October 1998. Inclusion criteria: infant 31 weeks of gestation who had an
arterial blood sample obtained in the first 3 hours after birth.
Exclusion criteria: infants were excluded if they had any medical
condition that may affect the red blood cell indices (eg, twin-to-twin
transfusion or fetomaternal hemorrhage).
Results. Of 428 infants, 188 who met both inclusion and exclusion criteria were classified into 3 gestational age groups: group 1 = 23 to 25 weeks of gestation (n = 40); group 2 = 26 to 28 weeks (n = 60); and group 3 = 29 to 31 weeks (n = 88). There were statistically significant differences between groups 1 and 3 in HCT, Hb, mean corpuscular Hb (MCH), and mean corpuscular volume (MCV). No differences in HCT and Hb values were noted in relation to sex, mode of delivery, multiple gestation, antenatal steroids, or maternal smoking. In group 3, the mean Hb, HCT, and MCV values were higher in white infants than in black infants (16.7 ± 1.6 g/dL vs 15.4 ± 1.7 g/dL; 50.0 ± 5.0 vs 45.5 ± 4.6; and 112 ± 5 fL vs 107 ± 8 fL, respectively).
Conclusions. Hb, HCT, and MCH values are described for
premature infants 31 weeks of gestation born in North America. Hb and
HCT increased, whereas MCV decreased with gestational age. Hb, HCT, and
MCV values are statistically higher in white infants than in black
infants.
Key words:
hematocrit,
hemoglobin,
premature,
race,
red blood cell indices.
Normative data for hematologic values in very low birth
weight (VLBW) infants are both limited and inconsistent. For example, the reported mean hematocrit (HCT) in these infants varies from 43.5%
to 60%.1,2 In addition, there is lack of agreement in the
medical literature regarding the effect of gestational age on the
hematologic values.3,4
The reported differences in the normative data may be secondary to the
types of patients studied, as well as the method of blood sampling.
Some of the studies had minimal exclusion criteria,4,5
whereas other studies included blood samples obtained beyond the
immediate postpartum period (>3 hours).6 This may have
resulted in the inclusion of patients with medical conditions that
could affect the red cell indices. Some of the sampling methods used in
previous studies include fetal cord blood,7 postpartum
cord blood,5 and capillary blood.4 Fetal
blood sampling precedes the intrapartum events, such as mode of
delivery and timing of cord clamping, which can affect the hematologic
values. Postnatal cord blood may underestimate the values through
dilution with Wharton's jelly. In relation to arterial
sampling, capillary blood sampling overestimates the hematologic
values.8,9
The purposes of this study were to determine the normative hematologic
values in low birth weight infants from arterial blood sample obtained
in the immediate postnatal period, and to evaluate the effects of race,
gestational age, sex, and mode of delivery on these values.
This is a retrospective chart review of all premature infants at
MetroHealth Medical Center, Cleveland, Ohio who were born between June
1994 and October 1998.
Inclusion Criteria
Infants were included if they were born at 31 weeks of gestation
or less and if an arterial blood sample was obtained in the first 3 hours after birth.
Exclusion Criteria
Infants were excluded if they had any underlying condition at
the time of blood sampling that may affect the red blood cell (RBC)
indices such as: 1) hydrops fetalis, 2) intrauterine transfusions, 3)
twin-to-twin transfusion (monozygotic twins with a difference in their
hemoglobin (Hb) of >5 g/dL), 4) maternal medications that may affect
the fetal hemopoietic system, 5) emergency caesarian section secondary
to significant antenatal bleeding, 6) low Apgar score (<5) associated
with tight nuchal cord, 7) intravenous fluid boluses given before
obtaining the blood sample,10 8) grade 3 or 4 intraventricular hemorrhage detected by the third postpartum day, 9)
small for gestational age (birth weight <10th percentile for
gestational age), 10) discordant twins (difference in birth weights
between the twins that is >20% of the weight of the larger twin), 11)
fetomaternal hemorrhage, 12) chromosomal anomalies, 13) cyanotic
congenital heart disease, 14) any shock-like state in the immediate
postpartum period, 15) disseminated intravascular coagulation, and 16)
positive antibody titers other than those attributable to RhoGAM (Bayer
Biological, West Haven, CT). In addition, infants without a first
trimester ultrasound were excluded if there was a difference of >2
weeks between the modified Ballard scoring method and the best
antenatal dates.
