What investigations are appropriate for a child under 5 years with fever and no obvious source of infection?

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Posted: 16 August 2025Updated: 16 August 2025 Guideline-Aligned (High Confidence) Clinically Reviewed
Dr Kola Tytler MBBS CertHE MBA MRCGPClinical Lead • iatroX

In children under 5 years presenting with a fever and no obvious source of infection, appropriate investigations include:

  • Full blood count (FBC)
  • Blood culture
  • C-reactive protein (CRP)
  • Urine testing for urinary tract infection (UTI)

In children with 'red' features, lumbar puncture and chest X-ray should also be considered, and serum electrolytes and blood gas may be indicated based on clinical assessment .

For children with 'amber' features, urine testing, blood tests (FBC, CRP, blood cultures), and lumbar puncture (especially in children under 1 year) are recommended, with chest X-ray if fever exceeds 39°C and WBC >20×10/L .

Children with 'green' features generally do not require routine blood tests or chest X-ray unless indicated by clinical assessment, but urine testing for UTI is advised if no focus of infection is apparent .

Related Knowledge Topics

Educational content only. Always verify information and use clinical judgement.