Fever is a frequent reason for emergency department (ED) visits in infants under 3 months of age. Although viral infections are the most common etiologies, the prevalence of severe bacterial infections (SBI) is high (10%). While in infants with SBI, establishing the diagnosis and initiating rapid intravenous antibiotic therapy is necessary, every effort should be made to avoid it in those at low risk of SBI.

Clinical examination and biomarkers are still sub-optimal for assessing the risk of SBI. As a result, the vast majority of these children receive inpatient intravenous antibiotic therapy.

Flow cytometry is a technique for measuring the expression of biomarkers on the cell surface of leukocytes in response to infection. A French team has identified a combination of some fifteen leukocyte cell surface markers that perform excellently in discriminating between bacterial and viral infections in a population of adults presenting to the emergency department with a suspected infection. However, there are no similar studies in children.

The objective of the study is to assess the diagnostic performance of a combination of biomarkers on the cell surface of leukocytes in discriminating between bacterial and viral infection. Infants less than 3 months of age, visiting the ED for fever will have an extra blood sample in order to measure those biomarkers. The performance of those biomarkers to identify SBI will be assessed.

Inclusion Criteria:

• Children aged between 7 and 90 days
• Visiting the emergency department with fever (Temperature greater than or equal to 38° measured rectally in the emergency department or reported by parents) OR whose fever is detected during the emergency department visit (Temperature greater than or equal to 38° measured rectally in the emergency department)
• Consent signed by one of the two parents/guardians

Exclusion Criteria:

• Weight less than 2,500 grams
• Chronic illness (heart failure...)
• Known immune deficiency
• Congenital anomaly significantly modifying the probability of bacterial infection (pulmonary malformation, esophageal atresia...)
• Antibiotic therapy in the previous 48 hours
• Clinically evident source of fever: skin infection, joint infection, etc.
• Parents or guardians unable to understand French
• Non-affiliation with a social security scheme (including AME)

All infants aged between 7 and 90 days presenting to emergency departments with fever.

The procedures carried out with the French data privacy authority (CNIL, Commission nationale de l'informatique et des libertés) do not provide for the transmission of the database, nor do the information and consent documents signed by the patients.

Consultation by the editorial board or interested researchers of individual participant data that underlie the results reported in the article after deidentification may nevertheless be considered, subject to prior determination of the terms and conditions of such consultation and in respect for compliance with the applicable regulations

Beginning 3 months and ending 3 years following article publication. Requests out of these time frame can also be submitted to the sponsor

Researchers who provide a methodologically sound proposal.