What are the signs of osteomyelitis?

Signs of Osteomyelitis

The key signs of osteomyelitis include exposed or palpable bone through a wound, "sausage toe" (erythematous and indurated toe), large ulcers (>2 cm²) especially over bony prominences, and ulcers that fail to heal after 6 weeks of proper care. 1, 2

Clinical Signs to Actively Look For

Local Wound Findings

• Exposed bone visible in the wound base or bone that can be felt with a sterile metal probe (positive likelihood ratio 9.2) 3, 1, 2
• Ulcer area greater than 2 cm² (positive likelihood ratio 7.2) 3, 1, 2
• Ulcer located over a bony prominence (metatarsal heads, calcaneus, malleoli) that fails to heal despite adequate off-loading 3, 1, 2
• Non-healing ulcer after at least 6 weeks of proper wound care and off-loading 1, 2

Soft Tissue Signs

• "Sausage toe" appearance: an erythematous, swollen, and indurated toe, highly suggestive of underlying osteomyelitis, particularly in diabetic patients 3, 1, 2
• Erythema extending ≥2 cm from the wound margin 3
• Local swelling or induration around the wound 3
• Local tenderness or pain (though this may be blunted by neuropathy) 3
• Local increased warmth 3
• Purulent discharge 3

Early Disease Presentation

• Subtle or absent erythema and swelling may be the only visible signs before bone destruction occurs 2
• Soft tissue edema may be present before radiographic bone changes appear 2

Important Clinical Caveats

What May Be Absent

• Systemic signs are often absent: fever, elevated white blood cell count, and other systemic inflammatory markers may be normal, especially in diabetic patients 3, 1
• Local inflammatory signs may be blunted by neuropathy or ischemia 3
• The absence of wound infection signs or leukocytosis does not rule out osteomyelitis 1

Diagnostic Pitfalls

• The actual depth of an ulcer is often unclear clinically, making it essential to probe every foot ulcer with a blunt metal probe at every consultation 1
• Neuro-osteoarthropathy (Charcot foot) can mimic osteomyelitis and may coexist with infection 1
• Clinical suspicion alone has reasonable diagnostic accuracy (positive likelihood ratio 5.5, negative likelihood ratio 0.54) 3

Probe-to-Bone Test

Perform a probe-to-bone test on every foot ulcer by gently inserting a blunt sterile metal probe through the wound 3, 1, 2:

• Positive test: probe strikes bone with a hard, gritty feel (positive likelihood ratio 7.2 in high-risk patients, 9.2 when bone is exposed) 3, 1, 2
• Negative test: essentially rules out osteomyelitis in low-risk patients (≤20% prevalence) with negative likelihood ratio 0.48 3
• Test accuracy is higher when performed by experienced clinicians and in ulcers located on the hallux or central metatarsals 3

Laboratory Markers

Inflammatory Markers

• Erythrocyte sedimentation rate (ESR) >70 mm/h substantially increases likelihood of osteomyelitis (positive likelihood ratio 11), while lower levels reduce likelihood (negative likelihood ratio 0.34) 3
• Elevated C-reactive protein (CRP) may be suggestive but has less robust data than ESR 3

Infection Classification

When signs of infection are present, classify severity 3:

• Mild (Grade 2): At least two local signs (swelling, erythema 0.5-2 cm, tenderness, warmth, purulent discharge) without systemic manifestations 3
• Moderate (Grade 3): Erythema ≥2 cm from wound margin and/or involvement of deeper tissues (tendon, muscle, joint, bone) 3
• Severe (Grade 4): Any foot infection with systemic inflammatory response syndrome (≥2 SIRS criteria: temperature >38°C or <36°C, heart rate >90 bpm, respiratory rate >20 breaths/min, WBC >12,000/mm³ or <4,000/mm³) 3
• Add "(O)" designation when osteomyelitis is present 3