Blood Cultures in the Emergency Department Evaluation of Childhood Pneumonia

Samir S. Shah, MD, MSCE; Maria H. Dugan, BA; Louis M. Bell, MD; Robert W. Grundmeier, MD; Todd A. Florin, MD; Elizabeth M. Hines, BA; Joshua P. Metlay, MD, PhD

Disclosures

Pediatr Infect Dis J. 2011;30(6):475-479. 

In This Article

Discussion

This study is the first to determine the prevalence of bacteremia in children with CAP evaluated in the ED in the post-pneumococcal conjugate and H. influenzae type b vaccine eras. The overall prevalence of bacteremia was low, although patients with complicated pneumonia such as empyema had relatively high rates of bacteremia. Positive blood culture results were often associated with meaningful changes in clinical management; however, the ultimate impact of blood cultures on clinical management was small given the low prevalence of bacteremia.

The prevalence of bacteremia was 2.1% (95% CI: 0.8%–4.4%) in an ambulatory cohort evaluated in the ED setting. Certain subsets of patients had even lower rates of bacteremia including those discharged from the ED and those with a normal chest radiograph. Among the 4 cases of bacteremia caused by S. pneumoniae, none of the serotypes were included in the heptavalent pneumococcal vaccine available at the time of the study, but 3 of the serotypes are included in the now available 13-valent pneumococcal conjugate vaccine.[20] Widespread use of this vaccine may further decrease the rate of bacteremia in children with CAP.

Researchers in Italy recently reported the prevalence of bacteremia to be 3.8% among a subset of children with blood cultures obtained nested within a larger study describing the use of real-time polymerase chain reaction for the diagnosis of CAP.[21] However, this study was conducted exclusively in hospitalized patients. Furthermore, despite the fact that it was conducted in the post-pneumococcal vaccine era, the authors noted that vaccination was limited by region and covered <30% of the country. Both of these factors may account for the reported higher prevalence of bacteremia compared with our study.

Most pediatric studies that examined bacteremia and pneumonia-associated complications focused on empyema.[22–24] Byington et al found that approximately one-third of children hospitalized for parapneumonic empyema were bacteremic in 2 different retrospective studies.[22,23] However, both of these studies were conducted in Utah, which was noted by the investigators to have an unusually high rate of empyema and perhaps a unique epidemiology of S. pneumoniae. The prevalence of bacteremia was 13.9% in a recent randomized control trial conducted by St Peter et al to compare treatment modalities for empyema.[24] The present study did not distinguish empyema from other types of complicated pneumonia, but we found a similar prevalence of bacteremia among all patients with a complicated pneumonia. We examined the pneumonia-associated complications more broadly than previous studies by including other types of pneumonia-associated complications, including organ dysfunction and metastatic infections in our analysis.

The proportion of contaminants in this study was considerably lower than previous studies conducted in the pediatric ED setting.[10–13] Institutional variability in collection procedures may have resulted in a higher rate of false-positive blood cultures at other institutions. The greater emphasis in the past decade on strategies to decrease contamination rates may also explain the lower rate in our study compared with previous studies. Furthermore, unlike studies that described many unnecessary diagnostic tests, treatments, and hospitalizations as the result of false-positive blood cultures,[12–14] only 1 additional blood culture and 1 day of an unnecessary antibiotic were attributed to a false-positive culture result in this study. Our results suggest that the impact of contaminated blood cultures among patients with CAP, who require hospitalization, may be less important than previously argued.

Given the low yield, blood culture results had a meaningful effect on clinical management in only a small minority of patients from whom a blood culture was obtained. Previous studies conducted in both the pediatric[1] and adult[5,6,9] populations with CAP found that culture results led to changes in management in less than half of patients with blood cultures positive for pathogenic bacteria. In contrast, our study found that culture results led to changes in antibiotic therapy in 5 of the 6 bacteremic patients. Two arguments are proposed in the adult CAP literature[6,9] for why blood cultures infrequently alter management. First, patients often received broad-spectrum empiric antibiotics and a positive culture result rarely necessitated further broadening of antibiotic coverage. Second, physicians were reluctant to narrow antibiotic therapy despite results of susceptibility testing. Our study provides support for the first argument, as nearly all bacteremic patients had pathogens susceptible to empiric therapy started in the ED. However, we found that physicians did appropriately narrow antibiotic therapy in most cases.

This study had several limitations. First, blood cultures were obtained in approximately one-third of patients during initial ED evaluation. Blood cultures were obtained more frequently in children who appeared ill and presumably were at higher risk for bacteremia. Among children without a blood culture obtained, most were discharged home from the ED. Therefore, our study may overestimate the true prevalence of bacteremia among children with CAP evaluated in the ED setting. However, it is likely that in certain subgroups, such as those with pneumonia-associated complications, the prevalence is accurate, as most children in this group had a blood culture obtained on initial evaluation. Second, the volume of blood inoculated for culture was not documented. The detection of bacteremia is improved with larger blood volumes.[25,26] If blood inoculated for culture was inadequate, we would underestimate the prevalence of bacteremia. This scenario is unlikely because standard practice at our institution is to achieve a blood:broth ratio between 1:5 and 1:10 (2–4-mL of blood). Third, because of the retrospective study design, changes in management attributed to blood culture results could not be proven despite best efforts to draw reasonable conclusions from the medical record. It is possible that there were unmeasured or undocumented factors that drove or contributed to changes in management. Fourth, we defined pneumonia by physician-assigned discharge diagnosis codes. It is challenging to distinguish bacterial from nonbacterial pneumonia. It is likely that patients with viral pneumonia were included, causing us to underestimate the true prevalence of bacteremia among children with bacterial pneumonia. Nevertheless, given the lack of a gold standard to diagnosis bacterial pneumonia this limitation is unavoidable and also mirrors clinical practice. Finally, the unique patient population of an ambulatory cohort was both a strength and limitation. We included only children followed up in a large ambulatory care network to minimize the bias of a tertiary referral center caring for the sickest children. However, for this same reason, these results cannot be broadly generalized to referral-based EDs. Despite the comparatively large sample size, the low prevalence of bacteremia led to relatively wide CI around the prevalence of bacteremia in certain subsets of patients such as those in whom chest radiographs were not obtained.

In summary, children from an ambulatory cohort presenting to the ED for evaluation of CAP are at low-risk for bacteremia. This risk may become even smaller with widespread use of the 13-valent pneumococcal vaccine. Despite the increased frequency of culture directed management changes found in our study, the impact on clinical management among all patients with a blood culture obtained was small because of the low prevalence of bacteremia. Our data do not support the routine use of blood cultures in children with mild CAP evaluated in the ED. Blood cultures may be useful in the evaluation of moderate-to-severe pneumonia requiring hospitalization.

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