Current Insights On Imaging Techniques For Diagnosing Infection
- Volume 24 - Issue 12 - December 2011
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Why The Clinical Setting Is A Key Factor In Interpreting Radiology Studies
Consideration of the clinical context in which your patients present is paramount to interpreting the results of any of the aforementioned radiology studies. In recent years, it has become increasingly clear just how important osteomyelitis prevalence (a.k.a. pre-test probability) is when interpreting test results in patients with diabetic foot infections.12,13
Consider, for example, a typical outpatient setting, where the prevalence of underlying osteomyelitis accompanying infected neurotrophic ulcers may only be 20 percent.1,3 In this scenario, a negative foot X-ray will be fairly supportive of establishing the absence of bone involvement. This would effectively lower the post-test probability for having osseous infection to just 0.15.12 However, in the inpatient setting, where the prevalence of osteomyelitis may be as high as 66 to 95 percent in infected neurotrophic ulcers, patients will still be more likely than not (post-test probability > 0.5) to have an underlying bone infection when their foot X-rays are negative or unremarkable.12
What is abundantly clear is that highly sensitive tests (e.g., bone scan and MRI) will always go a long way to rule out underlying osteomyelitis in situations when there is low clinical suspicion (e.g., outpatient setting) and you get a negative test result. The converse is also true. Highly specific tests (e.g., MRI) will go a long way to rule in osteomyelitis in settings (e.g., inpatient) where there is high clinical suspicion provided you get a positive test result.
Currently, there is no gold standard as it applies to diagnostic imaging of infection. The true diagnosis of infection is based on biopsy and culture of infecting organisms, and the reliability of these methods has even been called into question recently.1,7 Often, plain film imaging is all that is necessary for achieving a diagnosis of infection. We cannot stress enough the need for early radiographic assessment as well as follow-up evaluation (i.e., serial X-rays).
We believe the use of the aforementioned radiographic checklist will be beneficial in maximizing the analysis of imaging films. Moving beyond plain films, MRI is generally most useful in further assessment for possible infection involving the foot and ankle. It is important for podiatric physicians to utilize imaging centers that are comfortable in the assessment of pedal images. It is also important for podiatric physicians to review these images. Implementing this action can greatly enhance the care you provide for your patients.
Dr. Evans is a Professor in the Department of Podiatric Medicine and Radiology at the William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago. He is a Fellow of the American College of Foot and Ankle Orthopedics and Medicine, and a Fellow of the American College of Podiatric Radiologists.
Dr. Fleischer is an Assistant Professor of Radiology and Medicine at the William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago. He is a Fellow of the American College of Foot and Ankle Surgeons.
Dr. Skratsky is an Assistant Professor in the Department of Podiatric Medicine and Radiology at the William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago.