When dealing with pressure sores, there is no definitive way of making a diagnosis of osteomyelitis short of a biopsy of the involved bone¹. 

In fact, only about a third of stage IV pressure ulcers (those extending to the bone) may be associated with osteomyelitis². In a study of pressure sores related to spinal cord injury or cerebrovascular accident, the clinical judgement of physicians with respect to the presence of osteomyelitis was accurate in only 56% of patients.  Only 3 of 21 patients with exposed bone had a diagnosis of osteomyelitis confirmed on biopsy³.

The “Probe to the Bone” bedside procedure has been studied primarily in diabetic foot infections with a recent systematic review reporting pooled sensitivity and specificity of 0.87 (95% confidence interval [CI], .75-.93) and 0.83 (95% CI, .65-.93), respectively⁴. Its performance in non-diabetic patients or those without a foot infection needs further study.

So in our patient, we should not assume a diagnosis of osteomyelitis; a bone biopsy is necessary for a definitive diagnosis.

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References

1. Larson DL, Gilstrap J, Simonelic K, et al. Is there a simple, definitive, and cost-effective way to diagnose osteomyelitis in the pressure ulcer patient? Plast Reconstr Surg 2011; 127:67.  https://www.ncbi.nlm.nih.gov/pubmed/21285771
2. Bodavula P, Liang SY, Wu J et al. Pressure ulcer-related pelvic osteomyelitis: a neglected disease? Open Forum Infect Dis 2015. DOI:10.1093/ofid/ofv112. https://www.ncbi.nlm.nih.gov/pubmed/26322317
3. Darouiche RO, Landon GC, Klima M et al. Osteomyelitis associated with pressure sores. Arch Intern Med 1994;154:753-58. https://www.ncbi.nlm.nih.gov/pubmed/8147679
4. Lam K, van Asten SA, Nguyen T, et al. Diagnostic accuracy of probe to bone to detect osteomyelitis in the diabetic foot: a systematic review. Clin Infect Dis 2016;63:944-8. https://www.ncbi.nlm.nih.gov/pubmed/27369321