What is the purpose of a probe to bone test of an ulceration?

Purpose of the Probe-to-Bone Test

The probe-to-bone test is a bedside clinical tool used to diagnose underlying osteomyelitis in foot ulcers by detecting whether bone can be palpated through the wound with a sterile blunt metal probe. 1

How the Test Works

• A sterile blunt metal probe is gently inserted through the ulcer until resistance is met 1
• A positive test occurs when the probe strikes bone, detected by a hard, gritty sensation 1
• The test should be performed at every consultation for chronic foot ulcers to monitor for development of osteomyelitis 2

Diagnostic Performance

The probe-to-bone test substantially increases the likelihood of osteomyelitis when positive (positive likelihood ratio 7.2) in high-prevalence populations (>60% pretest probability). 1

When the Test is Most Useful:

• Positive test in high-risk patients: When bone is palpable in a patient with high pretest probability (>60%), osteomyelitis is highly likely (positive predictive value 89-95%), making specialized imaging unnecessary 1, 3, 4
• Negative test in low-risk patients: When bone is not palpable in a patient with low pretest probability (≤20%), osteomyelitis is essentially ruled out (negative likelihood ratio 0.48) 1

Test Performance Characteristics:

• Sensitivity: 66-98% (varies by population and operator experience) 3, 4, 5
• Specificity: 78-85% 3, 4, 5
• Positive predictive value: 87-95% 3, 4, 5
• Negative predictive value: 56-91% 3, 4, 5

Clinical Context for Suspecting Osteomyelitis

The probe-to-bone test should be performed when clinical features suggest possible bone infection:

• Ulcers overlying bony prominences 1
• Ulcers failing to heal after 6 weeks of adequate off-loading and wound care 2
• Large ulcers (area >2 cm²) 1, 6
• Deep ulcers (>3 mm depth) 7
• Erythematous and indurated toes ("sausage toe") 1
• Visible or exposed bone (positive likelihood ratio 9.2) 1

Important Caveats and Pitfalls

Operator Experience Matters:

• Inter-observer variability is high among inexperienced clinicians but low among experienced clinicians 1
• Test correlation is stronger for ulcers in the hallux and central metatarsals compared to lesser toes 1

Combining Tests Improves Accuracy:

• Combining probe-to-bone results with plain radiography improves overall diagnostic accuracy 1
• Adding erythrocyte sedimentation rate >70 mm/h (positive likelihood ratio 11) further increases diagnostic certainty 1, 5, 6
• However, normal inflammatory markers do not exclude osteomyelitis—up to 50% of severe diabetic foot infections present with normal ESR/CRP 7, 8

When Additional Testing is Needed:

• If the probe-to-bone test is positive in a high-risk patient, proceed directly to treatment without additional imaging 1, 3
• If diagnostic uncertainty remains after probe-to-bone testing and plain radiographs, obtain MRI (the imaging study of choice) 2, 6
• MRI has high accuracy (89%) and a negative MRI makes osteomyelitis much less likely (negative likelihood ratio 0.14) 6

Practical Application Algorithm

1. Perform probe-to-bone test at initial evaluation and every follow-up visit for chronic ulcers 2
2. If positive in high-risk patient (large/deep ulcer, failed healing, ESR >70): diagnose osteomyelitis and initiate treatment 1, 4, 6
3. If negative in low-risk patient (small/shallow ulcer, no systemic signs): osteomyelitis unlikely, continue wound care 1
4. If uncertain (intermediate risk or discordant findings): obtain plain radiographs and consider MRI 2, 6
5. Reassess at 6 weeks if ulcer not healing despite appropriate care 2