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