What are the test characteristics and treatment guidelines for osteomyelitis diagnosed using the bone probe test?

Bone Probe Test Characteristics for Osteomyelitis Diagnosis

The probe-to-bone test is a highly useful clinical tool for diagnosing osteomyelitis, with a sensitivity of 0.87 and specificity of 0.83 when performed correctly, making it an excellent initial diagnostic test that should be incorporated into the evaluation of all diabetic foot ulcers. 1

Probe-to-Bone Test Technique and Interpretation

Proper Technique:

• Use a sterile, blunt metal probe
• Gently insert through the wound
• Positive test: Feeling a hard, gritty structure (bone)
• The test should be performed by experienced clinicians as inter-observer variability is higher among inexperienced practitioners 2

Test Characteristics:

• Sensitivity: 0.87 (95% CI, 0.75-0.93) 1
• Specificity: 0.83 (95% CI, 0.65-0.93) 1
• Positive Likelihood Ratio: 7.2 2
• Negative Likelihood Ratio: 0.48 2

Interpretation Based on Pre-test Probability:

• High prevalence setting (>60%): Positive test strongly suggests osteomyelitis (PPV up to 89%) 3
• Low prevalence setting (≤20%): Negative test effectively rules out osteomyelitis (NPV 98%) 4
• Intermediate prevalence: Consider additional diagnostic tests

Factors Affecting Test Accuracy

• Anatomical location: Better correlation for ulcers in the hallux and central metatarsals compared to lesser toes 2
• Clinician experience: Lower inter-observer variability among experienced clinicians 2
• Ulcer characteristics: Larger ulcers (>2 cm²) have higher likelihood of underlying osteomyelitis (positive LR 7.2) 2
• Exposed bone: Presence of visible bone has a positive LR of 9.2 for osteomyelitis 2

Diagnostic Algorithm for Suspected Osteomyelitis

1. Initial evaluation: Perform probe-to-bone test on all diabetic foot ulcers

2. If probe-to-bone positive:

   • In high-risk patients: Strongly consider osteomyelitis diagnosis
   • Combine with plain radiography to improve diagnostic accuracy 2
   • Consider bone biopsy for definitive diagnosis in select cases 2

3. If probe-to-bone negative:

   • In low-risk patients: Osteomyelitis unlikely
   • In high-risk patients: Consider additional testing

4. Additional diagnostic tests:

   • Plain radiography: Sensitivity 0.54, specificity 0.68 5
   • MRI: Sensitivity 0.90, specificity 0.79 (most accurate imaging test) 5
   • Erythrocyte sedimentation rate: ESR >70 mm/h increases likelihood of osteomyelitis (positive LR 11) 2

Enhancing Diagnostic Accuracy

• Combine diagnostic modalities: Probe-to-bone test + plain radiography improves overall accuracy 2
• Laboratory markers: ESR >70 mm/h has high positive LR (11) for osteomyelitis 2, 6
• Consider MRI: When diagnosis remains uncertain after initial evaluation, MRI provides the highest diagnostic accuracy among imaging tests 5

Common Pitfalls and Limitations

• False negatives may occur in early osteomyelitis
• False positives may occur with exposed bone without infection
• Test accuracy varies with anatomical location and clinician experience
• Diagnostic accuracy is highly dependent on the prevalence of osteomyelitis in the population being tested 2
• The IWGDF/IDSA recommends using a combination of probe-to-bone test, plain X-rays, and inflammatory markers (ESR, CRP, or PCT) for initial diagnosis 2

Treatment Considerations

• Bone biopsy remains the gold standard for definitive diagnosis and pathogen identification 2
• Consider bone biopsy in cases of suspected antibiotic-resistant organisms, progressive bony deterioration, or failure to respond to empiric therapy 2
• Midfoot or hindfoot lesions warrant stronger consideration for bone biopsy due to higher risk of above-ankle amputation 2

The probe-to-bone test is a simple, cost-effective bedside test that should be incorporated into the initial assessment of all diabetic foot ulcers to help guide further diagnostic and therapeutic decisions.