* Values for Hb, Hct, MCV and MCH are based on reference ranges in Christensen et al.63 Values for leucocytes and NRBCs are based on reference ranges in references 64–67 and our own hospital laboratory data (unpublished); values for peripheral blood blast cells and platelets are based on Roberts et al.68
Hb, haemoglobin concentration; Hct, haematocrit; MCH, mean cell haemoglobin; MCV, mean cell volume, NRBC, nucleated red blood cell.
Table 1.3 Impact of postnatal age on Hb and Hct values in healthy term and preterm neonates*
| Postnatal age | |||
|---|---|---|---|
| Birth | 2 weeks | 4 weeks | |
| Gestation at birth 35–42 weeks | |||
| Hb (g/l) | 140–215 | 110–180 | 100–170 |
| Hct (l/l) | 0.43–0.65 | 0.32–0.55 | 0.27–0.48 |
| Gestation at birth 29–34 weeks | |||
| Hb (g/l) | 130–215 | 100–170 | 80–135 |
| Hct (l/l) | 0.40–0.42 | 0.30–0.48 | 0.24–0.42 |
* Values are based on reference ranges in Christensen et al. 2009.63
Hb, haemoglobin concentration; Hct, haematocrit;
In the absence of red cell transfusion, the Hb and haematocrit fall over the first few weeks of life due to the physiological reduction in red blood cell production. In term babies, the average Hb falls from 180 g/l at birth to 140 g/l at the age of 4 weeks,69 reaching a nadir of around 100 g/l at 2 months of age. Studies of healthy preterm infants carried out almost 50 years ago reported a more rapid fall in Hb than in term babies, reaching a mean of 65–90 g/l at 4–8 weeks postnatal age (reviewed in reference 35). However, these differences are difficult to interpret because of the variable clinical course of preterm infants and the effects of red cell transfusion, particularly in neonates of less than 26 weeks’ gestation at birth. More recently, data from non‐transfused neonates have confirmed the lower Hb nadir in preterm neonates, with a mean Hb 28 days after birth of approximately 105 g/l in neonates with a gestational age at birth of 29–36 weeks and 130 g/l for term neonates.69
The mean cell volume (MCV) of red blood cells in healthy neonates at birth is higher than that in older children and adults and is inversely proportional to gestational age (Table 1.2).63,68 The average MCV of a term neonate is about 105 fl, while extremely preterm neonates with a gestational age of less than 26 weeks typically have an MCV averaging about 120 fl.63 Similarly, the mean cell haemoglobin (MCH) at birth is higher in preterm neonates than in term neonates, averaging 40 pg in a preterm neonate of less than 26 weeks’ gestation and about 36 pg in a term neonate.63 The MCV, but not MCH, has been reported to be significantly lower in black preterm compared with white preterm neonates, although this may reflect a higher prevalence of α thalassaemia trait in these neonates as this was not specifically investigated.70 In contrast to the MCV and MCH, the mean cell haemoglobin concentration (MCHC) does not change significantly during gestation70 and, unlike in older children, changes in MCHC are not very useful for diagnostic purposes in neonates. In term babies, the MCV and MCH fall slowly over the first few weeks of life, with a lower limit of normal of 77 fl and 26 pg, respectively.63 The same pattern is seen in preterm neonates although the high frequency of red cell transfusions means that reliable data are not available.
Two newly available automated red cell parameters generated as part of a full blood count, MicroR and HYPO‐He, have recently been evaluated in neonates.71 In adult red blood cells, MicroR provides an automated measure of the percentage of red blood cells with an MCV of <60 fl and HYPO‐He measures the percentage of red blood cells with an MCH of <17 pg. Bahr and colleagues created new reference ranges for neonates based on analysis of more than 11 000 blood counts and used them to show that a combination of MicroR and HYPO‐He was more sensitive than MCV/MCH in identifying iron deficiency at birth.71 Further validation of these results in prospective studies and assessment of the impact of gestational age will be needed to determine the value of these new parameters in the diagnosis of neonatal iron deficiency and anaemia.
Reticulocytes and circulating nucleated red blood cells
The reticulocyte count falls rapidly after birth as erythropoiesis declines. In term babies it then starts to increase at 7–8 weeks of age, reaching 35–200 × 109/l (1–1.8%) at 2 months of age; in preterm babies it increases at 6–8 weeks of age.35 Neonatal reticulocytes, like mature neonatal red blood cells, have a larger volume and lower Hb than adult reticulocytes.35 Manual reticulocyte counts have now largely been replaced by automated reticulocyte counts based on cell size and ribonucleic acid (RNA) content. Automated reticulocyte counts also provide a measure of the fraction of reticulocytes with the highest RNA content (Immature Reticulocyte Fraction [IRF]), which has been used in neonates to determine whether or not there is increased erythropoietic activity, for example in response to EPO treatment or as a diagnostic aid to haemolysis.72–74
Normal ranges for the numbers of circulating nucleated red blood cells (NRBC) in neonates have been compiled by Christensen et al.66 and are generally higher in preterm neonates (2–3 × 109/l) than in term neonates (about 1 × 109/l) (see Table 1.2). Although modern analysers are increasingly able to generate accurate absolute NRBC counts, a useful guide from examination of blood films is that the presence of up to 5 NRBC/100 white blood cells in a term baby and up to 25 NRBC/100 white blood cells in a preterm baby can be considered normal for the first 1–2 days of life. In fact, there seems to be
