Will X-ray Show if Infection Has Spread to Bone?
Plain X-rays are useful but unreliable for detecting early bone infection—they should always be obtained first, but a normal X-ray does NOT rule out osteomyelitis, especially in the first 2 weeks of infection. 1
Key Limitations of Plain Radiographs
Plain X-rays are insensitive to acute osteomyelitis and may be completely normal in early infection (<14 days), showing only soft tissue swelling. 1, 2 The sensitivity of plain radiographs for detecting osteomyelitis ranges from only 43-75%, with specificity of 75-83%. 3
- Radiographic changes typically take 2-3 weeks to become visible after infection onset 1, 4
- Early osteomyelitis often shows no bony changes, only nonspecific soft tissue swelling 1, 2
- The positive likelihood ratio for plain X-rays is only 2.3, and negative likelihood ratio is 0.6—meaning they are only marginally predictive 1
When Plain X-rays ARE Helpful
Despite these limitations, plain radiographs should always be the first imaging study obtained because they: 1, 2
- Demonstrate findings of chronic osteomyelitis (bone sclerosis, sequestrum formation) 1
- Show characteristic features including cortical erosion, periosteal reaction, bone destruction, and trabecular changes 2
- Detect gas in soft tissues and radiopaque foreign bodies 1
- Exclude alternative diagnoses (fracture, tumor, neuropathic arthropathy) 1
- Provide baseline for comparison with future imaging 1
- Are inexpensive, widely available, and cause minimal harm 1
Critical Follow-Up Strategy
If initial X-rays are normal but clinical suspicion for osteomyelitis remains high, repeat plain radiographs in 2-3 weeks. 1, 2 This is particularly important because:
- Serial radiographs showing new or evolving changes (progressive bone erosion, periosteal reaction, demineralization) are highly suggestive of osteomyelitis 1
- Comparing sequential films over time improves diagnostic accuracy 1
When Advanced Imaging Is Needed
MRI with contrast is the imaging modality of choice when diagnosis remains uncertain after plain radiographs, with 97% sensitivity, 93% specificity, and 94% accuracy. 1, 2 MRI should be obtained when:
- Plain X-rays are negative but clinical suspicion is high 2
- Distinguishing infection from non-infectious conditions (Charcot arthropathy) is difficult 1
- Detecting soft tissue abscesses, extent of bone involvement, or associated complications is needed 1, 2
- A normal marrow signal on MRI reliably excludes infection with 100% negative predictive value 2
Practical Clinical Approach
Combine plain X-rays with clinical findings and inflammatory markers for optimal diagnostic accuracy: 1, 2
- ESR >60 mm/hr or CRP >3.2 mg/dL combined with radiographic findings increases diagnostic confidence 2
- Probe-to-bone test (in diabetic foot infections) combined with X-rays improves accuracy 1
- Visible bone through a wound has a positive likelihood ratio of 9.2 for osteomyelitis 2, 5
- Large ulcers (>2 cm²) significantly increase likelihood of underlying bone infection 1, 2
Common Pitfall to Avoid
The most critical error is assuming a normal X-ray rules out osteomyelitis. 1, 2 In early acute infection, radiographs are frequently normal and should never be used alone to exclude bone infection when clinical suspicion is moderate to high. Always proceed to MRI or repeat radiographs in 2-3 weeks if suspicion persists. 1, 2