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By Brian McCurdy, Senior Editor

How Effective Is The PTB Test In Diagnosing Osteomyelitis?

     There has been some recent debate within the profession about the effectiveness of the probe-to-bone (PTB) test in diagnosing osteomyelitis. A new study in Diabetes Care has found that the PTB test has a relatively low positive predictive value when it is utilized for diabetic patients with foot wounds.      The two-year study tracked 1,666 patients with diabetes who underwent regular foot exams and were instructed to come to the clinic if they developed signs of lower-extremity complications. For those with infections, researchers compared PTB test results to a culture of infected bone. Study authors said the PTB test was positive if the bone or joint were palpable and they defined osteomyelitis as a positive bone culture.      Over 27.2 months, 247 patients developed foot wounds and 151 patients had 199 foot infections, according to the study. Researchers found osteomyelitis in 30 patients or 12 percent of patients with foot wounds and 20 percent of patients with foot infections. For all diabetic foot wounds, the study says the PTB test was “highly sensitive” (0.87) and specific (0.91). The positive predictive value was 0.57 but the negative predictive value was 0.98. Investigators concluded that while the PTB test has a low predictive value for those with diabetic foot ulcers, a negative PTB test may exclude the diagnosis of osteomyelitis.      Study co-author Benjamin Lipsky, MD, says one should conduct the PTB test as part of a thorough evaluation of any open foot wound. As one probes to find the ulcer depth and searches for any foreign bodies, he notes the ease of checking for the presence of palpable bone.      Study co-author Lawrence Lavery, DPM, MPH, says although another simple bedside test would be effective, there is nothing in the pipeline. In many cases, he says diagnosing osteomyelitis comes down to clinical judgment and clinical correlation is “an essential component.” He notes that testing is one aspect of the process and labs, imaging studies, patient characteristics and the wound all contribute to the diagnosis of osteomyelitis.

When Are Additional Diagnostic Tests Needed?

     As Dr. Lavery notes, bone biopsy is still the gold standard for osteomyelitis diagnosis and MRI or bone scans must meet or compare against the bone biopsy standard. “The risk is that osteomyelitis will often be over-diagnosed with imaging techniques. Previous surgery, Charcot arthropathy and trauma can all show false positives,” says Dr. Lavery, a Professor in the Department of Surgery at Texas A&M Health Science Center College of Medicine.      If a DPM is assessing for osteomyelitis in the office, study co-author David G. Armstrong, DPM, PhD, advises considering other tests, such as serial radiography, MRI or bone biopsy, along with the PTB, depending on the circumstance. Dr. Armstrong notes that the majority of the time, osteomyelitis by itself is not an acute emergency and infection control, along with biopsy, MRI or serial radiographs (depending on the circumstance), will “win the day.”      Often, one will make a presumptive diagnosis long before securing an expensive test, says Dr. Armstrong, a Professor of Surgery, Chair of Research and Associate Dean at the William M. Scholl College of Podiatric Medicine at Rosalind Franklin University of Medicine.       “The clinician must weigh the expense of failing to make the right diagnosis, treating with the wrong antibiotic or wrong duration, or encouraging antibiotic resistance against the cost of any imaging studies,” says Dr. Lipsky, a Professor of Medicine at the University of Washington School of Medicine.      For almost all patients with a diabetic foot ulcer, he feels plain X-rays are useful. However, when one has a reasonably high suspicion of osteomyelitis, perhaps a greater than 30 percent likelihood, and plain X-rays are not helpful, Dr. Lipsky says it is appropriate to consider more expensive imaging studies. In many instances, he says a bone biopsy is the best way both to definitively diagnose osteomyelitis and to determine the etiology pathogen and their antibiotic susceptibilities.

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