What diagnostic exams should be ordered for suspected osteomyelitis?

Diagnostic Exams for Suspected Osteomyelitis

For suspected osteomyelitis, initial plain radiographs should be obtained first, followed by MRI as the gold standard imaging modality with >90% sensitivity and specificity, along with blood cultures and inflammatory markers (ESR, CRP) to guide diagnosis and treatment. 1, 2

Initial Diagnostic Approach

Laboratory Tests

• Blood cultures: Obtain two sets (aerobic and anaerobic) 2
• Inflammatory markers:
  • ESR (elevated >27 mm/h)
  • CRP (elevated >0.93 mg/L)
  • Fibrinogen (cutoff 432 mg/dL)
  • Interleukin-6 (higher predictive value than other markers) 2

Imaging Algorithm

1. Plain Radiographs (First-line)

   • Always start with radiographs of the affected area 1
   • Provides anatomic overview and can detect:
     • Bone erosions
     • Periosteal reaction
     • Soft tissue swelling
     • Bone sclerosis (in chronic osteomyelitis)
   • Caveat: Early osteomyelitis (<14 days) may show normal radiographs or only mild soft tissue swelling 1
   • Low sensitivity (54%) and specificity (68%) for early disease 2

2. MRI (Second-line, gold standard)

   • Highest accuracy for osteomyelitis detection 1, 2
   • Protocol: MRI without and with IV contrast is preferred 1
   • Look for:
     • Decreased T1-weighted bone marrow signal
     • Increased signal on fluid-sensitive sequences
     • Soft tissue involvement
     • Abscess formation
     • Fistulas 1, 2
   • Provides excellent evaluation of disease extent for surgical planning 1

3. Alternative Imaging (If MRI contraindicated)

   • CT with IV contrast:

     • Useful for detecting:
       • Sequestrum (dead bone fragment)
       • Bone erosions
       • Soft tissue gas
       • Foreign bodies 1
     • Less sensitive than MRI for acute osteomyelitis 1

   • Nuclear Medicine Studies:

     • Three-phase bone scan:
       • High sensitivity (83%) but low specificity (45%) 1
       • Can rule out osteomyelitis if negative 1
     • Labeled leukocyte scan + Sulfur colloid marrow scan:
       • Increases specificity for infection diagnosis
       • Particularly useful with recent surgery/fracture 1
     • FDG-PET/CT:
       • High sensitivity (81-100%) and specificity (87-100%)
       • Useful for multifocal infection detection 1

   • Ultrasound:

     • Limited value for osteomyelitis diagnosis
     • Useful for detecting:
       • Fluid collections
       • Periosteal involvement
       • Soft tissue abnormalities
       • Guiding aspiration/biopsy 1, 3

Definitive Diagnosis

• Bone biopsy with culture and histopathology:
  • Gold standard for definitive diagnosis 2
  • Should be performed with image guidance
  • Submit samples for both microbiologic and pathologic examination 2

Special Considerations

Hardware/Prosthetic-Associated Infection

• Use MRI with metal artifact reduction sequences when possible 1
• Consider nuclear medicine studies if MRI limited by hardware artifact 1
• Joint aspiration with cell count, differential, and culture should be performed 2

Chronic Osteomyelitis

• CT may be more useful than in acute cases for detecting sequestrum 1
• MRI remains highly accurate but may show different patterns 4

Draining Sinus

• High clinical suspicion for chronic infection 1
• MRI and CT both sensitive for diagnosis 1

Common Pitfalls

1. Relying solely on radiographs for early diagnosis (changes may not appear until 3-6 weeks after symptom onset) 2

2. Misinterpreting bone scan results in patients with recent trauma, surgery, or arthritis (false positives) 1

3. Failing to obtain appropriate cultures before antibiotic administration (leads to false-negative results) 2

4. Not distinguishing between neuropathic arthropathy and osteomyelitis in diabetic patients:

   • Osteomyelitis typically involves a single bone beneath an ulcer
   • Neuropathic arthropathy often involves multiple midfoot bones 4

5. Overlooking the need for definitive diagnosis through bone biopsy when imaging results are equivocal 2