A fever in an infant under 60 days is concerning. Young babies can't wall off infection like older kids, so bacteremia can silently become meningitis. You can't reassure parents and go home based on how the kid looks. You need a system.
How to Work Them Up
Start simple. Check rectal temp (fever = ≥38.0°C), know the vaccines they got, look at them. Well-appearing kids have normal alertness, good color, normal cry, respond to interaction. Ill-appearing kids look sick, lethargy, poor perfusion, weird cry, that feeling you get.
The Labs
Get CBC, CRP, procalcitonin, and urinalysis. These thresholds matter:
- WBC >15,000 or <5,000: Could be infection
- Left shift (bands high) or I/T ratio >0.2: Suggests acute infection, but low specificity (lots of viral infections do this)
- CRP >10 mg/L: Some inflammation, but bacterial and viral both cause it
- Procalcitonin <0.5 ng/mL in a well-appearing kid: Pretty reassuring
- Procalcitonin >2 ng/mL: Real risk of bacterial infection
Plug them into the Febrile Infant Markers calculator to see how they integrate.
Check the Urine
UTIs are in 5–10% of febrile infants under 90 days, the most common serious infection in this age group. Get a catheterized specimen (bag urine is garbage). Look for positive leukocyte esterase, positive nitrites, pyuria (>5 WBC/hpf), or bacteriuria.
Use UTICalc to estimate probability. Urine culture takes 24–48 hours, so if you're treating, start antibiotics and adjust when culture comes back.
Meningitis
Infants don't have typical meningitis signs. A fever, poor feeding, and irritability? Could be meningitis. Neck stiffness? You don't see that in babies.
Risk factors: age <30 days, ill appearance, procalcitonin high. If any of these fit, do a lumbar puncture, CSF pleocytosis, positive gram stain, seizures confirm it.
Empiric coverage depends on age. For neonates ≤28 days, use ampicillin plus gentamicin (ceftriaxone is avoided in this age group due to calcium-ceftriaxone precipitation risk and inadequate Listeria/Enterococcus coverage). For infants 29–60 days, use ceftriaxone plus ampicillin (ampicillin covers Listeria). Start antibiotics immediately when meningitis is on your differential.
Use Meningitis Score to guide LP decisions.
The Sepsis Score
Maternal risk factors (fever, prolonged rupture of membranes >18 hours, chorioamnionitis, GBS colonization) and baby factors (premature, low birth weight) go into the sepsis calculation.
Use Neonatal Sepsis Risk calculator.
What to Do
Very low-risk (well-appearing, normal labs, normal urine, low risk scores, can follow up reliably): Send home with no antibiotics. Call same-day, mandatory re-check in 24 hours.
Lower-risk: Oral antibiotics (cephalexin or amoxicillin-clavulanate), observe as outpatient, cultures pending.
Higher-risk (ill-appearing, abnormal labs, high risk scores): Admit, IV antibiotics, lumbar puncture, full workup.
Critical
If you send a baby home, the parents need to know what to watch for, when to come back, and what the cultures showed. Same-day phone call is mandatory. 24-hour re-evaluation is not optional. Don't give antibiotics to truly low-risk kids just to hedge; that kid's parents will never bring them back in, and you've created antibiotic resistance.
Know how to risk-stratify. Use the calculators. Decide on antibiotics and admission based on risk, not just labs. Follow up ruthlessly.