What is the best test to confirm osteomyelitis?

Best Test to Confirm Osteomyelitis

MRI is the best test to confirm osteomyelitis, with the highest sensitivity (95.6%) and specificity (80.7-96%) among all imaging modalities, and a 100% negative predictive value for excluding the diagnosis. 1, 2

Diagnostic Algorithm

Step 1: Initial Radiographs

• Begin with plain radiographs to exclude alternative diagnoses (fracture, tumor, degenerative changes), identify foreign bodies, and detect soft-tissue gas 1, 3
• Radiographs have limited sensitivity (70.4%) for acute osteomyelitis but are essential for baseline evaluation 2
• Critical pitfall: Normal radiographs do NOT exclude osteomyelitis—proceed to MRI if clinical suspicion remains high 3

Step 2: MRI Without and With IV Contrast

• MRI is the definitive test showing decreased T1-weighted bone marrow signal and increased signal on fluid-sensitive sequences (T2-weighted fat-saturated, STIR) 1
• Noncontrast MRI alone has high sensitivity and specificity for peripheral osteomyelitis 1
• IV contrast improves evaluation of soft tissue infections and abscesses but does not improve diagnosis of bone infection itself 1
• MRI provides excellent anatomic detail, multiplanar imaging, and superior soft tissue resolution without ionizing radiation 1, 4

Step 3: Alternative Tests When MRI is Contraindicated

FDG-PET/CT (second-line option):

• High sensitivity (81-100%) and specificity (87-100%) for osteomyelitis detection 1
• Particularly useful for multifocal infection, hardware-associated infection, or when hematogenous spread is suspected 1
• Limitation: Cannot distinguish infection from inflammation in acute post-operative/post-traumatic settings until 3-4 months after surgery 1

Combined Nuclear Medicine Studies (third-line option):

• WBC scan combined with sulfur colloid scan achieves up to 90% accuracy 1
• Technetium-99m HMPAO WBC scintigraphy shows 87.3% sensitivity and 94.7% specificity 2
• Most useful with orthopedic hardware or altered bone marrow distribution 1

CT with IV Contrast (limited role):

• Generally insensitive for acute osteomyelitis 5
• Superior for detecting sequestra (dead bone), cortical destruction, and sinus tracts in chronic osteomyelitis 1, 5
• Better than MRI for identifying foreign bodies and soft-tissue gas 1

Special Clinical Scenarios

Chronic Osteomyelitis with Draining Sinus

• MRI remains superior for extent of involvement 1
• CT with contrast useful for detecting sequestra and cortical thickening 1, 5
• FDG-PET shows 91% accuracy in chronic osteomyelitis 1

Hardware-Associated Infection

• Combined WBC scan with sulfur colloid scan preferred over MRI due to metal artifact 1
• FDG-PET/CT shows 96% accuracy for infection with orthopedic hardware 1
• CT with contrast helps evaluate hardware complications and periprosthetic osteolysis 1, 5

Diabetic Foot Osteomyelitis

• MRI diagnostic accuracy similar to overall results 2
• Cortical erosion and confluent T1-weighted signal are independent risk factors for osteomyelitis 6

Key Diagnostic Pitfalls to Avoid

• Do not rely on radiographs alone—early osteomyelitis frequently has normal radiographs 3
• Avoid misinterpreting post-surgical/post-traumatic changes as infection on MRI, particularly within 3-4 months of surgery 1
• Three-phase bone scan has poor specificity (<50%) in the setting of recent surgery or trauma due to osseous remodeling 1
• Ultrasound has limited benefit for diagnosing osteomyelitis itself, though useful for detecting soft tissue abscesses and guiding aspiration 1

Confirmatory Testing

• Bone biopsy remains the gold standard for microbiological diagnosis when imaging is equivocal or to guide antibiotic therapy 6
• Image-guided aspiration of fluid collections should be performed when abscesses are detected 3