What is the most appropriate initial investigation for a diabetic patient presenting with a 2-cm ulcer over the first metatarsal area, swelling of the big toe, and warmth and tenderness of the calf?

Initial Investigation for Diabetic Foot Infection with Suspected Osteomyelitis

The most appropriate initial investigation is a combination of probe-to-bone test, plain radiographs in three views, and inflammatory markers (ESR or CRP). 1, 2

Immediate Bedside Assessment

• Perform a probe-to-bone (PTB) test immediately using a sterile blunt metal probe gently inserted through the ulcer to feel for hard, gritty bone. 1
  • A positive PTB test (palpable bone) has a sensitivity of 0.87 and specificity of 0.83 for osteomyelitis, with a positive likelihood ratio of 7.2 in high-risk patients. 1
  • In this patient with a 2-cm ulcer over the first metatarsal (a bony prominence), a positive test substantially increases the likelihood of osteomyelitis. 1
  • The "sausage toe" appearance (swelling, warmth, erythema of the big toe) further suggests underlying bone infection. 1

Plain Radiography

• Obtain plain radiographs of the foot in three views with a radio-opaque marker at the ulcer site. 2, 1
  • This is the mandatory initial imaging study for all non-superficial diabetic foot infections. 2, 1
  • Look for cortical erosion, periosteal reaction, mixed lucency and sclerosis, or bone destruction—findings highly suggestive of osteomyelitis. 1
  • Plain radiographs have only 61.9% sensitivity for early osteomyelitis, so normal films do not exclude the diagnosis. 2, 1
  • If initial radiographs are negative but clinical suspicion remains high, repeat plain films in 2-3 weeks to detect progressive bony changes. 1

Laboratory Testing

• Check inflammatory markers: ESR and/or CRP. 1, 2
  • An ESR >70 mm/hour has a positive likelihood ratio of 11 for osteomyelitis, while lower values reduce the likelihood (negative LR 0.34). 1
  • Markedly elevated CRP also suggests osteomyelitis, though the threshold is less well-defined. 1, 2
  • Combining clinical findings (ulcer depth, PTB test) with elevated inflammatory markers improves diagnostic accuracy. 1

Wound Culture Collection

• Obtain deep tissue specimens or bone cultures after debridement, not superficial wound swabs. 2, 3
  • Superficial swabs yield contaminated results and should never be obtained. 2, 3
  • Use curettage or biopsy from the debrided ulcer base for reliable microbiological data. 3

When to Consider Advanced Imaging

• MRI is NOT needed as a first-line investigation but should be reserved for specific situations. 1, 2
  • Order MRI only when the diagnosis remains in doubt after PTB test, plain radiographs, and inflammatory markers. 2, 1
  • MRI has the highest diagnostic accuracy (96.4% sensitivity, 83.8% specificity) and is the gold standard when advanced imaging is required. 2, 1
  • Consider MRI if plain radiographs show equivocal findings (not clearly normal or clearly osteomyelitis) or if distinguishing from Charcot neuro-osteoarthropathy is necessary. 1

Critical Pitfalls to Avoid

• Do not delay antibiotic therapy to obtain imaging. 2, 3

  • This patient has signs of moderate infection (warmth and tenderness extending to the calf), requiring immediate empirical antibiotics after obtaining cultures. 2, 3

• Do not assume normal early radiographs exclude osteomyelitis. 2, 1

  • Bony changes may not appear for 2-3 weeks, so clinical judgment and inflammatory markers are essential. 1

• Do not order vascular imaging based solely on diabetes. 2

  • This patient has intact distal pulses, so duplex ultrasound or angiography is not indicated. 2

Diagnostic Algorithm Summary

In this high-risk patient (2-cm ulcer over bony prominence, "sausage toe," warmth extending to calf):

1. Perform PTB test at bedside (likely positive given clinical presentation). 1
2. Order plain radiographs immediately in three views. 2, 1
3. Check ESR and/or CRP. 1, 2
4. Obtain deep tissue cultures after debridement. 2, 3
5. If PTB test OR plain radiographs are positive, combined with elevated inflammatory markers, the diagnosis of osteomyelitis is highly probable (>90% post-test probability). 1, 4
6. Reserve MRI for cases where all three initial tests are negative or equivocal but clinical suspicion remains high. 1, 2

The combination of PTB test and plain radiographs has a sensitivity of 0.97, specificity of 0.92, and positive likelihood ratio of 12.8 in high-prevalence settings like this one. 4 Only 6.6% of patients with negative results on both tests have osteomyelitis. 4