Objective To analyse the impact of blood enterovirus and human parechovirus PCR (ev-PCR) testing in young infants with fever without a source (FWS).

Design Observational study, subanalysis of a prospective registry.

Setting Paediatric emergency department.

Patients Infants ≤90 days of age with FWS seen between September 2015 and August 2019 with blood ev-PCR, blood and urine cultures and urine dipstick test performed.

Main outcome measures Prevalence of invasive bacterial infection (IBI: bacterial pathogen in blood or cerebrospinal fluid) in infants with positive or negative ev-PCR test results. Secondarily, we also compared length of stay and antibiotic treatment in hospitalised infants.

Results Of 703 infants, 174 (24.7%) had a positive blood ev-PCR and none of them were diagnosed with an IBI (vs 2.6% (95% CI 1.3% to 4.0%) of those with a negative result, p=0.02). Prevalence of non-IBI (mainly urinary tract infection) was also lower among infants with a positive blood ev-PCR (2.3% (95% CI 0.1% to 4.5%) vs 17.6% (95% CI 14.3% to 20.8%), p<0.01).

Overall, 258 infants were hospitalised (36.6%) and 193 (74.8%) of them received antibiotics. Length of hospital stay and antibiotic treatment were shorter in those with a positive blood ev-PCR (median: 3 days vs 5 days and 1 day vs 5 days, respectively; p<0.01). Differences remained statistically significant among well-appearing infants >21 days old with normal urine dipstick.

Conclusion Blood ev-PCR identifies a group of infants under 90 days of age with FWS at very low risk of IBI. This test may help to guide clinical decision making in young febrile infants.