Definitions
The primary method for defining gestational age was by first
trimester ultrasound. If the ultrasound was not performed, then gestational age was defined by best antenatal dates provided that they
were in concordance (± 2 weeks) with the modified Ballard scoring
method. The following categories by gestational age were defined a
priori: group 1 (23-25 weeks), group 2 (26-28 weeks), and group 3 (29-31 weeks). Early cord clamping (defined as <30 seconds) is the
standard practice of our obstetric colleagues at our institution.
Blood Sampling and Analysis
Within 3 hours after delivery, a blood sample consisting of 2 mL
of blood was obtained, of which .5 mL were sent for a complete blood
count in an ethylenediamine-tetraacetic acid-coated plastic microcontainer bottle. The blood sample was obtained percutaneously from a peripheral artery (dorsalis pedis or radial) or from an indwelling umbilical arterial catheter. To avoid dilution with intraarterial infusate, 3 mL of blood were withdrawn before sampling. Before May 1996, the blood samples were analyzed with an STKS Coulter
analyzer using the cyanohemoglobin method (Coulter Corp, Miami, FL).
After May 1996, the blood samples were analyzed with the HFP Sysmex
SE-9500 (Sysmex Corporation of America, Long Grove, IL) using the
sulfolyser Hb method. The change in methodology was secondary to the
ability to analyze multiple samples rapidly coupled with a reduction in
the production of toxic wastes. All analyses were performed in the
clinical laboratories at MetroHealth Medical Center.
Technique of Automated Blood Cell Counting
For both machines (Sysmex and Coulter), the size and number of
blood cells are determined by changes in electrical impedance. As
individual blood cells pass through a narrow aperture, the increased
resistance of the cells impedes the flow of current between 2 electrodes. Each momentary drop in conductance represents a cell with
the magnitude of the drop being proportional to the volume of cell.
Hence, RBC counts and mean corpuscular volume (MCV) are measured
directly, whereas the HCT is calculated from the above values. The
white blood cell (WBC) counts are determined in a similar manner;
however, the cell membranes are initially lyzed, which eliminates the
red cells from the counting chamber. The remaining nuclei are counted
as WBC. Hb is measured by a spectrophotometric assay.
Statistical Analysis
Interval level data are reported as either the mean ± standard deviation (Table 1) or as the
median with the corresponding 5th and 95th percentiles. A 1-way
analysis of variance (ANOVA) was used with the dependent variable being
the RBC indices (ie, HCT and MCV) and the independent variable being
gestational age (group 1: 23-25 weeks; group 2: 26-28 weeks; and
group 3: 29-31 weeks). In addition, a 2-way ANOVA was used with the
independent variables being race (white vs black) and gestational age.
If the overall P value from the ANOVA was <.05 (2-tail),
then posthoc statistical testing was performed using the Bonferroni
correction for testwise error. Statistical significance was defined a
priori as a P value <.05 (2-tail). Data were analyzed using
SPSS/PC, version 6.1, statistical software package (SPSS Inc, Chicago, IL).
TABLE 1
METHODS
Top
Abstract
Methods
Results
Discussion
Conclusion
References
Patient
Characteristics*
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RESULTS |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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Patient Characteristics (Table 1)
Of a possible 428 premature infants 31 weeks of gestation, 188 met both our inclusion and exclusion criteria. Their distribution by
gestational age were group 1 (23-25 weeks; n = 40),
group 2 (26-28 weeks; n = 60), and group 3 (29-31
weeks; n = 88). These 3 groups were similar with
respect to sex, mode of delivery, use of antenatal steroids, and
history of maternal smoking (P > .05). The groups were
different with a higher frequency of whites in group 3 than in group 1 (56.8% vs 37.5%; P = .017).
