What is the workup for osteomyelitis?

Workup of Osteomyelitis

The diagnostic workup for osteomyelitis should begin with blood cultures, inflammatory markers (ESR, CRP), and MRI as the primary imaging modality, followed by bone biopsy for definitive diagnosis when necessary. 1, 2

Initial Clinical Assessment

• Look for specific indicators of osteomyelitis:
  • Ulcer overlying bony prominence (especially if deep or chronic)
  • Exposed bone in wound (positive probe-to-bone test)
  • Ulcer area >2 cm²
  • Erythematous and indurated "sausage toe" appearance
  • Nocturnal pain of moderate intensity
  • Fever (present in up to 45% of patients)

Laboratory Studies

• Obtain two sets of blood cultures (aerobic and anaerobic) 1
• Inflammatory markers:
  • ESR (elevated >27 mm/h suggests infection) 1
  • CRP (elevated >0.93 mg/L suggests infection) 1
  • Consider fibrinogen (cutoff 432 mg/dL) 1
  • Interleukin-6 (higher predictive value than other markers) 1
• Consider specialized tests:
  • Alpha-defensin in synovial fluid (97% sensitivity, 96% specificity) 1
  • Combination of synovial fluid alpha-defensin and CRP (97% sensitivity, 100% specificity) 1

Imaging Algorithm

1. Plain Radiographs (first-line)

   • Low sensitivity (54%) and specificity (68%) for early osteomyelitis 2
   • Look for: loss of bone cortex, focal loss of trabecular pattern, periosteal reaction, bone sclerosis
   • Consider serial radiographs 2-4 weeks later if initial films are negative 2

2. MRI (second-line if diagnosis uncertain after radiographs)

   • Gold standard imaging with >90% sensitivity 2
   • Look for: decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences 1
   • 100% negative predictive value - normal marrow signal reliably excludes infection 1
   • Particularly valuable for evaluating extent of disease and soft tissue involvement

3. Alternative Imaging (if MRI contraindicated)

   • Combination spine gallium/Tc99 bone scan 1
   • CT scan (better for detecting sequestra, foreign bodies, and gas) 1
   • PET scan 1

Definitive Diagnosis

• Bone biopsy with culture and histopathology is the gold standard 2
  • Image-guided or intraoperative aspiration/biopsy of disc space or vertebral endplate 1
  • Submit samples for both microbiologic and pathologic examination
  • Look for inflammatory cells and osteonecrosis on histopathology

Special Considerations

• For vertebral osteomyelitis:

  • Perform PPD test or interferon-γ release assay if tuberculosis is suspected 1
  • Consider fungal blood cultures in at-risk patients 1
  • Brucella serologic tests for patients in endemic areas 1

• For prosthetic joint infection:

  • Joint aspiration with cell count, differential, and culture 1
  • If initial aspiration is negative but infection still suspected, repeat aspiration 1

• For diabetic foot osteomyelitis:

  • Consider wound cultures from tissues closest to bone when ulcer/sinus tract is present 1
  • Avoid wound swab cultures due to contamination risk 1

Common Pitfalls

• Radiographic changes may not appear until 3-6 weeks after symptom onset 1
• Demineralization alone is not specific for osteomyelitis (can be seen in Charcot neuro-osteoarthropathy) 2
• Serologic tests may be difficult to interpret in patients with underlying inflammatory arthropathy 1
• Staphylococcus aureus is the most common causative organism, but Klebsiella pneumoniae and other pathogens should be considered 3
• Prior antibiotic use can lead to false-negative cultures 1

By following this systematic approach, clinicians can efficiently diagnose osteomyelitis and guide appropriate treatment decisions based on the specific pathogen and extent of disease.