In a child under five years old with fever for five days and at least four clinical criteria of Kawasaki disease, which baseline laboratory panel and imaging studies should be obtained to confirm inflammation, assess complications, and guide treatment?

Baseline Laboratory and Imaging Evaluation for Kawasaki Disease

In a child under five with fever for five days and at least four clinical criteria of Kawasaki disease, obtain a comprehensive inflammatory panel including CBC with differential, ESR, CRP, albumin, ALT, urinalysis, and an echocardiogram to confirm systemic inflammation, assess for coronary artery involvement, and guide treatment decisions. 1

Essential Laboratory Studies to Confirm Inflammation

The American Heart Association guidelines specify supplemental laboratory criteria that support the diagnosis and assess disease severity 1:

Inflammatory Markers

• ESR and CRP: Both should be obtained to document ongoing inflammation and guide treatment decisions, particularly for children beyond day 10 of fever 1
• Elevated inflammatory markers help distinguish Kawasaki disease from other febrile illnesses 2

Complete Blood Count with Differential

• White blood cell count: Leukocytosis >15,000/mm³ supports the diagnosis 1
• Hemoglobin/Hematocrit: Anemia for age is a common finding 1
• Platelet count: Thrombocytosis after day 7 (>450,000/mm³) is characteristic, though initial counts may be normal 1
• Neutrophil and band counts: Elevated counts help predict coronary artery lesion risk 1

Hepatic and Metabolic Assessment

• Albumin: Hypoalbuminemia (≤3.0 g/dL) is a key supplemental criterion 1
• ALT (alanine aminotransferase): Elevation indicates hepatic involvement 1

Urinalysis

• Sterile pyuria: >10 white blood cells per high-power field without bacterial growth is characteristic 1
• This finding is often mistaken for partially treated urinary tract infection, representing a common diagnostic pitfall 1

Critical Imaging Study

Echocardiography

An echocardiogram must be obtained in all suspected Kawasaki disease cases to assess for cardiac complications and guide treatment urgency 1, 2:

• Coronary artery assessment: Measure z-scores of left anterior descending (LAD) and right coronary artery (RCA); z-score ≥2.5 is considered positive 1
• Early vasculitis signs: Perivascular brightness, lack of tapering, and coronary ectasia may precede aneurysm formation 1
• Cardiac function: Assess for decreased left ventricular function, which indicates myocarditis 1
• Valvular involvement: Evaluate for mitral regurgitation or other valvular abnormalities 1
• Pericardial effusion: Document if present as part of acute inflammatory process 1

The echocardiogram is considered positive if any of three conditions are met: z-score of LAD or RCA ≥2.5, coronary arteries meet Japanese Ministry of Health criteria for aneurysms, or ≥3 suggestive features exist 1

Clinical Context and Timing Considerations

When Laboratory Testing is Especially Critical

For infants ≤6 months with fever ≥7 days without other explanation, laboratory testing and echocardiography should be performed even without clinical criteria, as this age group has the highest risk for coronary abnormalities yet may present with minimal clinical features 1, 2

Treatment Can Proceed Before Complete Evaluation

Treatment with IVIG and aspirin can be initiated before performing the echocardiogram if clinical suspicion is high, as early treatment (within 10 days of fever onset) significantly reduces coronary artery aneurysm risk from 15-25% to much lower rates 1, 3, 4

Common Pitfalls to Avoid

• Don't dismiss sterile pyuria as treated UTI: This is a characteristic finding in Kawasaki disease, not evidence of bacterial infection 1
• Don't wait for all clinical criteria simultaneously: Features may appear sequentially over days; careful history-taking about prior symptoms is essential 3, 2
• Don't overlook incomplete presentations: Children with only 2-3 clinical features but laboratory evidence of inflammation warrant full evaluation including echocardiography 1, 2
• Don't attribute rash to antibiotic reaction: Children initially treated for bacterial lymphadenitis who develop rash and mucosal changes likely have Kawasaki disease 1