Effect of Gestational Age on RBC Indices (Table 2)
With increasing gestational age, both the HCT and Hb rose, whereas the MCV and mean corpuscular Hb (MCH) decreased (1-way ANOVA, P < .05). With posthoc testing, the differences were statistically significant between groups 1 and 3 for HCT, Hb, MCH, and MCV and between groups 2 and 3 for HCT, Hb, and MCV (P < .01). For example, the mean HCT was 48% in group 3 versus 43.5% and 45% in groups 1 and 2, respectively. The red cell distribution width (RDW) was similar among the 3 groups.
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Effect of Race on RBC Indices (Table 3)
In the 2-way ANOVA, the main effects of both race and gestational age were statistically significant for HCT, Hb, and MCV (P < .01). With posthoc testing, the mean Hb, HCT, and MCV values for group 3 white infants (16.7g/dL, 50.0%, and 112 fL, respectively) were higher than for black infants (15.4g/dL, 45.5%, and 107 fL, respectively). There were no statistically significant interactions noted between race and gestational age with any of the RBC indices. There were also no statistical differences noted in RDW or MCH between the 2 races.
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Effect of Other Variables on RBC Indices
None of our patients had an uncorrected WBC count (WBC + nucleated red blood cell [NRBC]) exceeding 100 × 109/L, or a platelet count exceeding 1000 × 109/L. The median (5th percentile; 95th percentile) for WBC count, NRBC count, uncorrected WBC count (WBC + NRBC), and platelet count in our patients were 9.0 × 109/L (4.1 x 109/L; 23.0 × 109/L), 1.26 × 109/L (.0 x 109/L; 9.3 × 109/L), 11.0 × 109/L (4.65 x 109/L; 28.7 × 109/L) and 225 × 109/L (128 x 109/L; 329 × 109/L), respectively.
Hb and MCV values were similar during the 2 periods in which different automated instruments (Coulter STKS and Sysmex HFP-9500) were used to analyze the blood samples (Hb, 16.2 ± 1.6 g/dL vs 16.2 ± 1.8 g/dL; P = .859; MCV, 110.1 ± 7.0 fL vs 110.7 ± 6.5 fL; P = .683, respectively). There was no effect of maternal smoking, antenatal steroids, multiple gestations, sex, and mode of delivery on any of the RBC indices.
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DISCUSSION |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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This study was undertaken because of the paucity and inconsistency of normative hematologic data in the premature infants.11 Besides reporting normative data in VLBW infants, we have also noted an effect of gestational age and race on HCT, Hb, and MCV.
There is marked inconsistency in the reported normative data for RBC indices in VLBW infants with HCTs ranging from 60% (capillary)4 to 43.5% (fetal sampling)12 at 29 to 31 weeks of gestation. This variability is probably secondary to problems inherent to each sampling technique with hemoconcentration occurring with capillary sampling8-9 and probable hemodilution with either amniotic fluid or Wharton's jelly in fetal cord blood sampling. In a venous sampling study of Arabic infants,13 the median HCT at 30 weeks of gestation was 47% to 48%. The strengths of this study were consecutive enrollment and accurate gestational dating (ultrasound, best antenatal dates, and Ballard method). The problems with this study are possible hemoconcentration with tourniqueted venous sampling, greater chance of postpartum events (fluid boluses)10affecting RBC indices with sampling performed at 4 to 6 hours after birth, and less strict exclusion criteria. In an arterial sampling study of North American infants,14 the mean HCT was 44.7% at 30 to 32 weeks of gestation. The strength of the study was the arterial sampling. However, the weaknesses were convenience sampling, sampling in the first 12 hours of life, small sample size in infants 24 to 32 weeks of gestation (n = 44), less accuracy in gestational dating (only Ballard method was used), and less stringent exclusion criteria. The mean HCT in our study for infants of 29 to 31 weeks of gestation was 48%. Our study incorporated the strengths of the previous 2 studies that include arterial sampling, stricter exclusion criteria, sampling in the first 3 hours after birth, accurate gestational dating (first trimester ultrasound, best antenatal dates, and modified Ballard scoring method), and consecutive sampling. Although the median HCT in our study at 29 to 31 weeks of gestation is similar to that of the Arabic infant study, the 5th and the 95th percentiles are different (39.4% vs 32% and 55.9% vs 62%, respectively). The narrower range noted in our study is a reflection of our sampling methods and tighter exclusion criteria, which may have resulted in the selection of a more normal, homogenous population.
