In a diabetic patient with a foot ulcer, what diagnostic studies are recommended to rule out osteomyelitis?

Diagnostic Studies to Rule Out Osteomyelitis in Diabetic Foot Ulcers

Initial Diagnostic Approach

All diabetic patients with foot ulcers should immediately undergo plain radiographs of the affected foot to evaluate for cortical erosion, periosteal reaction, mixed lucency/sclerosis, soft tissue gas, and foreign bodies. 1, 2

Probe-to-Bone (PTB) Test

• Perform a probe-to-bone test at the bedside for any open diabetic foot wound; when properly conducted, a positive test strongly supports osteomyelitis when clinical suspicion is high, while a negative test helps exclude it when suspicion is low. 1, 3, 2
• The PTB test combined with plain radiographs provides effective first-line diagnostic assessment for diabetic foot osteomyelitis. 4
• All patients with ulcers exposing bone should be presumed to have osteomyelitis. 5

Plain Radiograph Interpretation

• If plain radiographs show classic changes of osteomyelitis (cortical erosion, active periosteal reaction, mixed lucency and sclerosis), initiate presumptive treatment after obtaining appropriate cultures. 1, 2
• Plain radiographs have relatively low sensitivity (28-93%) and specificity (25-92%), and changes may not appear until 2-4 weeks after infection onset. 1, 2, 6
• If initial radiographs are normal but clinical suspicion persists, treat the soft tissue infection for 1-2 weeks with appropriate wound care and off-loading, then repeat radiographs in 2-4 weeks; serial films significantly improve diagnostic accuracy. 1, 2

Advanced Imaging When Diagnosis Remains Uncertain

Magnetic Resonance Imaging (MRI)

MRI is the imaging study of choice when the diagnosis remains uncertain after plain radiographs, when soft tissue abscess is suspected, or when surgical planning is required. 1, 3, 2

• MRI demonstrates 90-97% sensitivity and 85-100% specificity for diagnosing diabetic foot osteomyelitis. 2, 6, 7
• A normal MRI reliably excludes osteomyelitis with 100% negative predictive value; normal bone marrow signal on T1-weighted sequences effectively rules out infection. 2
• Standard MRI protocol should include T1-weighted and T2-weighted (or STIR) sequences; decreased bone marrow signal on T1 is highly specific for osteomyelitis. 2, 8
• Request interpretation by an experienced musculoskeletal radiologist when available, as specialist reads improve diagnostic accuracy. 8

Important MRI Limitations

• Bone marrow edema alone is not specific—reactive edema from trauma, prior surgery, or Charcot neuroarthropathy can mimic infection and reduce specificity. 2, 8
• Metal hardware produces significant artifact that may reduce interpretability. 2

Alternative Imaging When MRI Is Unavailable or Contraindicated

When MRI cannot be performed, labeled white blood cell scintigraphy combined with a bone scan is the preferred alternative. 1, 2

• FDG-PET/CT offers high specificity (92-93%) and good sensitivity (81%) and should be considered as an alternative to MRI. 2, 8
• Three-phase technetium bone scans alone have unacceptably high false-positive rates (specificity only 18-83%) and should not be used as a standalone test. 2, 6, 7
• Indium-labeled leukocyte scintigraphy demonstrates 89% sensitivity but only 29% specificity in diabetic foot osteomyelitis. 6, 5

Definitive Diagnosis: Bone Culture and Histology

The most definitive way to diagnose diabetic foot osteomyelitis is by combined findings on bone culture and histology. 1, 3, 2

Indications for Bone Biopsy

• Consider bone biopsy when the diagnosis remains uncertain after imaging, when the causative organism or antibiotic susceptibilities are unpredictable, or when mid- or hindfoot lesions are present (these carry higher amputation risk). 1, 2
• Bone biopsy can be performed percutaneously under fluoroscopic or CT guidance (traversing uninvolved skin when possible) or surgically; complications are exceedingly rare. 1, 2
• Cultures of bone specimens provide more accurate microbiologic data than soft tissue cultures and should guide definitive antibiotic therapy. 1, 3
• Obtain 2-3 bone specimens when possible, sending at least one for culture and another for histological analysis. 1

Supportive Laboratory Studies

• Obtain inflammatory markers (ESR, CRP, or procalcitonin) to support the diagnosis, recognizing that they are nonspecific and must be interpreted alongside imaging and clinical findings. 2, 4
• Elevated erythrocyte sedimentation rate is particularly associated with diabetic foot osteomyelitis. 4

Common Diagnostic Pitfalls

• Delayed or inadequate imaging leads to missed diagnoses; osteomyelitis underlies 68% of diabetic foot ulcers, with the majority being clinically unsuspected. 5
• Relying solely on clinical examination misses the diagnosis in 68% of cases, as most osteomyelitis in diabetic foot ulcers occurs without exposed bone or obvious inflammation. 5
• Using soft tissue cultures instead of bone cultures results in inappropriate antibiotic selection and treatment failure. 3
• Failing to repeat plain radiographs after 2-4 weeks when initial films are normal but suspicion persists misses early osteomyelitis that has not yet produced radiographic changes. 1, 2