• Andrew T Krack, Michelle Eckerle, Prashant Mahajan, Octavio Ramilo, John M VanBuren, Russell K Banks, T Charles Casper, David Schnadower, Nathan Kuppermann, and Febrile Infant Working Group of the Pediatric Emergency Care Applied Research Network (PECARN).
• Department of Pediatrics, Children's Hospital Colorado, Aurora, Colorado, USA.
• Acad Emerg Med. 2024 Sep 1; 31 (9): 903914903-914.

Background And ObjectiveSerum procalcitonin (PCT) is a highly accurate biomarker for stratifying the risk of invasive bacterial infections (IBIs) in febrile infants ≤60 days old. However, PCT is unavailable in some settings. We explored the association of leukopenia and neutropenia with IBIs in non-critically ill febrile infants ≤60 days old, with and without PCT.MethodsWe conducted a secondary analysis of a prospective observational cohort consisting of 7407 non-critically ill infants ≤60 days old with temperatures ≥38°C. We focused on the risk of IBIs in patients with leukopenia (white blood cell [WBC] count <5000 cells/μL) or neutropenia (absolute neutrophil count [ANC] <1000 cells/μL), categorized to extremes of lower values, and the impact of PCT on these associations. Multiple logistic regression was used to identify independent predictors of IBIs.ResultsFinal analysis included 6865 infants with complete data; 45% (3098) had PCT data available. Of the 6865, a total of 111 (1.6%) had bacteremia without bacterial meningitis, 18 (0.3%) had bacterial meningitis without bacteremia, and 19 (0.3%) had both bacteremia and bacterial meningitis. IBI was present in four of 20 (20%) infants with WBC counts ≤2500 cells/μL and four of 311 (1.3%) with ANC ≤1000 cells/μL. In multivariable logistic regression analysis not including PCT, a WBC count ≤2500 cells/μL was significantly associated with IBI (OR 13.48, 95% CI 2.92-45.35). However, no patients with leukopenia or neutropenia and PCT ≤0.5 ng/mL had IBIs.ConclusionsLeukopenia ≤2500 cells/μL in febrile infants ≤60 days old is associated with IBIs. However, in the presence of normal PCT levels, no patients with leukopenia had IBIs. While this suggests leukopenia ≤2500 cells/μL is a risk factor for IBIs in non-critically ill young febrile infants only when PCT is unavailable or elevated, the overall low frequency of leukopenia in this cohort warrants caution in interpretation, with future validation required.© 2024 Society for Academic Emergency Medicine.

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