The effects of the COVID-19 pandemic on the frequency of serious bacterial infections, clinical, and laboratory characteristics in febrile children aged 1-3 months without a focus

INTRODUCTION AND OBJECTIVE: Fever is the most common reason for presentation to pediatric outpatient clinics and pediatric emergency departments. Evaluating febrile infants under 3 months of age is particularly challenging due to difficulties in obtaining an accurate medical history and the similarity of physical examination findings across different clinical conditions. The evaluation becomes even more complicated in patients without any symptoms or signs indicative of a specific focus of infection and with no positive findings on physical examination. In this study, we aimed to determine the frequency of underlying serious bacterial infections (SBI) in febrile infants aged 1-3 months, investigate the impact of the COVID-19 pandemic on the frequency of SBI, assess the reliability of the Rochester criteria in detecting SBIs, and examine the effects of the COVID-19 pandemic on this age group with incomplete immunity. To the best of our knowledge, this is the first study to comprehensively compare this population before and after the pandemic in terms of various data and laboratory parameters. METHODS: Data from febrile infants aged 31-90 days who presented between January 1, 2017, and May 31, 2023, were retrospectively analyzed. Infants with poor general appearance, signs of focal infection, complaints suggestive of an infection focus other than fever, or those without a complete blood count sample were excluded. Statistical analysis was performed on descriptive data and laboratory parameters of 248 infants meeting these criteria. RESULTS: Of the included patients, 45.2% were female, and 55.8% were male. SBIs were detected in 23% of the patients. The prevalence of SBI was similar between the pre-pandemic and post-pandemic periods. Patients with WBC >15,000/mm³ had a higher frequency of SBI (p = 0.005). No statistically significant difference in SBI detection rates was observed in patients with WBC levels of 5,000-15,000/mm³ or <5,000/mm³. Patients classified as high risk based on Rochester criteria had a significantly higher frequency of SBI compared to the low-risk group (p < 0.001). After the COVID-19 pandemic, the association between WBC >15,000/mm³ and SBI lost its significance (p = 0.244). Although the ability of the Rochester criteria to detect SBI decreased after the pandemic, it remained statistically significant (p < 0.001 vs p = 0.014). The sensitivity of the Rochester criteria was low (33.8%), but its specificity was high (89.0%). The positive predictive value was 77.2%, and the negative predictive value was 52.0%. Patients with multiple risk factors based on the Rochester criteria had a significantly higher frequency of SBI compared to those with a single risk factor (p = 0.026). WBC and ANC were higher in the SBI group (p < 0.001 and p = 0.002, respectively). No association was found between SBI and IG, IG%, MLR, ESR, lactate levels, ALS, or AMS. After the COVID-19 pandemic, the association between ANC and SBI weakened to marginal significance (p = 0.052). NLR was higher in the SBI group (p = 0.021). However, the relationship between NLR and SBI lost its significance in the post-pandemic period (p = 0.061). Higher CRP and procalcitonin levels were associated with an increased frequency of SBI (p < 0.001). In the ROC analysis, the cutoff points were determined as follows: WBC at 10,335/mm³, ANS at 8,840/mm³, NLR at 1.73, CRP at 13.69 mg/L, and procalcitonin at 0.195 μg/L. Among these, CRP demonstrated the highest discriminative power in the ROC analysis (AUC = 0.712), followed by procalcitonin (AUC = 0.665), WBC (AUC = 0.643), and ANS (AUC = 0.614). NLR, however, did not exhibit a significant discriminative power. Patients with 10 or more leukocytes detected in the urinalysis had a higher rate of CBE detection (p < 0.001).After the COVID-19 pandemic, the strength of the association between procalcitonin levels and SBI decreased (p = 0.176). Patients with positive urine cultures constituted 75.7% of the SBI group, with Escherichia coli being the most frequently isolated pathogen (38%). CONCLUSION: In febrile infants aged 1-3 months, higher levels of WBC, ANC, CRP, and NLR are associated with an increased frequency of SBI. After the COVID-19 pandemic, the significance of WBC >15,000/mm³, ANC, NLR, and procalcitonin levels decreased. No relationship was found between SBI and ESR, IG, IG%, lactate, ALS, or AMS. There was no significant association between the frequency of SBI and the COVID-19 pandemic. Among infants with SBI, 75.1% had urinary tract infections, with E. coli being the most frequently isolated pathogen. Keywords: serious bacterial infection, febrile infant, COVID-19, pandemic, IG, IG%, NLR, MLR, Rochester criteria