Issues And Answers In Diagnosing Diabetic Foot Osteomyelitis

Kelly Pirozzi, DPM, and Andrew J. Meyr, DPM

Any podiatric medical student could tell you that the “gold standard” for the diagnosis of osteomyelitis is bone biopsy. This is obviously an important diagnosis to reach with a high degree of confidence. From a clinical standpoint, we utilize bone biopsy results: to decide on the course and duration of antibiotic therapy; to determine the need for subsequent wound debridement; as an indication for hyperbaric oxygen therapy; to assess the timing of wound closure; and to ascertain the level of lower extremity amputation.1-13

   From a research standpoint, we use bone biopsy as the standard reference marker when examining the reliability of other diagnostic techniques for osteomyelitis including: clinical findings (such as the “probe to bone” test); laboratory values (such as erythrocyte sedimentation rate (ESR) and C-reactive protein); and advanced imaging analyses (such as magnetic resonance imaging (MRI) and bone scans).

   The somewhat surprising problem is that there is actually no standardized or uniform definition for the term “bone biopsy.” To demonstrate this point, consider the classic 1995 study by Grayson and colleagues.14 That same podiatric medical student from our introductory sentence could probably also tell you that this study demonstrated an 89 percent positive predictive value of the “probe to bone” test for the diagnosis of osteomyelitis.

   However, think about the study design utilized by the authors in this study. There were really two variables for the authors to consider: (1) whether the pedal ulcer probed to bone and (2) whether the underlying bone had osteomyelitis. Grayson and co-workers did not utilize the probe to bone test to primarily diagnose osteomyelitis in this study so the authors needed some other “standard” way to arrive at the diagnosis of osteomyelitis in the first place. In this study, the authors chose to utilize the histopathologic analysis of bone as their standard reference marker and definition for the term bone biopsy.14 In other words, a pathologist looked at a specimen of bone under a microscope and arrived at a diagnosis.

   In recent years, several studies (most notably one by Lavery and co-authors in 2007) have attempted to replicate the findings of the Grayson study.15 Lavery and colleagues concluded that although the probe to bone test may remain an important diagnostic tool, it probably doesn’t have as high a positive predictive value as initially thought. Which standard reference marker did these authors utilize? Bone biopsy of course. However, in this case, the authors defined bone biopsy as the microbiologic analysis of bone. In other words, they sent a specimen of bone for culture and sensitivity analysis. No pathologist was required.

   Accordingly, we have two important studies with both analyzing bone biopsy as the standard reference marker for the diagnosis of osteomyelitis but utilizing two different definitions and diagnostic tests for the term “bone biopsy.” The intention of this brief column is to review the intrinsic problems with both the microbiologic and histopathologic analysis of bone, and emphasize a comprehensive diagnostic approach to diabetic foot osteomyelitis.

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