What is the initial imaging study to order for a 43‑year‑old man with type 2 diabetes mellitus, peripheral neuropathy, a 2‑month‑old third‑degree plantar ulcer on the left foot, a positive probe‑to‑bone test, leukocytosis and an elevated erythrocyte sedimentation rate?

Initial Imaging for Suspected Diabetic Foot Osteomyelitis

Order a plain radiograph of the left foot as the initial imaging study.

Rationale for Plain Radiography First

Plain radiographs should be the first-line imaging modality in this clinical scenario, despite the availability of more advanced imaging options 1, 2. Here's the algorithmic approach:

Step 1: Clinical Assessment Strongly Suggests Osteomyelitis

Your patient has multiple high-probability features for osteomyelitis:

• Positive probe-to-bone test (positive likelihood ratio 6.4, meaning osteomyelitis is 6.4 times more likely) 2
• ESR of 45 mm/h (while not >70 mm/h which would give a likelihood ratio of 11, this is still elevated and supportive) 2
• Third-degree ulcer with 2-month duration 2
• Leukocytosis (WBC 12.6) indicating active infection 1

Step 2: Plain Radiograph as Initial Imaging

Plain radiographs should be obtained first because:

• An abnormal plain radiograph doubles the odds of osteomyelitis (positive likelihood ratio 2.3) 2
• Plain radiographs demonstrated statistically more significant power than MRI in predicting diabetic foot osteomyelitis in hospitalized patients undergoing partial foot amputations (after adjusting for age, ESR, and CRP) 3
• They are immediately available, low-cost, and provide essential baseline information 1
• They help exclude alternative diagnoses such as Charcot foot, foreign bodies, or gas in soft tissues 1

Step 3: Role of Advanced Imaging

MRI should be reserved for specific situations:

• When plain radiographs are negative but clinical suspicion remains very high 2
• When surgical planning requires detailed anatomic information 1
• MRI has excellent negative predictive value (negative likelihood ratio 0.14), meaning a normal MRI makes osteomyelitis much less likely 2
• However, MRI's overall accuracy is 89%, and it should not replace initial plain radiography in the diagnostic algorithm 2

What NOT to Order Initially

Serum CRP

• While CRP can be useful, it does not replace imaging 4
• CRP/albumin ratios showed no significant advantage over standard inflammatory markers alone 4
• You've already obtained ESR, which provides similar diagnostic information 4

Bone Scan

• Not recommended as first-line imaging 1
• Less specific than plain radiographs or MRI for diabetic foot osteomyelitis 1
• Takes longer to obtain results, delaying definitive management 1

Critical Pitfalls to Avoid

• Do not wait for advanced imaging before starting antibiotics – you correctly obtained blood cultures and are starting IV antibiotics, which should not be delayed 1
• Plain radiographs may be negative early – bone changes typically require 10-14 days and 30-50% bone destruction to become visible, but given your patient's 2-month history, radiographic changes are likely present if osteomyelitis exists 2
• Do not rely solely on inflammatory markers – ESR >70 mm/h has a likelihood ratio of 11, but your patient's ESR of 45 still supports infection without being diagnostic 2

Next Steps After Plain Radiograph

If plain radiographs show osteomyelitis (periosteal reaction, bone destruction, or sequestrum), proceed with treatment. If radiographs are negative but clinical suspicion remains high (positive probe-to-bone, elevated ESR, ulcer >2 cm²), then obtain MRI for definitive diagnosis 2.