**3.2. Possible limitations of the use of IG**

White blood cell differential counts are not only influenced by infection. The gestational as well as the infant's age in hours at the time of blood collection, the method of blood sampling and the infant's gender might affect the results as well as the method of delivery, or maternal risk factors like hypertension (Chirico et al., 1999; Christensen et al., 2009; Escobar, 2003; Kayiran et al., 2003; Newman et al., 2010; Schelonka et al., 1995; Schmutz et al., 2008). Even the influence of the sea level on ANC in neonates has been described leading to a wider range of reference values with isolated elevated counts of absolute neutrophil granulocytes but normal IT-ratios in healthy term babies (Lambert et al., 2009).

A prospective longitudinal study observed higher cord blood cortisol levels, as a sensitive marker of intrauterine stress, in infants born by vaginal delivery compared to elective cesarean section in term neonates and a significant positive correlation between total leukocyte and neutrophil counts. After 12 hours of life no differences in the variation of leukocyte counts remained. Although a significant increase of immature neutrophil counts in vaginally delivered infants or after long labour has been previously described (Hasan et al., 1993; Schelonka et al., 1994), no significant differences in the IT-ratio were detected between the two groups (Chirico et al., 1999). In a prospective observational study including 60 preterm infants with a gestational age < 32 weeks prenatal growth retardation has been shown to be an independent factor for significantly lower counts of leukocytes, total neutrophil and immature neutrophil counts in very immature preterm infants immediately after birth when compared with AGA counterparts. It has been assumed that these low numbers of circulating white blood cells might reect the reduced bone marrow reserves (Wirbelauer et al., 2010). As the median granulocyte count in the SGA group was with a count of 1.058/µL near to absolute granulopenia one could consider this as a risk factor for sepsis, as previous studies had reported an association between early onset neutropenia and sepsis (Christensen et al., 2006). But low numbers of inammatory cells could also represent a possible protection mechanism for pulmonary and central-nervous disease by reducing inammatory events postnatally (Wirbelauer et al., 2010). Among extremely low birth weight (ELBW) neonates low neutrophil counts (< 1000/µL) have been observed with a rate of ve times higher than reported in the general NICU population. Most cases were present in the rst days of life and represented a transient phenomenon. SGA or maternal pregnancy induced hypertension were common causes for these alterations, whereas in over one third of cases no cause had been detected. Except in proven early onset bacterial infection, the presence, severity and duration of low counts showed no relationship with mortality rate, whereas neutropenia within the first 3 days of life showed an association with necrotizing enterocolitis (NEC) or nosocomial infection (Christensen et al., 2006).Using IGC in a clinical context should incorporate these factors as well as the likelihood of infection in every individual patient.

As for the blood cell count it has been shown that the performance characteristics in distinguishing between infants with and without infections improve significantly during the first 4 hours after birth. The AUC of the WBC, ANC and IT-ratio showed an increase from 0 hours of 0.52, 0.55 and 0.73, respectively to 0.87, 0.85 and 0.82, respectively 4 hours after birth (Newman et al., 2010). In a review article on new technologies for a diagnostic approach in neonatal sepsis Srinivasan and Harris considered the future development of computerized algorithms including these variables as possibly useful to estimate the probability of sepsis (Srinivasan & Harris, 2012). Taking this fact into account it might be advisable to perform serial IGCs, especially in cases where sepsis had to be ruled out and the result of the test will have a therapeutic consequence (i.e. discontinuing of antibiotic treatment).

The clinical applicability of automated IG counting might be limited by the relatively poor sensitivity of this method (Ansari-Lari et al., 2003; Nigro et al., 2005). Considering the satisfactory specificity and the high NPV this parameter could represent a valuable additional aid in combination with other laboratory markers or diagnostic algorithms. However, as higher values of IG% of more than 3% have been shown to predict blood culture positive results (Ansari-Lari et al., 2003), this subgroup of patients should probably be revaluated for potential infection even in the absence of specific symptoms.
