What is the diagnostic approach for osteomyelitis?

Diagnostic Approach to Osteomyelitis

The gold standard for diagnosing osteomyelitis is bone biopsy with histopathological examination showing inflammatory cells and osteonecrosis, along with isolation of bacteria from reliably obtained bone samples. 1, 2

Clinical Assessment

• Suspect osteomyelitis when an ulcer lies over a bony prominence, particularly when it fails to heal despite adequate treatment, or when a toe is erythematous and indurated ("sausage toe") 1
• An ulcer area larger than 2 cm² significantly increases the likelihood of underlying osteomyelitis (positive likelihood ratio 7.2) 1, 3
• The probe-to-bone test, where a sterile blunt metal probe gently inserted through a wound strikes bone (detected by its hard, gritty feel), is a useful clinical tool with a positive likelihood ratio of 7.2 when the prevalence of bone infection is high (>60%) 1, 3
• Clinical examination findings alone have low sensitivity (22-33%) for diagnosing osteomyelitis 1

Laboratory Studies

• Obtain two sets of aerobic and anaerobic blood cultures before starting antibiotics 2
• Elevated inflammatory markers support the diagnosis:
  • Erythrocyte sedimentation rate (ESR) >70 mm/h significantly increases the probability of osteomyelitis (likelihood ratio 11) 2, 3
  • C-reactive protein (CRP) is recommended but has lower specificity 2
• Note that inflammatory markers may be elevated due to various factors, making them non-specific for osteomyelitis 1

Imaging Studies

Initial Imaging

• Plain radiographs should be the first imaging test for all patients with suspected osteomyelitis 4, 2
• Early findings include periosteal thickening/elevation and soft tissue swelling; late findings include cortical erosion, periosteal reaction, mixed lucency and sclerosis, sequestrum, or involcrum 1, 4
• Plain radiographs have limited sensitivity (18-68%) in early osteomyelitis but can double the odds of diagnosis when abnormal (likelihood ratio 2.3) 1, 3

Advanced Imaging

• MRI is the preferred advanced imaging modality with sensitivity of 90-98% and specificity of 22-98% 1, 2
• MRI findings include decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences 2
• A normal marrow signal on MRI reliably excludes infection with a negative likelihood ratio of 0.14 and 100% negative predictive value 4, 2, 3
• Triple-phase bone scans have high sensitivity (64-100%) but low specificity (0-57%) and are most appropriate when MRI is contraindicated 1, 4
• CT scanning depicts cortical bone well but has relatively low sensitivity (11-67%) and moderate specificity (56-90%) for osteomyelitis 1, 4

Microbiological Diagnosis

• Bone biopsy for culture and histopathology is the definitive diagnostic method 1, 2
• Obtain deep specimens for culture of tissue and bone at the time of surgical debridement or biopsy 1
• Percutaneous biopsy should preferably be done under fluoroscopic or CT guidance, traversing uninvolved skin if possible 1
• Collect 2-3 specimens if possible, sending at least one for culture and another for histological analysis 1
• Bone cultures have high sensitivity (76-100%) but variable specificity (8-67%) for diagnosing osteomyelitis 1

Diagnostic Algorithm

1. Perform clinical assessment including probe-to-bone test and measure inflammatory markers (ESR, CRP) 1
2. Obtain plain radiographs as initial imaging 4, 2
3. If radiographs are negative but clinical suspicion remains, wait 2-4 weeks and repeat radiographs 1
4. If repeat radiographs remain negative or equivocal but suspicion persists, proceed to MRI 1, 2
5. If MRI is contraindicated, consider nuclear medicine studies (triple-phase bone scan, labeled leukocyte scan) 4
6. For definitive diagnosis, obtain bone biopsy for histopathology and culture, especially when:
   • Diagnosis remains in doubt after imaging 1
   • Osteomyelitis is likely but the causative organism or antibiotic susceptibilities are not predictable 1
   • Infection has failed to respond to initial empirical antibiotic therapy 2

Common Pitfalls to Avoid

• Relying solely on radiographs to rule out early osteomyelitis 4
• Interpreting exposed bone alone as diagnostic of osteomyelitis in pressure ulcers 1
• Failing to obtain contrast-enhanced MRI when evaluating soft tissue involvement 4
• Not considering that pressure-related changes, fractures, and soft tissue infections can mimic osteomyelitis on imaging 1, 4
• Overlooking that bone sampling and histopathology are essential for definitive diagnosis, especially in cases with equivocal imaging findings 1