Fever Without Source in Young Infants
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Direct answer
Fever without source in a young infant is a rectal temperature of 38 degrees Celsius or more with no localising focus, where bacteraemia, urinary tract infection or meningitis must be excluded. Infants under about 28 days carry the highest risk and need full sepsis evaluation with lumbar puncture. Rochester criteria help decide whether a well-looking older infant can be managed as low risk.
What you must remember
- Definition: a rectal temperature of 38 degrees Celsius or more in an infant with no focus found on examination.
- Serious causes: urinary tract infection, bacteraemia, occult pneumonia, meningitis and enteric infection, with herpes simplex virus considered in neonates.
- Rochester criteria: a term, previously well, well-appearing infant up to 60 days old with a white cell count of 5,000 to 15,000 per microlitre, band count below 1,500, normal urinalysis and normal chest film where taken.
- Boston criteria: a similar low-risk approach for older young infants, about 28 to 89 days, which uses lumbar puncture and a white cell count below 20,000 per microlitre; cut-offs differ between sources.
- Lumbar puncture: done in neonates and in infants with toxicity or abnormal markers, because meningitis can be silent in young infants.
- Empirical antibiotics: an IV third-generation cephalosporin, with ampicillin added for Listeria cover in young infants and aciclovir when herpes is suspected, per unit protocol.
- Urine sampling: a catheter or suprapubic specimen for culture, since a dipstick alone does not establish or exclude urinary infection.
A stepwise approach to a febrile infant
A 20-day-old term baby has a temperature of 38.4 degrees Celsius, feeds poorly and has no obvious focus on examination. The steps below apply the age-based pathway.
- Define the age band. Neonates up to 28 days receive a full sepsis work-up and empirical IV antibiotics regardless of how well they look.
- Assess appearance and risk. A well-looking term infant may be low risk, whereas toxicity, poor feeding or respiratory distress makes the infant high risk.
- Run the bloods and urine. Take a full blood count, CRP, blood culture, and a catheter urine sample for culture and urinalysis, with CRP useful but not decisive.
- Decide on lumbar puncture. Perform it in neonates and in infants with toxicity or abnormal markers, provided the clinical state and platelets allow.
- Treat and review. Give IV cephalosporin with ampicillin where Listeria cover is needed, add aciclovir if herpes is suspected, and narrow the regimen when cultures return.
Where the exam tries to catch you
Three traps recur on this topic.
- Trap: labelling a well-looking neonate as low risk Correct: neonates under 28 days always need a full work-up, because a well appearance is unreliable at this age.
- Trap: trusting a negative urine dipstick Correct: a urine culture from a catheter or suprapubic sample is needed before urinary infection is excluded.
- Trap: treating the Rochester criteria as a diagnostic test Correct: they stratify risk in a well-looking infant and do not exclude sepsis when the infant looks unwell.
Frequently asked questions
What are the Rochester criteria for fever without source?
The Rochester criteria identify low-risk fever without source in well, previously healthy term infants up to 60 days old. They require a white cell count of 5,000 to 15,000 per microlitre, a band count below 1,500, a normal urinalysis and a normal chest film where taken.
When is lumbar puncture indicated in fever without source in infants?
Lumbar puncture in fever without source in infants is indicated in neonates under 28 days and in infants with toxicity or abnormal markers. It checks for meningitis, which can be silent in young infants.
Which empirical antibiotics are used for fever without source in a young infant?
Empirical antibiotics for fever without source in a young infant usually include an IV third-generation cephalosporin, with ampicillin added for Listeria cover and aciclovir when herpes is suspected. Exact regimens follow unit protocol and local resistance patterns.
Can a febrile infant without source be sent home?
A febrile infant without source can be discharged only if risk stratification classes the infant as low risk and reliable follow-up is arranged. Infants under 28 days are generally admitted for full evaluation and treatment.