In the literature, there are inconsistent findings on the effect of gestational age on the HCT in VLBW infants.3,7 In a capillary sampling study, the HCT remained unchanged from 63% to 60%2 from 24 to 31 weeks of gestation, whereas in a fetal blood sampling study, the HCT increased from 38.6% (22-25 weeks) to 43.6% (>30 weeks). The HCT also increased in both the venous blood sampling study13 and the arterial sampling study in North American infants.14 However, in the latter 2 studies, there was no formal hypothesis testing conducted to show that the increase in HCT with gestational age is statistically significant. In our report, the rise in HCT from 43.5% (23-25 weeks) to 48% (29-31 weeks) was statistically significant and substantiates the findings in the previous studies. In addition, we also noted an increase in Hb and a decrease in MCV with gestational age.
To our knowledge, the effect of race on hematologic values in the
newborn period has not been previously described. In older children
(5-9 years old), Dallman et al15 reported that Hb was .5 g/dL lower in black children, compared with white children. Similar
observations have been reported in adolescents16 and
adults.17 We now report that racial differences exist in
premature infants as well. The HCT, Hb, and MCV values were
significantly higher in white infants. In aggregate, this suggests that
the differences may be genetically mediated rather than acquired. The
physiologic basis for the racial differences in hematologic values is
unknown. One may speculate that these differences are secondary to the increased incidence of 
-thalassemia (Hb Barts) in black infants or
to a racial difference in the timing of when 
-synthesis is switched
over to 
-chain production. It is unlikely that this difference is
secondary to maternal nutrition or to the iron status of the mothers.
Previous studies have reported no correlation between maternal iron
status, HCT, and the newborn's iron status or Hb.18-25
Only if the maternal iron deficiency is severe, is it considered likely
to affect the fetal iron or Hb.26,27
Automated hematology counters have limitations. Because Hb is measured spectrophotometrically, the increase in the turbidity of the solution from an elevated uncorrected WBC count (>100 × 109/L) or from cellular debris (platelet count >1000 × 109/L) may falsely increase the Hb value. In addition, nucleated cells are counted as red cells for determining red cell indices. Because red cells usually out number nucleated cells by 500-fold, the effect of nucleated cells on RBC counts and MCV is negligible, unless the uncorrected WBC count exceeds 100 × 109/L.28-30 None of the patients in our study exceeded these values.
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CONCLUSION |
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Top
Abstract
Methods
Results
Discussion
Conclusion
References
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We report the normative data for RBC indices in preterm infants
31 weeks of gestation. In addition, we have noted the effect of
gestational age and race on these indices. Hb and HCT increased with
gestational age, whereas MCV decreased. White infants have higher HCT,
Hb, and MCV values, compared with black infants. Further research is
needed to physiologically explain these observed differences.
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FOOTNOTES |
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Received for publication Jul 29, 1999; accepted Dec 6, 1999.
This work was presented in part at the Pediatric Academic Society meeting; May 1-4, 1999; San Francisco, CA.
Reprint requests to (P.A.) Division of Neonatology, MetroHealth Medical Center, 2500 MetroHealth Dr, Cleveland, OH 44109. E-mail: pradeepalur{at}msn.com
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ABBREVIATIONS |
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VLBW, very low birth weight; HCT, hematocrit; RBC, red blood cell; Hb, hemoglobin; MCV, mean corpuscular volume; WBC, white blood cell; ANOVA, analysis of variance; MCH, mean corpuscular hemoglobin; RDW, red cell distribution width; NRBC, nucleated red blood cell.
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REFERENCES |
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