4.1 Fever Without a Source & Sepsis Evaluation

Why Fever Without a Source Matters

Fever without a source (FWS) is fever (temperature >=38.0C / 100.4F, measured rectally in infants) in a child whose history and physical examination do not reveal a focus of infection after a careful evaluation. FWS is one of the highest-yield topics on the ABHS Pediatrics Part 1 written exam because the correct action changes dramatically with age -- the same fever that triggers a full septic workup and empiric antibiotics in a 10-day-old neonate may be observed safely at home in a fully immunized, well-appearing 18-month-old.

The core exam concept is age-stratified risk: younger infants have immature humoral and cellular immunity, cannot mount reliable clinical signs of serious illness, and are exposed to different pathogens (particularly perinatally-acquired organisms) than older children. As age increases, the prevalence of invasive bacterial infection (IBI) falls and the reliability of clinical appearance as a screening tool rises.

The Three Classic Age Bands

Neonates (0-28 Days): No Shortcuts

In a neonate, fever without a source is treated as sepsis until proven otherwise, regardless of how well the infant looks. This is a repeated exam trap: examiners will describe a neonate who 'looks well' and 'is feeding normally' with a documented fever, and the correct answer is still a full septic evaluation: blood culture, catheterized urinalysis and urine culture, and cerebrospinal fluid (CSF) analysis (cell count, glucose, protein, Gram stain, culture; HSV PCR if risk factors such as maternal lesions, seizures, or vesicles are present). Empiric parenteral antibiotics (typically ampicillin plus gentamicin, or ampicillin plus a third-generation cephalosporin when meningitis is a concern) are started immediately after cultures are drawn, and the infant is admitted for observation. Listeria monocytogenes is intrinsically resistant to cephalosporins, which is why ampicillin is retained in the empiric regimen for this age group specifically.

29-90 Days: Where Risk-Stratification Begins

Once an infant passes the neonatal period, well-validated risk-stratification tools become usable because the immune system is more mature and inflammatory markers become more discriminating. Two generations of criteria are worth knowing:

• Classic criteria (Rochester, Boston, Philadelphia) -- historically used a low white blood cell count (roughly 5,000-15,000/mm3), a low band-to-total neutrophil ratio, a normal urinalysis, and (Boston/Philadelphia) a normal CSF cell count and chest radiograph to identify infants who could be safely managed without antibiotics or with outpatient follow-up.
• Newer inflammatory-marker-based approach (reflected in the 2021 American Academy of Pediatrics guideline for well-appearing infants 8-60 days old) -- stratifies risk using procalcitonin, absolute neutrophil count (ANC), and C-reactive protein (CRP) rather than the white count alone, because procalcitonin rises and falls faster and correlates better with invasive bacterial infection.

The exam-relevant takeaway is the concept, not a specific cutoff table: infants who are well-appearing, have reassuring inflammatory markers, and have a normal urinalysis are lower risk and may be candidates for close outpatient follow-up without an LP; infants with abnormal markers, an ill appearance, or age closer to the neonatal boundary still require CSF evaluation and admission. Urinalysis and blood/urine culture are obtained in essentially all infants in this band -- urinary tract infection remains the single most common serious bacterial infection at this age.

Older Infants and Toddlers (3-36 Months)

Beyond three months, the pretest probability of occult bacteremia has fallen sharply, largely due to widespread conjugate vaccination against Haemophilus influenzae type b (Hib) and Streptococcus pneumoniae (PCV13/PCV15). In a fully immunized, well-appearing child in this age range, the evaluation focuses on identifying a source (ears, throat, skin, lungs) and screening for UTI, which is now the leading occult bacterial diagnosis. A urinalysis is indicated for girls under 24 months and uncircumcised boys under 12 months (with age thresholds varying slightly by local protocol) who lack another explanation for fever. Blood culture and empiric antibiotics are reserved for children who are unimmunized or incompletely immunized, ill-appearing, or otherwise high-risk; lumbar puncture is reserved for infants with meningeal signs, a bulging fontanelle, or a toxic appearance.

Sepsis Evaluation: Escalation Beyond Source-Hunting

When a febrile infant or child shows signs of sepsis -- toxic appearance, poor perfusion, tachycardia or tachypnea out of proportion to fever, hypotension, or altered mental status -- management shifts from age-band risk stratification to immediate stabilization and broad evaluation:

• Resuscitate first: secure the airway if needed; establish IV access; give an isotonic fluid bolus (10-20 mL/kg) for suspected hypovolemia or shock, repeating while monitoring for fluid overload
• Obtain cultures before antibiotics when possible, but do not delay antibiotics in a critically ill child waiting on lumbar puncture or transfer
• Empiric broad-spectrum IV antibiotics appropriate to age and local epidemiology (e.g., ampicillin plus a third-generation cephalosporin or gentamicin in young infants; ceftriaxone or similar in older children pending culture data)
• Lumbar puncture when meningitis is suspected and the child is stable enough to tolerate the procedure; if LP must be deferred because of instability, start antibiotics and obtain CSF as soon as it is safe

The exam tests whether you recognize that appearance and perfusion, not fever height alone, determine whether a child needs a sepsis-style workup versus outpatient observation.

Red Flags That Override Any Age-Based Algorithm

Regardless of the age band, certain findings should immediately raise concern for serious bacterial infection (SBI) or sepsis and push toward the most aggressive evaluation and empiric treatment:

• Toxic/ill appearance -- lethargy, poor perfusion, weak or absent eye contact, abnormal cry, marked tachycardia or tachypnea out of proportion to fever
• Petechiae or purpura, especially with fever -- raises concern for meningococcemia
• Bulging fontanelle, neck stiffness, or seizure -- suggests meningitis
• Poor feeding, decreased urine output, or inconsolable irritability
• Temperature >=40.0C or hypothermia (<36.0C) in a young infant
• Prolonged capillary refill (>2-3 seconds) or mottled skin

An exam vignette that pairs a young infant's fever with any of these findings should be answered with immediate full evaluation and empiric antibiotics, even if the child is outside the neonatal window.