What is the initial workup for osteomyelitis?

Initial Workup for Osteomyelitis

The initial workup for suspected osteomyelitis should begin with plain radiographs of the affected area, followed by blood tests including C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and blood cultures, with subsequent MRI if the diagnosis remains uncertain. 1, 2

Clinical Assessment

When evaluating for osteomyelitis, look for these specific indicators:

• Ulcer overlying bony prominence
• Exposed bone in wound
• Ulcer area >2 cm²
• Erythematous and indurated "sausage toe" appearance (in foot infections)
• Nocturnal pain of moderate intensity
• Fever (present in up to 45% of patients)
• History of trauma, surgery, or underlying conditions like diabetes 2

Initial Diagnostic Tests

Imaging

1. Plain Radiographs (First Line)

   • Provides excellent anatomic overview
   • Can exclude fractures and tumors as causes of symptoms
   • Helps with interpretation of subsequent imaging studies
   • Caution: Early osteomyelitis (<14 days) may show only mild soft tissue swelling or appear normal 1
   • Typical findings in established disease: erosions, periosteal reaction (acute), bone sclerosis (chronic) 1

2. MRI (Second Line)

   • Gold standard with >90% sensitivity and specificity
   • Indicated when diagnosis remains uncertain after initial radiographs and laboratory tests
   • Look for decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences
   • Excellent for evaluating disease extent and soft tissue involvement 2, 3
   • Strong recommendation: Perform MRI when the diagnosis of osteomyelitis remains in doubt despite clinical, plain X-rays and laboratory findings 1

Laboratory Tests

1. Inflammatory Markers

   • C-reactive protein (CRP)
   • Erythrocyte sedimentation rate (ESR) (elevated >27 mm/h suggests infection)
   • Procalcitonin (PCT) in equivocal cases 1, 2

2. Blood Cultures

   • Obtain two sets (aerobic and anaerobic)
   • Collect before initiating antibiotics when possible 2

3. Bone Biopsy and Culture

   • Gold standard for definitive diagnosis
   • Bone samples (rather than soft tissue) should be obtained for culture, either intraoperatively or percutaneously 1
   • Use conventional microbiology techniques rather than molecular methods 1
   • Note: Aspiration of fluid at the time of biopsy and the presence of an open wound are the best predictors of positive bone cultures 4

Special Diagnostic Considerations

Probe-to-Bone Test

• Simple bedside test using a sterile blunt metal probe
• Positive when a hard, gritty structure is felt
• High sensitivity (87%) and specificity (83%) when performed correctly
• Particularly useful in diabetic foot infections 1

Alternative Imaging When MRI is Contraindicated

• CT with IV contrast: Useful for detecting sequestrum, bone erosions, soft tissue gas, and foreign bodies
• Nuclear medicine studies:
  • Three-phase bone scan
  • FDG-PET/CT (high sensitivity 81-100% and specificity 87-100%)
  • Leukocyte scintigraphy or SPECT as alternatives to MRI 1, 2

Common Pitfalls to Avoid

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

   • False positives common in patients with recent trauma, surgery, or arthritis 2

3. Collecting cultures after antibiotic administration

   • Prior antibiotic use can lead to false-negative cultures 2

4. Inadequate bone sampling

   • Soft tissue cultures may not reflect bone pathogens
   • Avoid wound swab cultures due to contamination risk 2, 5

5. Overlooking diabetic foot osteomyelitis

   • Consider specialized workup in diabetic patients with foot ulcers 1

By following this systematic approach to the initial workup of osteomyelitis, clinicians can improve diagnostic accuracy and guide appropriate treatment decisions to reduce morbidity and mortality associated with bone infections.