What is the next best step in diagnosing a patient with a suspected osteomyelitis of the left foot, who has a non-healing ulcer, positive probe-to-bone test, and is already on intravenous (IV) antibiotics?

Plain Radiographs of the Left Foot Should Be Ordered Now

In this patient with suspected diabetic foot osteomyelitis (DFO), plain radiographs of the left foot should be obtained immediately as the initial imaging study, despite the patient already being started on IV antibiotics. 1

Rationale for Plain Radiographs First

• The IWGDF/IDSA 2023 guidelines explicitly recommend obtaining plain X-rays of the foot in all cases of non-superficial diabetic foot infection as part of the initial diagnostic workup. 1

• Plain radiographs should be performed before advanced imaging because they provide essential baseline information about bone architecture, can identify characteristic features of osteomyelitis (cortical erosion, periosteal reaction, bone destruction), and help exclude other pathology such as foreign bodies or gas in soft tissues. 1, 2

• The combination of probe-to-bone test (which is positive in this patient), plain X-rays, and elevated ESR (45 mm/hr, though not markedly elevated) provides the recommended initial diagnostic approach for DFO. 1, 3

Why Not the Other Options Now

Serum C-reactive protein (Option A): While inflammatory markers like CRP can support the diagnosis of osteomyelitis, the patient already has an elevated ESR and leukocytosis documented. 1 The guidelines recommend using inflammatory markers when clinical examination is equivocal—but this patient has clear clinical signs (positive probe-to-bone, malodorous ulcer, surrounding erythema/swelling). 1 Additional inflammatory markers would not change immediate management when imaging is the critical next step.

Bone scan (Option B): Nuclear medicine studies are considered second-line imaging, reserved for situations when MRI is unavailable or contraindicated. 1 They should not be ordered before obtaining plain radiographs, which are the standard initial imaging modality.

MRI of the left foot (Option D): While MRI is the most sensitive and specific advanced imaging for DFO, the IWGDF/IDSA guidelines clearly state that MRI should be performed "when the diagnosis of diabetes-related osteomyelitis remains in doubt despite clinical, plain X-rays, and laboratory findings." 1, 4 Plain radiographs must be obtained first to establish baseline bone architecture and may be sufficient for diagnosis if classic osteomyelitis changes are present. 1, 2

Clinical Context Supporting This Approach

• This patient has high pretest probability for osteomyelitis based on: a 2-month non-healing ulcer, positive probe-to-bone test (positive LR 7.2 in high-risk patients), third-degree ulcer depth, elevated inflammatory markers (ESR 45, WBC 12,600), and thrombocytosis suggesting chronic inflammation. 1

• In high-prevalence settings (>60% likelihood), a positive probe-to-bone test is largely diagnostic of osteomyelitis, making imaging essential to confirm and define extent. 1

• Plain radiographs may show diagnostic changes in established osteomyelitis (this ulcer has been present for 2 months, allowing time for radiographic changes to develop), and combining probe-to-bone results with plain radiography improves overall diagnostic accuracy. 1

Algorithmic Next Steps After Plain Radiographs

• If plain X-rays show classic osteomyelitis changes (cortical erosion, periosteal reaction, bone destruction): The diagnosis is confirmed, proceed to obtain bone culture (intraoperatively or percutaneously) to guide antibiotic therapy, then continue treatment. 1

• If plain X-rays are negative or equivocal: Proceed to MRI to definitively confirm or exclude osteomyelitis, as plain radiographs have limited sensitivity (54%) in early infection and may take weeks to show changes. 4, 5, 2

• Bone biopsy should be obtained (not soft tissue cultures) for definitive microbiologic diagnosis, as soft tissue specimens do not accurately reflect bone culture results. 1

Common Pitfalls to Avoid

• Do not skip plain radiographs and proceed directly to MRI without first obtaining baseline imaging—this violates guideline-recommended diagnostic algorithms and may result in unnecessary costs. 1

• Do not rely on soft tissue or wound cultures to guide antibiotic therapy for osteomyelitis—bone cultures are required for accurate pathogen identification. 1

• Do not assume normal plain radiographs exclude osteomyelitis—if clinical suspicion remains high despite negative X-rays, proceed to MRI as sensitivity of plain films is only 54% for early infection. 1, 5, 2