What is the appropriate workup for a 1‑year‑old infant with febrile lymphadenitis and a C‑reactive protein of approximately 200 mg/L?

Workup for 1-Year-Old with Febrile Lymphadenitis and CRP 200 mg/L

This 1-year-old requires urinalysis with urine culture (via catheterization), complete blood count with differential, blood culture, and chest radiograph if respiratory symptoms are present; lumbar puncture is not routinely indicated at this age unless the child appears ill or has concerning neurologic findings. 1, 2, 3

Initial Risk Stratification

At 1 year of age, this child falls outside the highest-risk neonatal period (0-28 days) but still requires systematic evaluation for serious bacterial infection (SBI). 1, 3 The markedly elevated CRP of 200 mg/L (20 mg/dL) is highly concerning and significantly increases the probability of bacterial infection. 4, 5, 6

Key Clinical Context

• CRP >20 mg/L has moderate-to-good diagnostic accuracy for SBI, with studies showing pooled sensitivity of 77% and specificity of 79% for serious bacterial infections. 4
• CRP >25 mg/L predicts invasive bacterial infections with an odds ratio of 6.3 in febrile infants. 7
• This patient's CRP of 200 mg/L is exceptionally high, placing them at substantial risk for bacterial infection requiring aggressive workup. 6, 7

Essential Laboratory Tests

Mandatory Initial Workup

Urinalysis and urine culture (catheterized specimen):

• UTI is the most common SBI in this age group, with prevalence of 5-7% in febrile infants younger than 24 months. 2, 3
• Never use bag collection as this dramatically increases false-positive results. 3
• Obtain culture before starting antibiotics to preserve diagnostic accuracy. 2, 3

Complete blood count with differential:

• Elevated WBC count and absolute neutrophil count are associated with greater risk of SBI. 3
• Neutrophil percentage is higher in invasive bacterial infections. 7
• Lower hemoglobin levels have been associated with invasive bacterial infections. 7

Blood culture:

• Must be obtained before initiating antibiotics. 2, 3
• Critical for detecting bacteremia, which occurs in approximately 1.5-2% of febrile children aged 3-36 months. 2

Imaging Considerations

Chest radiograph:

• Indicated if respiratory signs or symptoms are present (tachypnea, cough, retractions, rales, rhonchi, wheezing, grunting). 1, 2
• Consider in this patient given the markedly elevated inflammatory markers (CRP 200 mg/L), as occult pneumonia can occur with CRP >20 mg/dL. 2
• Prevalence of pneumonia in febrile infants is approximately 7% overall, but higher with elevated WBC >20,000/mm³. 1

Lumbar Puncture Decision

Lumbar puncture is NOT routinely required for well-appearing 1-year-old infants with fever. 3 However, consider LP if:

• The child appears ill or toxic
• Neurologic symptoms are present
• The child does not meet low-risk criteria (which this patient does not, given CRP 200 mg/L)
• Bacteremia is confirmed on blood culture 2

The American Academy of Pediatrics reserves mandatory lumbar puncture for infants under 28 days of age. 8, 2 At 1 year, clinical judgment based on appearance and risk factors guides this decision. 3

Lymphadenitis-Specific Considerations

Given the presenting complaint of lymphadenitis with systemic inflammation:

• Evaluate the lymph node characteristics: size, location, tenderness, overlying skin changes
• Consider bacterial lymphadenitis (Staphylococcus aureus, Group A Streptococcus) as a primary source
• Ultrasound of the affected lymph node may help differentiate reactive adenopathy from suppurative lymphadenitis requiring drainage
• If lymph node appears fluctuant or abscess is suspected, surgical consultation for possible incision and drainage with culture

Differential Diagnosis to Exclude

While pursuing bacterial workup, consider:

• Kawasaki disease: Check for conjunctivitis, rash, mucous membrane changes, extremity changes (though CRP 200 mg/L is consistent with both bacterial infection and Kawasaki disease)
• MIS-C (if COVID-19 exposure): Though typically presents with multiorgan involvement and requires SARS-CoV-2 testing 1
• Other viral infections: Though CRP 200 mg/L makes pure viral etiology less likely 4, 5

Critical Pitfalls to Avoid

• Never rely on clinical appearance alone to exclude serious infection, as well-appearing infants can have bacteremia. 8, 3
• Never use urine collection bags instead of catheterization. 3
• Never administer antibiotics before obtaining cultures, as this compromises diagnostic accuracy. 2, 3
• Do not assume normal WBC rules out bacterial infection, especially with CRP this elevated. 2
• Do not miss the opportunity for source control if suppurative lymphadenitis is present—imaging and surgical consultation may be needed

Immediate Management Approach

1. Obtain catheterized urinalysis and urine culture
2. Draw blood for CBC with differential and blood culture
3. Assess for respiratory symptoms/signs; if present, obtain chest radiograph
4. Consider ultrasound of lymph node if suppurative process suspected
5. Initiate empiric antibiotics after cultures obtained if patient appears ill or has concerning vital signs
6. Hospitalize if patient appears ill, has concerning vital signs, or if close outpatient follow-up cannot be ensured 1, 3

The combination of febrile lymphadenitis with CRP 200 mg/L represents high-risk presentation requiring comprehensive bacterial workup and likely empiric antibiotic therapy pending culture results. 4, 5, 6, 7