What test should I order to rule out osteomyelitis?

Diagnostic Tests for Osteomyelitis

For ruling out osteomyelitis, a stepwise approach should be used starting with probe-to-bone test, plain radiographs, and inflammatory markers, followed by MRI when needed. 1

Initial Diagnostic Tests

• Perform a probe-to-bone test for any infected open wound - a negative test in a low-risk patient largely rules out osteomyelitis, while a positive test in a high-risk patient is highly diagnostic 1
• Obtain plain X-rays of the affected area as the first imaging test for all cases of suspected osteomyelitis 1, 2
• Check serum inflammatory markers, especially erythrocyte sedimentation rate (ESR) - a highly elevated ESR (>70 mm/h) increases the likelihood of osteomyelitis (positive LR of 11) 1
• Consider C-reactive protein (CRP) measurement - values >7.9 mg/dL have 80% specificity for osteomyelitis in diabetic foot infections 3

Diagnostic Algorithm

1. Clinical Assessment:

   • Suspect osteomyelitis when an ulcer lies over a bony prominence, fails to heal despite adequate care, or when a toe appears erythematous and indurated ("sausage toe") 1
   • Large ulcers (>2 cm²) have a higher likelihood of underlying osteomyelitis (positive LR 7.2) 1

2. First-line Tests:

   • Probe-to-bone test - if negative in low-risk patients, osteomyelitis is unlikely (negative LR 0.48) 1
   • Plain radiographs - while sensitivity is low in early disease, they are widely available and can detect bone changes 1, 4
   • ESR and CRP - an ESR <30 mm/h suggests low likelihood of osteomyelitis, while ESR >60 mm/h with CRP >7.9 mg/dL indicates high likelihood 3

3. Advanced Imaging:

   • MRI with and without contrast is the preferred advanced imaging test when diagnosis remains uncertain after initial evaluation (sensitivity 97%, specificity 93%) 1, 2
   • When MRI is unavailable or contraindicated, consider a white blood cell-labeled radionuclide scan, SPECT/CT, or fluorine-18-fluorodeoxyglucose PET/CT scans 1

Definitive Diagnosis

• The gold standard for diagnosis is bone biopsy with histological and microbiological examination 1
• Bone biopsy is particularly indicated when:
  • Definitive diagnosis is needed to justify treatment choices
  • Cultures suggest antibiotic-resistant organisms
  • There is progressive bone deterioration during therapy
  • The patient fails to respond to empiric antibiotics 1

Important Considerations

• Combining diagnostic tests improves accuracy - using probe-to-bone test with plain radiography enhances diagnostic precision 1
• Blood cultures should be obtained before starting antibiotics in patients with suspected osteomyelitis 1, 2
• Avoid using soft tissue or sinus tract specimens for selecting antibiotic therapy as they do not accurately reflect bone culture results 1
• In diabetic foot infections, ESR combined with probe-to-bone test (parallel testing) provides 96% sensitivity for early detection of osteomyelitis 5

Common Pitfalls

• Relying solely on plain radiographs can miss early osteomyelitis as changes may not be visible for 10-14 days 1, 4
• Misinterpreting bone destruction from Charcot neuroarthropathy as osteomyelitis - MRI can help differentiate between these conditions 1
• Failing to obtain bone cultures before initiating antibiotics, which can reduce culture yield 1
• Over-reliance on soft tissue cultures, which often do not correlate with bone pathogens 1