A 13‑month‑old infant refusing to bear weight, with a normal hip X‑ray, low C‑reactive protein (0.6 mg/dL) and mild hypocapnia (pCO₂ 20 mm Hg) – what are the likely serious bacterial infections and what immediate diagnostic and management steps are recommended?

Differential Diagnosis and Immediate Management

In a 13-month-old refusing to bear weight with normal hip X-ray and CRP 0.6 mg/dL, the most likely serious bacterial infection is septic arthritis of the hip, which can present with normal initial inflammatory markers and radiographs—immediate ultrasound of the hip and arthrocentesis are mandatory to exclude this orthopedic emergency. 1

Critical Assessment Framework

Septic Arthritis Remains the Primary Concern

• Septic arthritis is an orthopedic emergency that can rapidly destroy cartilage, and normal CRP does not exclude this diagnosis. 1
• The Kocher criteria (fever >38.5°C, ESR ≥40 mm/hr, WBC ≥12,000/mm³, inability to bear weight) are widely used, but CRP >2.0 mg/dL has been described as an accurate predictor—however, your patient's CRP of 0.6 mg/dL does NOT rule out septic arthritis, especially if presenting within the first 12-24 hours of symptom onset. 1, 2
• Low CRP levels do not exclude bacterial infection in children; initial CRP may be negative even in severe bacterial infections or within 12 hours of onset. 2
• Serial CRP measurements obtained 24 hours apart are far more sensitive than a single initial value—two CRP levels <1 mg/dL obtained 8-48 hours after presentation indicate bacterial infection is unlikely. 3

Immediate Diagnostic Steps

Perform hip ultrasound immediately to detect joint effusion, which X-ray cannot reliably identify. 1

• Ultrasound can confirm the presence of fluid in the joint space and guide arthrocentesis. 1
• If ultrasound demonstrates effusion, proceed directly to arthrocentesis for Gram stain, culture (including anaerobic bottle), cell count with differential, and glucose/protein analysis. 1
• Diagnosis of septic arthritis is made by arthrocentesis—clinical suspicion is paramount because imaging and laboratory markers can be falsely reassuring early in the disease course. 1

Additional Laboratory Evaluation

• Obtain blood cultures (including anaerobic bottle) before initiating antibiotics. 1
• Complete blood count with manual differential is essential—automated counters may miss abnormal cells, and WBC ≥12,000/mm³ is one component of the Kocher criteria. 1, 4
• Repeat CRP in 24 hours—a second CRP level has 92.9% sensitivity for proven or probable sepsis in early-onset infections, compared to only 39.4% for the initial value. 3
• ESR should be obtained if not already done, as ESR ≥40 mm/hr is part of the Kocher criteria. 1

Alternative Diagnoses to Consider

Osteomyelitis

• Osteomyelitis can present with refusal to bear weight and normal initial X-rays, as radiographic changes typically lag clinical symptoms by 7-14 days. 1
• In children <2 years, septic arthritis is more common than osteomyelitis (P = 0.0003), but osteomyelitis can spread to adjacent joints in infants due to transphyseal vessels. 1
• If hip ultrasound is negative for effusion, obtain MRI of the hip and proximal femur to evaluate for osteomyelitis, subperiosteal abscess, or bone marrow edema. 1

Transient Synovitis

• Transient synovitis is the most common cause of hip pain in children and is managed expectantly, but it is a diagnosis of exclusion that can only be made after septic arthritis has been ruled out by arthrocentesis. 1
• Meeting fewer Kocher criteria reduces the likelihood of septic arthritis, but does not eliminate it—clinical judgment must prevail. 1

Occult Bacteremia or Pneumonia

• The hypocapnia (pCO₂ 20 mmHg, normal low 21 mmHg) suggests mild hyperventilation, which could indicate early compensated metabolic acidosis from sepsis or pain-related tachypnea. 5
• Obtain pulse oximetry immediately—SpO₂ <92% mandates hospital admission and chest radiography to evaluate for pneumonia. 1, 6, 5
• If respiratory rate is elevated for age (≥50 breaths/min in a 13-month-old), obtain posteroanterior and lateral chest X-ray to evaluate for pneumonia. 1, 5
• However, in a well-appearing child without respiratory distress, routine chest radiography is not indicated. 1, 4

Management Algorithm

If Hip Ultrasound Shows Effusion

1. Proceed immediately to arthrocentesis with orthopedic surgery consultation. 1
2. Send synovial fluid for Gram stain, aerobic and anaerobic cultures, cell count with differential, glucose, and protein. 1
3. Initiate empiric IV antibiotics after cultures are obtained:
   • For a 13-month-old, the most common pathogens are Staphylococcus aureus and Kingella kingae (<4 years). 1
   • Start vancomycin 40-60 mg/kg/day IV every 6-8 hours PLUS ceftriaxone 50-100 mg/kg/day IV every 12-24 hours to cover MRSA and gram-negative organisms. 6
4. Arrange for arthrotomy, irrigation, and debridement—typical management of septic arthritis includes antibiotics plus surgical drainage. 1

If Hip Ultrasound Is Negative

1. Obtain MRI of the hip and proximal femur to evaluate for osteomyelitis or subperiosteal abscess. 1
2. Repeat CRP and ESR in 24 hours—serial measurements are far more sensitive than initial values. 3, 2
3. If MRI shows osteomyelitis, initiate IV antibiotics (vancomycin plus ceftriaxone) and consult orthopedic surgery for possible debridement if subperiosteal collection or necrosis is present. 1, 6
4. If MRI is negative and repeat CRP remains <1 mg/dL at 24 hours, bacterial infection is unlikely (negative predictive value 99.7%). 3

Common Pitfalls to Avoid

• Do not rely on a single normal CRP to exclude septic arthritis or osteomyelitis—initial CRP can be negative even in severe bacterial infections, especially within 12 hours of onset. 2
• Do not assume normal X-rays exclude osteomyelitis—radiographic changes lag clinical symptoms by 7-14 days. 1
• Do not delay arthrocentesis if ultrasound shows effusion—septic arthritis is an orthopedic emergency that requires immediate drainage. 1
• Do not attribute refusal to bear weight to recent viral illness without excluding bacterial infection first—many pediatric infections present without elevated inflammatory markers. 4, 2
• Do not use CRP alone to differentiate viral from bacterial infection—CRP >40 mg/L suggests bacterial infection, but values <40 mg/L do not rule it out. 7