What signs indicate infectious etiology in a child with a painful hip?

Signs Indicating Infectious Etiology in a Child with Painful Hip

All four signs listed—ESR >40 mm/hr, fever, non-weight-bearing status, and WBC >12,000—are validated predictors of septic arthritis in children with hip pain, collectively known as the Kocher criteria. 1

Key Clinical Predictors of Septic Arthritis

The following signs suggest an infectious etiology rather than benign conditions like transient synovitis:

Laboratory Markers

• ESR ≥40 mm/hr is a validated predictor of septic arthritis 1
• WBC count ≥12,000 cells/mm³ is part of the established diagnostic criteria 1
• C-reactive protein >2.0 mg/dL is an accurate predictor of septic arthritis 1
• ESR >36 mm/hr and CRP >60 mg/L are specifically associated with osteomyelitis or septic arthritis 2

Clinical Signs

• Fever >101.3°F (38.5°C) strongly suggests infection 1
• Inability to bear weight on the affected limb is significantly associated with bacterial infection including osteomyelitis, septic arthritis, or intramuscular abscess (p=0.016) 2
• The presence of fever, elevated WBC, elevated ESR, or elevated CRP collectively suggests infection 3

Diagnostic Algorithm

Initial Assessment

• Fever is the main distinguishing factor between harmless conditions and life-threatening septic arthritis 4
• Poor feeding and restlessness predict serious disease with sensitivity of 78% and 76%, respectively 5
• The combination of all four Kocher criteria (fever, non-weight-bearing, ESR ≥40, WBC ≥12,000) has high predictive value 1

Imaging Approach

• Ultrasound of the hip should be performed immediately to detect joint effusion, which can guide aspiration 3, 1
• Plain radiographs have low sensitivity for septic arthritis but should be obtained to rule out other pathology 3, 1
• MRI with decreased femoral head enhancement on early post-contrast imaging is a reliable feature of septic arthritis not seen in transient synovitis 1

Critical Clinical Pitfalls

Time-Sensitive Nature

• Septic arthritis is an orthopedic emergency because bacterial proliferation causes rapid cartilage damage through direct toxicity and inflammatory response 6, 7
• False-negative ultrasound can occur if performed within 24 hours of symptom onset 3

Associated Conditions

• In children with signs of infection but absence of hip effusion, consider pelvic osteomyelitis or pyomyositis 3
• MRI detected osteomyelitis in approximately 50% of children with clinically suspected septic arthritis, and septic arthritis was associated with osteomyelitis in about 70% of patients 3
• Concomitant joint and bone infections occur in >50% of pediatric cases 6

Age-Specific Considerations

• Group B streptococcus predominates in neonates 6
• Kingella kingae is common in children under 4 years of age 6
• In neonates and infants, septic arthritis may occur secondary to spread of osteomyelitis into the adjacent joint 6