Keys To The Diagnostic Workup For Patients With Diabetic Foot Infections

Author(s): 
Valerie L. Schade, DPM, AACFAS, and Nathan S. Higa, DPM, AACFAS

Assessing Imaging Studies

Plain film radiographs. Plain film radiographs are the first line in imaging evaluation of a diabetic foot infection. Bilateral films are recommended to assess for subtle differences using the contralateral foot as a “control.”28 Radiograph review should occur from the skin to the bones to ensure all soft tissue and osseous findings are noted.29 Soft tissue emphysema should prompt rapid surgical intervention.

   Osseous changes indicating possible osteomyelitis will not be evident on radiographs until there is a 35 to 50 percent reduction in osseous density. It can take a minimum of 10 to 14 days for initial osseous changes, such as periosteal reaction, to be evident.28,30 One can assume the duration of ulceration and infection to be more than 30 days when frank osseous destruction is present on radiographs that correlate to the osseous structures that lie beneath the ulceration on the foot.

   Plain film radiographs have a low sensitivity (54 percent) and specificity (68 percent) for detecting acute osteomyelitis. The primary factor for this is the time at which radiographs were taken due to the lag time from osseous infection to positive radiographic findings.30

   Nuclear medicine imaging studies (Tc99 and indium-111 WBC labeled scan). Three phase bone scans have an 81 percent sensitivity and a 28 percent specificity for detecting osteomyelitis of the lower extremity. The addition of an indium-111 labeled leukocyte scan decreases sensitivity to 74 percent and increases specificity to 68 percent. Despite this improvement from a three phase bone scan alone, an indium-111 labeled leukocyte scan still only has a low to moderate accuracy in diagnosing osteomyelitis.30

   Capriotti and colleagues performed a meta-analysis of nuclear medicine imaging studies in the diagnosis of osteomyelitis.31 Their results were divided into an imaging algorithm for forefoot and rearfoot ulcerations. The study authors found the most optimal use for nuclear medicine studies is in clinical situations in which there is a low suspicion for acute osteomyelitis. If radiographs were negative for any concerning findings, study authors recommended a three phase bone scan. If the results of the three phase bone scan were positive, the study authors recommended WBC scintigraphy. If this exam was positive, Capriotti and co-workers recommended obtaining an MRI or performing a bone biopsy to confirm the presence or absence of osteomyelitis.31

   Magnetic resonance imaging (MRI). Researchers have found MRI to be the gold standard for imaging acute osteomyelitis with a sensitivity of 90 percent and a specificity of 79 percent.30 It is the one imaging study with the best predictive value for acute osteomyelitis. A physician can feel confident in ruling out osteomyelitis if the MRI is negative for any findings concerning for osteomyelitis. The main limiting factor in obtaining MRI is the cost.1

Taking The Final Step Toward Determining Infection Severity

Once a thorough workup of a patient who presents with a diabetic foot infection is complete, all the information one has gathered can help in determining the severity of the infection, prompting initiation of appropriate medical and/or surgical therapy.

   Two classification systems exist in regard to determining the severity of infection. The International Working Group on the Diabetic Foot developed the first classification system in 2003.32 The Infectious Disease Society of America (IDSA) developed the second system in 2004.33 Both classification systems are similar and rely on local and systemic signs to assess for severity of infection.

   Lavery and colleagues validated the IDSA classification system in 2007.9 In their retrospective review of 1,666 patients with a diabetic foot infection, they found that patients who presented with a moderate or severe infection have an increased incidence of hospitalization and amputation.9

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