What is the initial workup for a patient suspected of having osteomyelitis?

Osteomyelitis Workup

Begin with plain radiographs of the affected area, obtain inflammatory markers (ESR and CRP), and collect blood cultures before starting antibiotics, followed by MRI with contrast if radiographs are negative or inconclusive and clinical suspicion remains high. 1, 2

Initial Clinical Assessment

• Perform a thorough medical and neurologic examination to identify potential sources of hematogenous seeding and assess motor/sensory function 1
• Evaluate for risk factors including diabetes mellitus, cardiovascular disease, recent trauma, surgery, or presence of hardware 2, 3
• Note that fever is often absent in osteomyelitis and should not be relied upon for diagnosis 4

Laboratory Studies

Inflammatory Markers

• Obtain ESR and CRP as first-line laboratory tests 1, 2
• In patients with low clinical suspicion, a normal age-adjusted ESR and CRP <5 mg/L effectively rules out osteomyelitis and no further urgent investigation is needed 5
• However, in high-risk patients (puncture wounds, foot ulcers, diabetic foot infections), normal ESR or CRP <5 mg/L does NOT exclude osteomyelitis 5
• ESR >30 mm/h and/or CRP >10-30 mg/L warrants further definitive investigation 5
• Consider procalcitonin when clinical examination is diagnostically equivocal 1, 2
• White blood cell count is NOT helpful in evaluating osteomyelitis 5

Important Caveat About Inflammatory Markers

• Laboratory values are significantly influenced by the infecting organism 4
• Culture-negative, fungal, and tuberculosis osteomyelitis demonstrate lower CRP, ESR, WBC, and neutrophil percentages compared to Staphylococcus aureus and antibiotic-resistant organisms 4
• This can contribute to delays in diagnosis when dealing with lower-virulence organisms 4

Blood Cultures

• Obtain two sets of aerobic and anaerobic blood cultures BEFORE starting antibiotics 1

Imaging Algorithm

Step 1: Plain Radiographs

• Perform plain radiographs as the first imaging test for ALL patients with suspected osteomyelitis 1, 2
• Recognize that radiographs have limited sensitivity in early disease and may be normal for weeks after symptom onset 1, 6
• Radiographic findings include bone sclerosis, areas of destruction, periosteal reaction, and trabecular coarsening, but these develop late 6

Step 2: MRI with Contrast

• Proceed to MRI with and without IV contrast when diagnosis remains uncertain after initial radiographs 1, 2
• MRI is the gold standard with 97% sensitivity, 93% specificity, and 94% accuracy 1
• Characteristic findings include decreased T1-weighted bone marrow signal with increased signal on fluid-sensitive sequences 1
• A normal marrow signal on MRI reliably excludes infection with 100% negative predictive value 1
• MRI is superior for detecting soft tissue involvement, abscesses, fistulae, and extent of bone involvement 6

Special Imaging Considerations

• For vertebral osteomyelitis, spine MRI is strongly recommended 1
• For chronic osteomyelitis, CT can evaluate periosteal reaction, cortical erosions, and fistulae when MRI is contraindicated 1, 6
• FDG-PET shows 91-94% accuracy for chronic osteomyelitis and is particularly useful when hardware is present 1, 6

Microbiological Diagnosis

• Obtain bone samples for culture (NOT soft tissue) either intraoperatively or percutaneously for definitive diagnosis 1, 2
• Intraoperative excisional biopsy during debridement is preferred over core needle biopsy because osteomyelitis can be focal 2
• Bone cultures have 76-100% sensitivity but only 8-67% specificity; combining with histopathology is recommended 2
• Conventional microbiology techniques are preferred for first-line pathogen identification 1

When to Obtain Bone Cultures

• Particularly indicated when clinical and imaging evaluations are uncertain 1
• When soft-tissue cultures are inconclusive 1
• When infection has failed to respond to initial empirical antibiotic therapy 1

Pediatric-Specific Considerations

• For children <5 years with suspected extremity osteomyelitis or septic arthritis, ultrasound, radiography, or MRI with contrast are all appropriate initial imaging options 7
• Ultrasound can detect joint effusions and guide aspiration 7
• If initial radiographs are normal but clinical suspicion persists, proceed to ultrasound, image-guided aspiration, or MRI 7

Common Pitfalls to Avoid

• Do not rely solely on radiographs for diagnosis, as they may be normal in early disease 6
• Do not use normal inflammatory markers to exclude osteomyelitis in high-risk patients or those with culture-negative/low-virulence organisms 4, 5
• Do not delay MRI when clinical suspicion is high despite negative radiographs 1, 6
• Do not misinterpret post-surgical or post-traumatic marrow changes as infection on MRI 6
• Do not fail to obtain contrast-enhanced MRI when evaluating soft tissue involvement 6