Objective To report the performance of clinical practice guidelines (CPG) in the diagnosis of serious/invasive bacterial infections (SBI/IBI) in infants presenting with a fever to emergency care in the UK and Ireland. Two CPGs were from the National Institutes for Health and Care Excellence (NICE guidelines NG51 and NG143) and one was from the British Society for Antimicrobial Chemotherapy (BSAC).

Design Retrospective multicentre cohort study.

Patients Febrile infants aged 90 days or less attending between the 31 August 2018 to 1 September 2019.

Main outcome measures The sensitivity, specificity and predictive values of CPGs in identifying SBI and IBI.

Setting Six paediatric Emergency Departments in the UK/Ireland.

Results 555 participants were included in the analysis. The median age was 53 days (IQR 32 to 70), 447 (81%) underwent blood testing and 421 (76%) received parenteral antibiotics. There were five participants with bacterial meningitis (1%), seven with bacteraemia (1%) and 66 (12%) with urinary tract infections. The NICE NG51 CPG was the most sensitive: 1.00 (95% CI 0.95 to 1.00). This was significantly more sensitive than NICE NG143: 0.91 (95% CI 0.82 to 0.96, p=0.0233) and BSAC: 0.82 (95% 0.72 to 0.90, p=0.0005). NICE NG51 was the least specific 0.0 (95% CI 0.0 to 0.01), and this was significantly lower than the NICE NG143: 0.09 (95% CI 0.07 to 0.12, p<0.0001) and BSAC: 0.14 (95% CI 0.1 to 0.17, p<0.0001).

Conclusion None of the studied CPGs demonstrated ideal performance characteristics. CPGs should be improved to guide initial clinical decision making.

Trial registration number NCT04196192.