The clinical presentation of LOS in ELBW infants is subtle, and as invasive bacterial and fungal infections are similar, this may cause diagnostic delay [9]. In this present study, we were unable to identify differences in clinical findings between microorganisms except in the age of onset, which was the lowest in the fungal group. At birth, most infants are uncolonized or have low colony counts of yeast. In the absence of antifungal prophylaxis, up to 60% of ELBW infants become colonized in the first 2 ~ 3 weeks after birth [10]. Clerihew et al. described a median age at diagnosis of 14 days, using 94 cases of invasive fungal infection in ELBW infants [11]. According to our study, the mean age of onset was 21 days after birth. These results are consistent with the fungal colonization timings mentioned above. Therefore, it is speculated that the fungal infection may occur as soon as colonization occurs in ELBW infants with compromised immunity, but the exact etiology needs to be investigated.
Although blood culture results are important for diagnosing neonatal sepsis, its unavailability within the 2 ~ 3 days of incubation and low culture rate make early diagnosis difficult. For the rapid identification of microorganisms causing sepsis, novel laboratory methods, such as cytokine and molecular analyses, have been developed; however, it is unlikely that these methods will be useful in the near future because they are not very cost effective [12, 13]. So far, reliable laboratory diagnosis has not been achieved.
The complete blood cell (CBC) count is a rapid, inexpensive, and widely available diagnostic test [14, 15]. However, the diagnostic accuracy of the CBC count is not well-defined in neonatal sepsis and the usefulness of the CBC count have reported conflicting results in preterm infants, especially in very low BW (VLBW) infants [16, 17]. Hornik et al. studied the diagnostic accuracy of CBC count in LOS in a large multicenter population and reported CBC parameters associated with LOS, including a total WBC count of < 5000/mm3 and an immature neutrophil/total neutrophil (I/T) ratio of > 0.10 [18]. According to some studies on the pathogenesis of organisms responsible for LOS, elevated I/T ratios were significantly more common in gram-negative infections than in gram-positive ones [19, 20]. Dogan et al. analyzed red-cell distribution width (RDW) during a LOS episode and its association with the type of growing microorganism [21]. According that study, RDW levels increased in preterm infants with LOS, which was especially evident in gram-negative infections. Unfortunately, we did not check I/T ratio and RDW levels in this study. Nevertheless, we found that higher WBC counts were associated with gram-negative LOS in ELBW infants. However, CBC count varies significantly with day of life and GA and are affected by non-infectious comorbidities. More specifically, VLBW infants are known to develop physiologic late onset neutropenia, which does not correlate with sepsis. Therefore, we have to cautiously interpret the WBC count during diagnosis of LOS in preterm infants.
Thrombocytopenia has been used as an early but nonspecific marker for sepsis [22]. Several studies have shown quantitative differences in the platelet response to infection with the three major categories of organisms causing sepsis in the VLBW infants. Daniel et al. reported that thrombocytopenia at the time of blood culture was a risk factor for candidemia [23, 24]. In our study, mean platelet count was 54,000/mm3 in the Candida group, which was significantly lower than those of the other two bacterial groups. This result was similar to previously published results. According to Benjamin et al., fungal sepsis is associated with a greater degree of thrombocytopenia than CoNS sepsis [25]. Although the mechanisms for this result have not yet been identified, it is known that administration of platelet activating factor is protective in a mouse model of C. albicans sepsis, suggesting that platelet activation plays a role in the host defense against fungal pathogens [26, 27]. On the other hand, Scheifele et al. demonstrated evidence of a relationship between gram-negative infections and thrombocytopenia [28]. Further work is needed to understand the basis for the effects of different species of microorganisms.
The most extensively studied laboratory marker for determining the diagnosis of neonatal sepsis has been CRP [29]. The increased survival of extremely premature infants has brought the diagnostic validity of the method to the point of great scientific controversy. Some clinicians suggest that CRP is an inappropriate method in extremely premature infants as they are incapable of sufficiently increasing CRP levels following a septic event. One possible explanation for this is that an extremely immature liver cannot respond to septic stimuli as those in more mature premature and full term infants [30, 31].
On the other hand, in a study of 123 ELBW infants with a mean GA of 27 weeks and mean BW of 1000 g, Wagle et al. concluded that infants with gram-negative sepsis are capable of mounting significant CRP responses similar to those of full term infants [32].
In addition, Dritsakou et al. found that gram-negative microorganisms were associated with higher CRP levels in ELBW infants [33]. On the contrary, some studies found that CRP could not serve to disclose CoNS infection. Previous studies have suggested that CoNS are associated with lower levels of inflammation than other bacteria [34–36]. These studies showed that CoNS, involving S. epidermidis, does not cause local inflammatory reactions or immunoglobulin G responses in animal models [36].
The results of our study are consistent with those of the abovementioned studies to some extent. According to our study, CRP increased in the gram-negative group, whereas CRP in the gram-positive group did not increase as much. However, in our study, the majority of the gram-positive group had S. aureus involvement, and not CoNS. CoNS are a common skin commensal and the most frequent organism associated with nosocomial infection in ELBW infants. However, recent studies have suggested that CoNS infection has decreased with central intravascular line care protocols and the reduction of the use and duration of central lines, which is consistent with our result [37, 38]. Therefore, we do not know exactly why the gram-positive group showed lower CRP level than the gram-negative group.
It has been recognized that hyperglycemia may be an important early sign in neonatal sepsis [39, 40]. It is known that inadequate hepatic and diminished pancreatic insulin secretory responsiveness increase the risk of hyperglycemia in stressful episodes, such as sepsis, in preterm infants [41]. Interestingly, blood glucose was higher in the fungal group than in the other two bacterial groups in our study. This result is consistent with another study by Manzoni et al, which revealed that hyperglycemia is more frequent in fungal LOS sepsis than in bacterial LOS sepsis in preterm infants [40, 42]. They suggested that hyperglycemia could be a possible surrogate marker predictor of invasive fungal infection in preterm infants. However, Levit et al. reported that sepsis-related death was associated with hypoglycemia in VLBW infants and that there was no specific correlation between glucose disturbances and the type of pathogen [43]. We do not know exactly why the Candida group showed higher glucose level than the bacterial groups in our study. Further studies are needed to know this association.
This study has several limitations, including its retrospective design and small number of patients. Furthermore, we only included blood culture-proven sepsis and the first LOS episode. Unfortunately, culture-negative sepsis is common in neonates. Therefore, we were unable to retrieve the complete information for all cases of LOS in ELBW infants.
In conclusion, we have shown that there are some differences in laboratory findings according to causative microorganisms in LOS of ELBW infants. WBC and CRP were increased in gram-negative infection, and thrombocytopenia and hyperglycemia were predominant in fungal infection. These data may be helpful to choose empirical antibiotics when sepsis is suspected.