Diagnosing diabetic foot osteomyelitis: is the combination of probe-to-bone test and plain radiography sufficient for high-risk inpatients?

Aims: To investigate the accuracy of the sequential combination of the probe-to-bone test and plain X-rays for diagnosing osteomyelitis in the foot of patients with diabetes.

Methods: We prospectively compiled data on a series of 338 patients with diabetes with 356 episodes of foot infection who were hospitalized in the Diabetic Foot Unit of La Paloma Hospital from 1 October 2002 to 31 April 2010. For each patient we did a probe-to-bone test at the time of the initial evaluation and then obtained plain X-rays of the involved foot. All patients with positive results on either the probe-to-bone test or plain X-ray underwent an appropriate surgical procedure, which included obtaining a bone specimen that was processed for histology and culture. We calculated the sensitivity, specificity, predictive values and likelihood ratios of the procedures, using the histopathological diagnosis of osteomyelitis as the criterion standard.

Results: Overall, 72.4% of patients had histologically proven osteomyelitis, 85.2% of whom had positive bone culture. The performance characteristics of both the probe-to-bone test and plain X-rays were excellent. The sequential diagnostic approach had a sensitivity of 0.97, specificity of 0.92, positive predictive value of 0.97, negative predictive value of 0.93, positive likelihood ratio of 12.8 and negative likelihood ratio of 0.02. Only 6.6% of patients with negative results on both diagnostic studies had osteomyelitis.

Conclusions: Clinicians seeing patients in a setting similar to ours (specialized diabetic foot unit with a high prevalence of osteomyelitis) can confidently diagnose diabetic foot osteomyelitis when either the probe-to-bone test or a plain X-ray, or especially both, are positive.