What is the role of hip Point-of-Care Ultrasound (POCUS) and laboratory tests, such as Complete Blood Count (CBC), Erythrocyte Sedimentation Rate (ESR), and C-Reactive Protein (CRP), in evaluating a limping child?

Hip POCUS and Laboratory Testing in Limping Children: Discharge vs. Further Investigation

Hip ultrasound combined with inflammatory markers (ESR, CRP, WBC) provides a powerful risk stratification tool to safely discharge low-risk limping children while identifying those requiring urgent intervention for septic arthritis. 1, 2, 3

Critical Red Flags Requiring Immediate Hip POCUS and Laboratory Testing

Any child with fever >101.3°F (38.5°C) AND refusal to bear weight requires immediate hip ultrasound to assess for joint effusion, as septic arthritis causes permanent joint damage within hours. 1, 2, 4, 3

The following clinical predictors mandate urgent evaluation:

• Temperature >101.3°F (38.5°C) 1, 3
• Refusal to bear weight or move the joint 1, 2, 4
• ESR >40 mm/hour 3
• WBC >12,000/mm³ 3
• CRP >20 mg/L 3

Algorithmic Approach Using POCUS and Laboratory Tests

Step 1: Initial Risk Stratification with Clinical Assessment + Labs

Obtain CBC, ESR, and CRP on all limping children with fever or inability to localize symptoms. 3, 5

• If fever present OR elevated inflammatory markers → proceed immediately to hip ultrasound 1, 2
• If afebrile with normal exam and improving symptoms → plain radiographs only, observation with NSAIDs 1, 4

Step 2: Hip POCUS Interpretation

Hip ultrasound should be performed immediately if symptoms localize to the hip or if infection is suspected. 1, 2

• If joint effusion present → ultrasound-guided aspiration for cell count, Gram stain, and culture 1, 2
• If no effusion with normal labs → consider transient synovitis, provide reassurance and NSAIDs 1
• If no effusion but elevated inflammatory markers → consider MRI to evaluate for osteomyelitis 1, 2, 4

Step 3: Safe Discharge Criteria

Children can be safely discharged home if ALL of the following are met:

• Afebrile (<101.3°F) 1, 3
• Able to bear weight 1, 4
• Normal or mildly elevated inflammatory markers (ESR <40, WBC <12,000, CRP <20) 3
• No hip effusion on ultrasound (if performed) 1, 2
• Normal or improving clinical examination 1, 4

Discharge management includes:

• Short-course NSAIDs for analgesia 1, 4
• Follow-up in 1-2 weeks 4
• Return precautions for fever, worsening pain, or inability to bear weight 1

Step 4: Further Testing Required

Obtain MRI if normal radiographs and ultrasound but persistent symptoms with elevated inflammatory markers, as osteomyelitis may be present. 1, 2, 4

• MRI with and without IV contrast is the gold standard for diagnosing osteomyelitis 1, 2
• Follow-up radiographs in 7-10 days if initial films normal but symptoms persist, as 10% of fractures only appear on repeat imaging 2, 4

Critical Pitfalls to Avoid

Never assume normal hip ultrasound excludes all pathology—osteomyelitis requires MRI for diagnosis and may present with normal joint ultrasound. 1, 2, 4

Never delay evaluation when fever is combined with inability to bear weight, even if inflammatory markers are pending—septic arthritis is a clinical diagnosis requiring urgent intervention. 1, 2, 4, 3

Never focus solely on the hip when symptoms are present—up to 30% of hip pathology presents as referred thigh or knee pain in young children. 2, 4

Never discharge a child with persistent symptoms and normal initial radiographs without arranging close follow-up and repeat imaging, as occult fractures appear on follow-up films in 10% of cases. 2, 4

Practical Integration of POCUS and Laboratory Tests

The combination of Kocher criteria (fever >101.3°F, non-weight bearing, ESR >40, WBC >12,000) with hip ultrasound provides the highest diagnostic accuracy for septic arthritis. 3

• 0-1 Kocher criteria + no effusion on ultrasound → discharge with close follow-up 3
• 2-3 Kocher criteria OR effusion on ultrasound → joint aspiration required 3
• 4 Kocher criteria → presumptive septic arthritis, immediate orthopedic consultation and aspiration 3

Hip POCUS is operator-dependent and requires adequate training—the European Society of Paediatric and Neonatal Intensive Care recommends specific competency standards before independent use. 6