Updated October 2025

Pediatric Acute Care

Approach to Fever Without an Identifiable Source (FWS) in Children

Most children with FWS have self-limited viral infections, but a small subset—particularly young infants—harbor serious bacterial infections (SBI). A structured approach prioritizes immediate stabilization, age- and risk-based testing for UTI/bacteremia/meningitis, judicious use of biomarkers, and clear safety-netting.

Clinical question

How should clinicians evaluate and manage a child with fever without an identifiable source in the first 7 days of illness?

PediatricsEmergency MedicineInfectious DiseasesPrimary Care

Key points

Define FWS and stratify by age

FWS is acute fever <7 days with no focus after exam. Risk and testing intensity vary markedly by age groups: ≤28 days, 29–60 days, 2–24 months, and >24 months ^[3], ^[4], ^[2].

Identify red flags immediately

Toxic appearance, hemodynamic instability, petechiae/purpura, bulging fontanelle, meningeal signs, focal neurologic deficits, respiratory distress mandate urgent sepsis pathway and empiric antibiotics ^[3], ^[4].

Use targeted testing

Prioritize urinalysis/urine culture for 2–24 months, consider blood tests and cultures based on age/appearance. Lumbar puncture in neonates and when meningitis suspected; not routine in older well-appearing infants ^[3], ^[2].

Leverage biomarkers judiciously

CRP and procalcitonin can refine SBI risk stratification when available, but do not replace clinical assessment; serial exams remain pivotal ^[3].

Safety-net and follow-up

Provide explicit return precautions, reliable follow-up within 24–48 hours if discharging well-appearing children without antibiotics ^[3], ^[4].

Evidence highlights

≥38.0°C (100.4°F) by reliable measurement ^[1]

Fever definition

Neonates ≤28 days—full sepsis workup recommended ^[2]

Highest SBI risk

Predominantly viral; UTI is most common SBI in 2–24 months ^[3]

Common etiologies

Clinical Workflow

Stepwise Evaluation and Management

Ages and appearance drive test selection and disposition. The majority can be safely managed as outpatients with clear safety-netting.

1) Confirm fever and assess severity

Verify temperature ≥38.0°C (method and timing). Immediately assess ABCs and perfusion. Look for toxicity or sepsis indicators: altered mental status, poor perfusion, hypotension, cyanosis, petechiae/purpura, focal neurologic deficits ^[1], ^[4].

2) Focused history and exam

Onset, peak temp, antipyretic response, immunization status, urinary symptoms, GI/respiratory complaints, exposure history. Full exam including ENT, lungs, abdomen, skin, fontanelle, joints, genitourinary; attempt to localize a source while recognizing that most FWS remains viral ^[5], ^[3].

3) Age-stratified testing

Neonates ≤28 days: full sepsis evaluation (blood, urine, CSF cultures; CBC, CRP/Procalcitonin if available; chest radiograph if respiratory signs) and empiric IV antibiotics with admission ^[2]. Infants 29–60 days: if well-appearing and low-risk by exam and biomarkers, limited evaluation may allow outpatient management; otherwise broaden testing. Children 2–24 months: prioritize UTI testing; consider bloodwork/cultures if unimmunized, toxic, or biomarker-elevated. >24 months: testing guided by exam and risk; routine chest radiography not indicated without respiratory findings ^[3], ^[2].

4) Urine testing is high-yield

UTI is the most common SBI in 2–24 months; obtain clean-catch or catheterized urinalysis and culture in at-risk children (especially females <24 months and uncircumcised males) ^[3].

5) Biomarkers to refine risk

CRP and procalcitonin improve SBI risk stratification; elevated values increase likelihood of bacteremia/meningitis, while low values support observation in well-appearing infants. Interpret in context; they do not obviate urine culture in 2–24 months ^[3].

6) Imaging

Chest radiograph only with respiratory signs or persistent high fever with tachypnea; not routine in older infants if exam is normal. Abdominal imaging reserved for focal signs (appendicitis) ^[2].

7) Antimicrobial strategy

Empiric IV antibiotics for neonates and any toxic-appearing child. For well-appearing older infants and children, avoid empiric antibiotics unless a bacterial focus is identified (e.g., positive UA) or risk is high. Treat confirmed UTI per local resistance patterns ^[3], ^[2].

8) Disposition and follow-up

Admit neonates and any child with instability or concern for SBI. Well-appearing children with reassuring evaluation may be discharged with clear return precautions and 24–48 hour follow-up; ensure caregivers understand triggers for return (persistent fever, new focal signs, poor intake, lethargy) ^[3], ^[4].

At-a-Glance

Risk Stratification and Orders

Use these quick checks to standardize care and reduce missed SBIs.

Red Flags (Act Now)

Toxic appearance, hypotension, poor perfusion

Petechiae/purpura, bulging fontanelle, neck stiffness

Focal neurologic deficits, altered mental status

Severe respiratory distress or hypoxia

Age ≤28 days with any fever ^[2]

Core Tests by Age

≤28 days: Blood cultures, CBC, CRP/PCT (if available), UA and catheterized urine culture, lumbar puncture, consider chest radiograph; admit and start IV antibiotics ^[2]

29–60 days: At minimum UA and urine culture; consider CBC, CRP/PCT, blood culture; LP if ill-appearing or biomarkers markedly abnormal ^[3], ^[2]

2–24 months: UA and urine culture strongly considered; targeted bloodwork/cultures if unimmunized/toxic; no routine CXR without respiratory signs ^[3]

>24 months: Testing driven by exam and risk; UA if urinary symptoms or risk factors ^[3]

When to Consider LP in >28 Days

Ill-appearing or neurologic signs

Strongly elevated inflammatory markers with no source

Persistent high fever with irritability/lethargy

Antibiotics before cultures planned

Immunocompromised host ^[3], ^[2]

Antibiotic Triggers

Neonates ≤28 days: empiric IV (e.g., ampicillin + gentamicin/cefotaxime) after cultures ^[2]

Ill-appearing any age: empiric IV broad-spectrum after cultures ^[4]

Positive UA with pyuria/nitrites: start UTI therapy pending culture ^[3]

Radiographic pneumonia or bacterial focus: treat per guidelines ^[3]