Discussion

This is the first study showing that the association between NEC and transfusion is not similar among different BW groups. We expected to see a much higher association in the group that received transfusions the most: the extremely low BW infants with BW ≤1000 g. Our findings were completely the reverse. The exposure to transfusions was significantly less likely to be associated with NEC in ≤1000 infants and more likely associated with NEC in larger infants with BW of 1001–1500 g.

Because of their critical condition and finite blood volume, VLBW infants tend to have transfusion thresholds that are inversely proportional to their BW. In a systemic review of prospective randomised studies evaluating the haematocrit thresholds with morbidity outcomes, infants who were transfused more and maintained a higher haematocrit were less likely to develop NEC.10 Retrospective studies evaluating the association between anaemia, exposure and NEC also had similar conclusions.4 ,6 ,7 In our practice, VLBW infants were initially transfused when their haematocrit dropped below a predefined level in their first weeks of life. With age, these predefined levels changed from a preset haematocrit level to the presence of clinical signs combined with oxygen demands. Our practice has remained in parallel with the universal approach.11 In a recent survey, 1018 neonatologists from 11 countries responded with a minimal variability, that in the first week of life higher haematocrit thresholds were targeted irrespective of the degree of respiratory support. At the 4 weeks of life, however, decisions were more variable, with transfusion haematocrit thresholds being affected by the degree of oxygen requirements.12 By that time, variable levels of gut injury may already have been potentiating the development of NEC. The clinical signs such as tachycardia and increased frequency of apnoea could well represent preclinical NEC and not the severity of anaemia. This consensus among neonatologists of accepting lower haematocrit levels for larger infants and our findings of an increased risk of NEC postexposure only in the larger two BW groups strongly suggests that the severity of anaemia may have a significant impact on the development of NEC post-transfusion.

In some studies, infants with exposure associated NEC were reported as older by an average of 2–3 weeks compared with non-exposed NEC infants; they were also ‘stable’ with no clinical signs of illness.4 ,7 ,8 Being older may reflect a longer period of anaemia and lower haematocrit level. In our cohort, exposed infants with NEC were older by only 5 days compared with non-exposed NEC infants, much in contrast to the 2-week to 3-week period previously mentioned. We also did not observe different results when only infants with the late onset NEC were evaluated.

Severity of illness has been proposed as another factor affecting the association between exposure to PRBC and the development of NEC. Exposed infants were more likely to be ventilated,2 ,4 ,7 ,8 have a PDA,4 ,7 ,8 and have a lower BW.1 ,2 ,7 ,8 As all variables exhibit some interaction, interpretation of these results becomes difficult. We initially arrived at similar conclusions. In unadjusted analysis, longer ventilator days and longer UAC placement days increased the likelihood for developing NEC. A prolonged UAC period likely reflected poor intravascular access, frequent blood draws and/or haemodynamic instability. When both variables were stratified into four periods based on quartiles, infants who had longer periods on a ventilator and UAC placement were less likely to develop NEC. The linear change in the RR was very significant. These infants were also in the lowest BW group at ≤1000 g. Thus, and in contrast to current reports, severity of illness, represented here by infants with lowest BWs and longest ventilator days, was not a risk for exposure associated NEC.

Recently, a meta-analysis of reported case studies on the association of NEC and blood transfusion concluded that recent exposure to transfusion was associated with NEC.13 Case studies can produce an exaggerated effect size as compared with cohort studies.14 Kirpalani et al15 has recently reviewed the effects of study design on the outcome of the association between transfusions and NEC. Randomised control trials showed an opposite effect to observational trials; higher transfusions were associated with a lower incidence of NEC. Most observational trials had the problem of confounding or small sample size. Since there was not any matching performed in our study, we were able to evaluate BW as an independent variable and show its significance.

There were several limitations to our study. Bias due to its retrospective nature limited our ability to differentiate between the overlapping preclinical signs of NEC and those of anaemia. Information about haematocrit levels in the first half of the study period were inaccessible. This limited our ability to test for anaemia as a plausible explanation. Information about the use of antenatal steroids was also not available. The treatment with antenatal corticosteroids has been shown to be associated with an overall reduction in necrotising enterocolitis.16 Lower weight infants may have received antenatal steroids more frequently than larger infants, thereby lowering the risk for development of NEC. Another limitation pertains to the days to total feeds and formula versus breast milk use. Historically in our unit, breast milk use was less than 20% and only increased above 60% in the last 2 years of the study period. There are also concerns about the lack of increased superior mesenteric artery blood flow in response to a feeding, following a blood transfusion,17 suggesting that feeds should be held after a transfusion. These results could be attributed to using older blood as it has been shown that by 2 weeks of shelve life, stored PRBC lose the ability to cause regional vasodilation and to increase oxygen uptake by the tissue when compared with fresh PRBC stored for 3–7 days.18 ,19 In our practice, PRBC units are usually exhausted within 4–10 days of postcollection. Using fresh blood for transfusions may explain our study findings but with data not available, this would be a speculation.

A likely explanation of the study findings would be that as an infant's BW increases, there is a threshold crossed that changes the polarity of the association between exposure and NEC. This is nicely demonstrated in the progressive increase in the relative risk value with each weight group. The threshold could be a change in clinical practice like using a lower transfusion limit. It could also be some other factor related to gut maturity, to feeding practice, or altogether still to be discovered.

In summary, using cohort approach in the investigation eliminated the need for matching by BW and lead to new information about different levels of association between blood transfusions and NEC in different weight groups. Our findings show that exposure to transfusions does not increase the risk of NEC. Specifically, after exposure to a PRBC transfusion, ≤1000 g BW infants were less likely to develop NEC than were infants born 1001–1500 g BW. These larger infants were likely to have lower transfusion thresholds as they were less ill and probably more anaemic. We speculate that anaemia could be the cause of their exposure-associated NEC. Future studies evaluating this association should be prospective, should incorporate BW as a risk factor and should incorporate newer modalities such as near-infrared spectroscopy to evaluate hypoxia at the intestinal level.20