Point-Counterpoint: Probe To Bone: Is It The Best Test For Osteomyelitis?

By John S. Steinberg, DPM, and Warren S. Joseph, DPM

      Yes. John S. Steinberg, DPM notes that the test is minimally invasive and easy to perform with sensitivity, specificity and positive predictive values similar to those of MRI and bone scans. By John S. Steinberg, DPM       It comes down to patient selection and common sense. In the properly selected patient, the “probe to bone” test can be a very strong diagnostic tool in determining the presence of osteomyelitis at an ulcer site in the diabetic foot.       Grayson, et. al., formally popularized and documented this important clinical test in 1995. Their article, “Probing to Bone in Infected Pedal Ulcers: A Clinical Sign of Underlying Osteomyelitis in Diabetic Patients” was published in the Journal of the American Medical Association (JAMA). This widely quoted work took place at New England Deaconess Hospital/Harvard Medical School in Boston. The study helped to confirm the widely believed thought that if an ulcer site contained exposed bone, then it was very likely the bone was infected.1       Some would call Grayson’s work on the probe to bone test “proving the obvious.” In addition, I think it would be safe to say that properly evaluating the depth of an ulcer is the standard of care in any setting. In most instances, this proper evaluation would include the use of a sterile, blunt probe to determine the quantitative and qualitative depth of the wound. Long before this publication in 1995, our profession and others have been advocating the use of a probe to bone test for the very same reasons outlined in the JAMA article. It just makes sense that if a bone were exposed within the wound, it would have a higher likelihood of being infected bone. The Grayson article simply provided an evidence-based confirmation to that clinical question.       In patients with diabetes and an open foot ulceration, there is strong academic debate as to what diagnostic modalities one should routinely order. Much of the literature points to MRI as a key modality when attempting to differentiate osteomyelitis from Charcot neuropathic osteoarthropathy. Others rely on nuclear medicine for imaging with a combination of Tc99 bone scans and either Indium 111 or HMPAO-labeled WBC scans. Consistent and reliable results in the setting of open ulceration are very complicated. In my opinion, these tests are often the cause of a significant delay in care while various members of the healthcare team debate the results and their interpretation.       If there is exposed bone that one can probe through a foot ulceration, I would suggest that osteomyelitis is present at the ulcer base until proven otherwise. Plain film radiographs will assist in clinical decision making, particularly if serial radiograph views are available to show bone changes over the course of several weeks to months.       In our practice, we generally take patients with a high clinical suspicion for osteomyelitis to the operating room for surgical debridement and bone biopsy. If the bone is soft or shows visible signs of infection (purulence, discoloration, erosions or fragmentation), we debride away this problem area of bone and send for culture and histopathology specimen. The podiatrist should perform serial debridement of bone and soft tissue until he or she is confident that the margins are clear of infection. One can send a proximal “clean margin” biopsy of bone if desired.

Emphasizing Proper Patient Selection

      Proper patient selection is paramount in deciphering the value and utility of a positive probe to bone test. In the setting of plantar, chronic, non-healing problem diabetic foot ulceration, the positive probe to bone test should lend a very high suspicion of underlying bone infection. However, in patients with acute traumatic wounds or surgically exposed bone in a wound site, the relationship of a probe to bone test would be very different. These circumstances significantly shorten the length of time the bone has been exposed and generally involve much more controlled circumstances, thereby yielding a lower overall infection rate of the involved bone.       In April 2006, Jeffcoate, et. al., published a letter in Diabetes Care citing some concerns about the patient population used in the Grayson study. Their concerns were that the statistical indicators may have been skewed by the high pretest probability of osteomyelitis in the study population.2 Indeed, the Grayson paper focused on diabetic inpatients with clinically overt infected ulcerations.1       Jeffcoate published his results of using the probe to bone test in a consecutive series of outpatients attending a multidisciplinary clinic. In a study of 104 outpatient foot ulcers, Jeffcoate found a probe to bone test sensitivity of 38 percent and a specificity of 91 percent in correlation to the confirmed cases of osteomyelitis.2 Although this presents a significantly lower sensitivity in comparison to Grayson’s article (66 percent), there is obviously still great clinical utility found in this test.1,2

