Ulcer depth, probe-to-bone and osteomyelitis

Synopsis

Use wound depth, probe-to-bone testing, biomarkers, radiography, MRI and bone sampling as a coherent probability-based assessment for diabetic foot osteomyelitis.

Deep infection with sepsis, abscess, necrosis, gangrene or significant ischaemia needs immediate acute referral and surgical or vascular assessment; do not wait for MRI.
A negative probe-to-bone test, normal inflammatory markers or normal early radiograph does not exclude osteomyelitis when a wound is deep, chronic or clinically suspicious.
Begin with wound depth, probe-to-bone, plain radiographs and ESR or CRP; use MRI when doubt persists and the result will change management.

Key red flags

Systemic toxicity, haemodynamic instability, rapidly spreading inflammation or organ dysfunction requires immediate sepsis management and surgical assessment.

Fluctuance, purulent tracking, crepitus, bullae, gas on imaging or disproportionate pain suggests a deep collection or necrotising process needing urgent source control.

Gangrene, a cool foot, absent Doppler signals or severe perfusion deficit means infection and ischaemia must be managed together urgently.

Exposed bone, a sausage toe, a deep chronic wound over bone or recurrent breakdown at the same site markedly raises pre-test probability of osteomyelitis.

Complicated deep infection

Systemic illness, deep fluctuance, necrosis, gas, extensive tissue tracking or gangrene indicates an urgent surgical problem. Imaging should define extent only when it does not delay drainage, debridement or revascularisation.

Investigation priorities

Probe-to-bone with wound depth assessmentFirst step

Adjust pre-test probability rapidly and identify ulcers that communicate with deeper structures.

Management branches

Diagnostic sequenceStable suspected osteomyelitis

A deep, chronic or otherwise suspicious ulcer is present without an immediate need for drainage or sepsis intervention.

Document wound geometry, perfusion and infection grade; perform trained probe-to-bone testing, plain radiographs and ESR or CRP as the initial combined assessment.
If osteomyelitis remains uncertain and the answer will change care, obtain MRI; consider PET, labelled leukocyte scintigraphy or SPECT when MRI is unsuitable or unavailable.

Preferred branchDeep infection needing source control

Sepsis, abscess, necrotising features, gangrene, compartment infection or severe ischaemia accompanies suspected bone infection.

Key medicines

Culture-directed systemic antibioticSelect agent, route, dose and frequency from the organism, susceptibility, oral bioavailability and renal or hepatic function. After complete bone resection treat 2–5 days; consider up to 3 weeks after minor amputation or bone resection with positive margin culture or histology; use 6 weeks when there is no bone resection or dead bone remains. These low-certainty durations require specialist review rather than an inflexible stop rule.There is no single universal drug regimen. Check allergy, pregnancy, interactions, perfusion and prior antibiotics; use oral therapy for prolonged treatment when bioavailability and susceptibility permit, and stop or revise for toxicity, resistance or diagnostic reclassification.

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