What temperature defines fever in a newborn (<28 days) and what is the recommended immediate management?

Fever Definition in Newborns (<28 Days)

Fever in newborns less than 28 days old is defined as a rectal temperature ≥38.0°C (100.4°F), and these infants require immediate hospitalization with full sepsis evaluation due to a 13% risk of serious bacterial infection. 1

Temperature Thresholds by Specific Age

• Birth to 30 days: Fever is ≥38.0°C (100.4°F) rectal 2
• Normal rectal temperature range: 36.7-37.9°C 1
• Rectal temperature measurement is the gold standard for this age group, providing the closest approximation to core temperature 1

Immediate Management Protocol

All Febrile Neonates (<28 Days) Require:

• Immediate hospitalization with full sepsis evaluation—this is non-negotiable given the 13% serious bacterial infection risk 1

• Complete sepsis workup before any antibiotics, including:

  • Blood culture 3
  • Catheterized urine culture (never bag collection) 3
  • Lumbar puncture with cerebrospinal fluid studies 4
  • Complete blood count with differential 3

• Empiric parenteral antibiotics after cultures obtained—neonates <2 weeks have the highest treatment rates (67%) and highest bacteremia/meningitis risk (4.8%) 5

Critical Clinical Pitfalls

• Recent antipyretic use can mask fever severity and does not rule out serious infection—children may present with normal temperature after acetaminophen but still have severe bacterial infection 1
• Clinical appearance is unreliable: only 58% of infants with bacteremia or bacterial meningitis appear clinically ill 1
• Viral and bacterial infections coexist—positive viral testing does not exclude concurrent bacterial infection 3
• Home thermometer readings may be inaccurate; always document rectal temperature in the clinical setting 3

Age-Specific Risk Context

Infants <2 weeks of age have:

• Highest serious bacterial illness rate: 25% compared to 13% in weeks 3-4 5
• Highest bacteremia/meningitis rate: 4.8% compared to 2.1% in weeks 3-4 5
• Highest treatment intensity: 67% receive parenteral antibiotics compared to 39% in weeks 3-4 5

What Constitutes "Toxic Appearance"

Assess for these specific findings that mandate immediate intervention 3:

• Altered mental status or severe lethargy
• Poor perfusion or signs of shock
• Respiratory distress (tachypnea, retractions, hypoxia)
• Petechial or purpuric rash
• Refusal to feed

The threshold for intervention in neonates is intentionally low because delays in treatment of bacterial meningitis or sepsis increase morbidity and mortality risk. 4