Some Caveats To Keep In Mind

      A few words of caution should be mentioned when describing and advocating for the probe to bone test in diabetic foot ulcerations. First, the instrument should not create harm or have the chance to introduce infection to a clean wound base. It should be a sterile, blunt instrument such as a stainless steel nasal probe or the reverse plastic end of a cotton tipped applicator. Avoid wooden ended, cotton-tipped applicators to prevent breaking off shards of wood in the base of the wound.       A second point to mention about technique in probing a wound is in regard to the pressure that one should use. One can probe virtually any wound to bone by pressing hard enough and forcing one’s way through otherwise intact tissue planes. The intent of this clinical test is to identify bone that is already exposed but not readily visible within the wound base. Proper use of the test requires applying a minimal amount of pressure to the instrument and should not create new tunneling.

In Conclusion

      The probe to bone test is a low cost, minimally invasive and easy to perform clinical technique that requires a very small learning curve for success. The sensitivity, specificity and positive predictive values are similar to those of MRI and bone scans. While the increasing concerns of medical liability can lend to an increased frequency of ordering expensive lab and radiographic tests, the Grayson JAMA article gives substantial credibility to the simple bedside exam of probing a wound to see what the base contains. Common sense use of the test in proper patients is unquestionably helpful as a diagnostic tool.       Dr. Steinberg is an Assistant Professor in the Department of Plastic Surgery at the Georgetown University School of Medicine in Washington, D.C. References 1. Grayson ML, Gibbons GW, Baloh K, Levin E, Karchmer AW: Probing to bone in infected pedal ulcers: a clinical sign of underlying osteomyelitis in diabetic patients. JAMA 273:721-723, 1995. 2. Shone A, Burnside J, Chipchase S, Game F, Jeffcoate W: Probing the validity of the probe-to-bone test in the diagnosis of osteomyelitis of the foot in diabetes. Diabetes Care 29:945, 2006.       Editor’s note: For related articles, see “How To Detect Pediatric Osteomyelitis” in the July 2005 issue or “Point-Counterpoint: Is Osteomyelitis Primarily A Surgical Disease?” in the June 2003 issue.       No. Citing recent studies, Warren S. Joseph, DPM notes that while the probe to bone test can be useful for screening, the evidence does not support using the test alone to diagnose osteomyelitis. By Warren S. Joseph, DPM       While it has been called the “30-second bone scan” or the “$5 bone scan,” what it should not be called is “gospel.”       Ever since the paper by Grayson and colleagues was published in the Journal of the American Medical Association in 1995, the simple act of probing to bone has been generally mischaracterized as definitive evidence of the presence of osteomyelitis in a diabetic foot ulceration.1       Let me begin by saying that I personally probe any and all diabetic foot ulcerations that have evidence of a deep tract. However, I was doing this long before 1995. Just because I touch bone in the base, I do not commit my patients to the diagnosis of osteomyelitis and the requisite treatment thereof.

A Historic Perspective On The Probe To Bone Test

      Let me put this into perspective with just a little bit of history. In the early 1990s, I attended the Interscience Conference for Antimicrobial Agents and Chemotherapy (ICAAC). The diabetic foot team from the Boston-based New England Deaconess Medical Center (widely considered to be the foremost experts in the world at the time) was about to give an oral poster presentation on a study they had just completed on the concept of “probe to bone” as a diagnostic indicator of osteomyelitis. I was sitting in the back of the room with a few other infectious disease/diabetic foot authorities including my mentor, Jack LeFrock, MD, and the late Jim Tan, MD.       I had mixed feelings. My first thought was “You have got to be kidding … someone actually studied this?” Then I was excited to learn that a number of DPMs, including Geoff Habershaw, DPM, were actually represented on a study presented at this major conclave. (Remember, this was the early 1990s. A DPM on a poster presentation at a major medical meeting was an unusual event. How times have changed.) After the presentation, our group listening in the back of the room just looked at one other and shrugged our collective shoulders. We thought it was interesting but we were already doing this. It was not anything new. Little did we know that when the paper was eventually published, it would have such an impact and be so misinterpreted.       One must take the probe to bone test (PTB) into context with the patient’s overall clinical presentation. The original Grayson paper studied a very select group of more severely infected, hospitalized patients. They demonstrated that in this population, the probe to bone test had a high positive predictive value (PPV) of 89 percent.1 This was to be expected since the overall prevalence of osteomyelitis in their very select population was 66 percent. In other words, two-thirds of all of their patients actually had osteomyelitis so of course the probe was going to find osteomyelitis.       If one takes a probe, sticks it into an ulcer tract through an abscess and into mushy bone, there will be obvious osteomyelitis. In many cases, utilizing a simple X-ray, which would have been ordered anyway, would have also made the diagnosis.

