Probe-to-bone test for diagnosing diabetic foot osteomyelitis: reliable or relic?

OBJECTIVE: We sought to assess the accuracy of the probe-to-bone (PTB) test in diagnosing foot osteomyelitis in a cohort of diabetic patients with bone culture proven disease.

RESEARCH DESIGN AND METHODS: In this 2-year longitudinal cohort study, we enrolled 1,666 consecutive diabetic individuals who underwent an initial standardized detailed foot assessment, followed by examinations at regular intervals. Patients were instructed to immediately come to the foot clinic if they developed a lower-extremity complication. For all patients with a lower-extremity wound, we compared the results of the PTB test with those of a culture of the affected bone. We called PTB positive if the bone or joint was palpable and defined osteomyelitis as a positive bone culture.

RESULTS: Over a mean of 27.2 months of follow-up, 247 patients developed a foot wound and 151 developed 199 foot infections. Osteomyelitis was found in 30 patients: 12% of those with a foot wound and 20% in those with a foot infection. When all wounds were considered, the PTB test was highly sensitive (0.87) and specific (0.91); the positive predictive value was only 0.57, but the negative predictive value was 0.98.

CONCLUSIONS: The PTB test, when used in a population of diabetic patients with a foot wound among whom the prevalence of osteomyelitis was 12%, had a relatively low positive predictive value, but a negative test may exclude the diagnosis.