Diagnostic markers of acute infections in infants aged 1 week to 3 months: a retrospective cohort study.

OBJECTIVE: History and physical examination do not reliably exclude serious bacterial infections (SBIs) in infants. We examined potential markers of SBI in young febrile infants.

DESIGN: We reviewed white cell count (WBC), absolute neutrophil count (ANC), neutrophil to lymphocyte count ratio (NLR) and C reactive protein (CRP) in infants aged 1 week to 90 days, admitted for fever to one medical centre during 2012-2014.

RESULTS: SBI was detected in 111 (10.6%) of 1039 infants. Median values of all investigated diagnostic markers were significantly higher in infants with than without SBI: WBC (14.4 vs 11.4 K/µL, P<0.001), ANC (5.8 vs 3.7 K/µL, P<0.001), CRP (19 vs 5 mg/L, P <0.001) and NLR (1.2 vs 0.7, P<0.001). Areas under the receiver operating characteristic curve (AUC) for discriminating SBI were: 0.65 (95% CI 0.59 to 0.71), 0.69 (95% CI 0.63 to 0.74), 0.71 (95% CI 0.65 to 0.76) and 0.66 (95% CI 0.60 to 0.71) for WBC, ANC, CRP and NLR, respectively. Logistic regression showed the best discriminative ability for the combination of CRP and ANC, with AUC: 0.73 (95% CI 0.67 to 0.78). For invasive bacterial infection, AUCs were 0.70 (95% CI 0.56 to 0.85), 0.80 (95% CI 0.67 to 0.92), 0.78 (95% CI 0.68 to 0.89) and 0.78 (95% CI 0.66 to 0.90), respectively. CRP combined with NLR or ANC were the best discriminators of infection, AUCs: 0.82 (95% CI 0.70 to 0.95) and 0.82 (95% CI 0.68 to 0.95), respectively.

CONCLUSIONS: Among young febrile infants, CRP was the best single discriminatory marker of SBI, and ANC was the best for invasive bacterial infection. ANC and NLR can contribute to evaluating this population.