In an 8‑month‑old infant with fever of unknown origin and an isolated C‑reactive protein level of 3.8 mg/dL, what is the appropriate management?

Management of 8-Month-Old with Fever and CRP 3.8 mg/dL

This infant requires immediate evaluation for Kawasaki disease and urinary tract infection, with urgent echocardiography if inflammatory markers support incomplete Kawasaki disease.

Immediate Clinical Assessment

Perform a meticulous physical examination specifically targeting the five principal features of Kawasaki disease:

• Conjunctivae: Examine for bilateral non-purulent injection (bulbar, sparing the limbus) 1
• Oral mucosa: Inspect lips for cracking and tongue for "strawberry" appearance 1
• Skin: Look for polymorphous rash, often accentuated in the groin 1
• Extremities: Assess hands and feet for erythema or edema with sharp demarcation at wrists/ankles 1
• Lymph nodes: Palpate neck for cervical lymphadenopathy ≥1.5 cm diameter 1
• General appearance: Evaluate hydration status and overall clinical appearance 1

Critical Diagnostic Consideration: Kawasaki Disease

The CRP of 3.8 mg/dL (38 mg/L) meets the threshold for incomplete Kawasaki disease workup in an infant with prolonged fever. 1

Why Kawasaki Disease is the Priority:

• Incomplete Kawasaki disease is especially common in infants <1 year and carries a higher risk of coronary artery aneurysms if untreated 1
• When fever ≥5 days is accompanied by only 2–3 clinical features, CRP ≥3 mg/dL should trigger full workup including urgent echocardiography 1
• Delaying treatment beyond 10 days of fever onset increases coronary aneurysm risk from approximately 5% to 25% 1

Immediate Laboratory Workup:

Order the following tests immediately: 1

• Erythrocyte sedimentation rate (ESR): Expect ≥40 mm/hr (often >100 mm/hr) in Kawasaki disease 1
• Complete blood count with differential: Look for neutrophilia, lymphopenia, and thrombocytosis 1
• Comprehensive metabolic panel: Including albumin (hypoalbuminemia is common) and liver transaminases 1
• Catheterized urine specimen: For urinalysis and culture (UTI prevalence is 5% in febrile children without source) 2
• Blood culture: Obtain before any antibiotics 1

Urgent Imaging:

Obtain transthoracic echocardiography immediately if ≥2 principal Kawasaki disease features are present, even before completing the full 5-day fever duration 1. Early echocardiography may reveal perivascular brightness, coronary ectasia, lack of tapering, reduced left-ventricular contractility, or pericardial effusion before frank aneurysm formation 1.

Urinary Tract Infection Evaluation

For an 8-month-old infant, UTI risk factors include: 2

• Girls: Age <12 months, white race, temperature ≥39°C, fever ≥2 days, absence of another infection source 2
• Boys: Uncircumcised status, nonblack race, temperature ≥39°C, fever >24 hours 2

Obtain catheterized urine specimen (never bag collection due to high contamination rates) for both urinalysis and culture 2. Do not assume normal urinalysis excludes UTI—obtain culture if clinical risk factors are present 2.

Chest Radiograph Decision

Chest radiograph is NOT indicated unless respiratory symptoms are present (cough, hypoxia, rales, tachypnea disproportionate to fever) 3, 2. Multiple prospective studies show the yield of chest radiography is <3% in infants without respiratory signs 3.

Lumbar Puncture Consideration

Lumbar puncture is NOT routinely indicated at 9 months of age unless specific concerning features are present: toxic or ill appearance, altered mental status, or abnormal neurologic examination 2. At this age, the risk of bacterial meningitis is substantially lower than in neonates 3.

Interpretation of CRP 3.8 mg/dL

This CRP level (38 mg/L) has important clinical implications:

• It is elevated above normal (<10 mg/L) and suggests bacterial infection or inflammatory process 4, 5
• It meets the threshold (≥3 mg/dL) for incomplete Kawasaki disease workup when combined with fever ≥5 days and 2–3 clinical features 1
• In febrile infants, CRP has an area under the ROC curve of 0.79 for predicting serious bacterial infection 4
• However, it is NOT in the "extremely elevated" range (>30 mg/dL) that would strongly suggest bacterial pneumonia, cellulitis, or sepsis 6

Management Algorithm

If ≥2 Kawasaki Disease Features Present:

1. Obtain urgent echocardiogram immediately 1
2. Complete laboratory workup as above 1
3. If diagnosis confirmed (≥4 features or coronary abnormalities on echo):
   • Initiate IVIG 2 g/kg as single infusion 1
   • Start high-dose aspirin 80-100 mg/kg/day divided into four doses 1

If <2 Kawasaki Disease Features Present:

1. Complete laboratory workup including catheterized urine culture 2
2. Re-examine daily for emergence of additional Kawasaki disease features 1
3. Provide antipyretic therapy and ensure adequate hydration 2
4. Reassess within 24-48 hours if fever persists 2

Critical Pitfalls to Avoid

• Do NOT dismiss Kawasaki disease because "no other symptoms" are evident—incomplete presentation is common in infants and can lead to serious coronary complications 1
• Do NOT rely on bag-collected urine specimens—they cannot establish UTI diagnosis reliably due to contamination 2
• Do NOT obtain routine chest radiograph in a well-appearing infant without respiratory symptoms 3, 2
• Do NOT delay echocardiography if inflammatory markers are markedly elevated with unexplained fever, even if fewer than four clinical features are present 1

Safety-Netting Instructions for Caregivers

Advise immediate return if: 1, 2

• New conjunctival redness develops
• Lip cracking or tongue changes appear
• Swelling of hands or feet occurs
• Cervical lymphadenopathy emerges
• Altered mental status, poor perfusion, petechial rash, respiratory distress, or refusal to feed develops