Written by Alex Chen, MD.
The CBC is not a very good test for identifying invasive bacterial infections in febrile infants ≤60 days. If you used common normal ranges of WBC count (5000-14900) and absolute neutrophil count > 10k, you would have missed 63% and 82% of invasive bacterial infections, respectively.
Why does this matter?
Due to the lack of reliable physical exam findings and nonspecific symptoms of invasive bacterial infections (IBI), we commonly rely on blood tests such as the CBC to help us risk stratify febrile infants ≤60 days. There have been smaller studies showing suboptimal performance of the CBC in terms of WBC count and absolute neutrophil count in identifying patients with IBI but no large prospective studies.
Much ado about white counts
This was a prospective observational cohort study of 4313 infants from 26 different EDs in the PECARN group. Patients were enrolled in the study if they had a documented fever >38C, blood cultures, and CSF cultures or a 7d follow-up call to see if there was any evidence of bacterial meningitis. Infants ≤28d made up 31% of the population, and 74% of the patients were admitted to the hospital. A total of 97 infants (2.2%) had IBIs, with 1.7% having isolated bacteremia (excluding predetermined contaminants) and 0.6% having bacterial meningitis.
While there was a significant difference between the WBC counts (10.7k vs 9.6k), ANC (5.4k vs 3.1k), and platelet count (331k vs 383k) between infants with IBI and those without, they were unable to determine a threshold that reliably predicted for IBI. They attempted to clarify this with ROC curves and a calculated optimal threshold, but this still had suboptimal performance.
In their discussion they postulated that the poor discriminatory ability of CBC could be due to the change in pathogens that cause IBI in the post-pneumococcal vaccine era, namely E. coli, group B streptococcus (GBS), and S. aureus; E.coli and GBS bacteremia had previously been found to be less likely to cause peripheral leukocytosis than pneumococcal bacteremia. There may have also been some bias due to parents bringing in their children for fever earlier in the disease process before these labs were elevated.
Ultimately, this reaffirms my belief that lab values cannot be taken in isolation, and a normal CBC should not be used as reassurance that the patient is less sick than they are.
Accuracy of Complete Blood Cell Counts to Identify Febrile Infants 60 Days or Younger With Invasive Bacterial Infections. JAMA Pediatr. 2017 Sep 11:e172927. doi: 10.1001/jamapediatrics.2017.2927. [Epub ahead of print]