Objectives: The diagnosis of serious bacterial infection (SBI) is difficult due to a lack of clinical evidence. The purpose of this study was to determine which inflammatory markers can be used to detect SBI in febrile infants.

Methods: This retrospective cohort study included infants aged 29 to 90 days who visited a tertiary hospital emergency department in Korea between July 2016 and June 2018. The diagnostic characteristics of the neutrophil-to-lymphocyte ratio (NLR), procalcitonin (PCT), C-reactive protein (CRP), white blood cell (WBC) count, and absolute neutrophil cell (ANC) count for detecting SBI were described. Their cutoff values were calculated based on receiver operating characteristic (ROC) curve analysis.

Results: Among 528 infants, 199 were finally enrolled. SBI was detected in 68 (34.2%) of these infants. The median values of all investigated diagnostic markers were significantly higher in infants with SBI than the values in those without: WBC (12.72 vs. 9.91 k/μL), ANC (6.28 vs. 3.14 k/μL), CRP (26.6 vs. 2.8 mg/L), NLR (1.29 vs. 0.78), and PCT (0.5 vs. 0 ng/mL). The areas under the ROC curves for discriminating SBI were: 0.705 (95% confidence interval [CI], 0.629-0.781), 0.793 (95% CI, 0.731-0.856), 0.832 (95% CI, 0.775-0.889), 0.722 (95% CI, 0.651-0.792), and 0.695 (95% CI, 0.611-0.780) for WBC, ANC, CRP, NLR, and PCT, respectively. Using a cutoff value of 0.67 for NLR, the negative predictive value was 90.8% for identifying SBI.

Conclusions: CRP was the best single discriminatory marker of SBI, while NLR was the best parameter for considering discharge.

Key words: bacterial infection, urinary tract infection, clinical marker, discharge planning

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