What Does The Recent Literature Indicate?

      A number of recent studies have subsequently looked at a less severely infected population. In looking at this newer data, it is clear that not all bone that one can touch is infected. Lipsky, et. al., presented data at the Infectious Diseases Society of America meeting in 2004 that was a result of the study on the use of linezolid for treating infected diabetic foot ulcerations.2 In this population, the PPV of probe to bone was only 54 percent. Just 21 percent of the patients had osteomyelitis.       These data are extremely consistent with that presented by Lavery, et. al., at the American College of Foot and Ankle Surgeons meeting last year.3 In the Lavery study, the PPV was 57 percent and the osteomyelitis prevalence was 20 percent.       Finally, in the only other data that has been actually published to date, Shone, et. al., reported a PPV of 53 percent with an osteomyelitis prevalence of 20 pecent.4 As Shone writes in Diabetes Care, “These data emphasize that the predictive value of a positive PTB test in the original report was influenced by the high prevalence of osteomyelitis in the population studied.”4

Why The Probe To Bone Test Is Not The Final Word In Diagnosing Osteomyelitis

      What does all of this mean to the practicing podiatric physician? Probe to bone is still an easy, quick and useful screening test for the presence of osteomyelitis in a diabetic foot ulceration. However, just because one can touch bone does not mean that the diagnosis is definite and that the patient needs to be started on a long course of antibiotics and undergo possible surgical debridement.       The problem is that for the past 10-plus years, it has been taught as dogma that a positive PTB is an actual clinical diagnosis of osteomyelitis. That is dangerous. It has actually been used against doctors in malpractice actions. Most assuredly, there are patients who have undergone unnecessary antibiotic therapy based on a false positive of this “benign” test.       The misconceptions of this test are not limited to DPMs. In lecturing to all different specialists in the area of diabetic foot care, I have found this to be a pervasive impression. Just because something is thought to be immutable “fact” by a large and diverse population does not make it so. We are supposed to be practicing evidence-based medicine. The bottom line is that the evidence just does not support the use of the PTB, when used in and of itself, as a diagnostic marker for osteomyelitis.       Dr. Joseph is a Consultant in lower extremity infectious diseases and is a Fellow of the Infectious Diseases Society of America. He is an Attending Podiatrist at the Coatesville Veterans Affairs Medical Center in Coatesville, Pa. References 1. Grayson ML, Gibbons GW, Balogh K, Levin E, Karchmer AW. Probing to bone in infected pedal ulcers. A clinical sign of underlying osteomyelitis in diabetic patients. JAMA 1995 Mar 1;273(9): 721-3. 2. Lipsky BA, Armstrong D, Citron D, et. al. The SIDESTEP study of diabetic foot infections. IDSA Abstract 3776, 2004. 3. Lavery L, Armstrong DA, et. al. Osteomyelitis and the “probe to bone” test in persons with diabetes: relevant, reliable or relic? ACFAS Abstract 2006. 4. Shone A, Brunside J, Chipchase S Game F, Jeffcoate W. Probing the validity of the probe to bone test in the diagnosis of osteomyelitis of the foot in diabetes. Diabetes Care. 2006 April; 29(4) 945.       Editor’s note: For related articles, see the Wound Care Q&A column, “Roundtable Insights On Imaging Foot And Ankle Wounds,” in the May 2005 issue or the January 2003 supplement, “Managing Diabetic Foot Infections.”

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