SGEM#27: Bad to the Bone (Diabetic Osteomyelitis)

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Date: 10 March 2013

Case Scenario: 62 year-old man presents to the emergency department feeling weak. His vital signs at triage are normal but his glucometer reading is high. He is a known type 2 diabetic and states his sugars have been running a little high lately. After conducting an appropriate history and directed physical examination you have not yet determined the cause of his generalized weakness. There is nothing to suggest respiratory or urinary tract infection. Before leaving the room you take off his socks to check out his feet. What you see and smell is a diabetic foot ulcer on the plantar aspect of his left foot.

Question: Does this patient with diabetes have osteomyelitis of the lower extremity?

Background: Complications from diabetes are common presentations to the emergency department. These emergency department presentation will likely go up with the world wide prevalence of diabetes projected to increase to 333 million by 2025. More than 30% of diabetics in the US have lower extremity disease including 7.7% with ulcers. These ulcers can lead to infection, osteomyelitis and ultimately limb amputation. Diabetic patients are ten times more likely than non-diabetics to require osteomyelitis-related limb amputations. The first step in preventing such amputations would be identify and treating patients with diabetes. Milne WK and Carpenter RC Annals of Emerg Med, May 2009

Reference: Butalia et al. Does this patient with diabetes have osteomyelitis of the lower extremity? JAMA 2008;299:806-813

Population: Diabetic patients with foot infections and suspected osteomyelitis
Intervention: N/A
Comparison: N/A
Outcome: Diagnostic accuracy (sensitivity, specificity, likelihood ratio) for bedside physical exam, lab tests (WBC, ESR, CRP), plain film imaging, and other imaging tests

Results: No studies looked at the precision of signs or symptoms. Temperature was only reported in one poor quality study. It was possible to report the test characteristics of those shown below:

Authors’ Conclusions: “An ulcer area larger than 2cm, a positive probe-to-bone test result, an erythrocyte sedimentation rate of more than 70 mm/h, and an abnormal plain radiograph result are helpful in diagnosing the presence of lower extremity osteomyelitis in patients with diabetes. A negative MRI result makes the diagnosis much less likely when all of these findings are absent. No single historical feature or physical examination reliably excludes osteomyelitis. The diagnostic utility of a combination of findings is unknown.”

This review attempted to summarize the test characteristics of the history, physical examination, routinely available laboratory tests and imaging studies and MRI for diagnosing osteomyelitis in diabetic patients. The review had a number of limitations including a search strategy of English only manuscripts. Of the 21 studies included only 8 were prospective and 11 were judged to be of poor quality. “Clinical gestalt” was never clearly defined. No assessment of reliability (Kappa) for subjective measures were reported. None of these studies were ED based raising problems of external validity. No attempt was made to create a clinical decision rule/instrument using a combination of the tests. And finally, no patient oriented outcomes were assessed in this diagnostic accuracy study.

EBM Point: This review included studies with verification/work-up bias. The diagnostic performance of a test is determined by comparing it to the gold standard or reference standard. This is most accurate established test for the disease in question. Bone biopsy was considered the reference standard for osteomyelitis in this review relative to ulcer size.

However, only those patients believed to have a high likelihood of disease are fully worked up (ie, undergo bone biopsy). This may mean that those patients with a positive result on the test being evaluated (ulcer size) are more likely to have the full evaluation, including bone biopsy, which leads to false “verification” of ulcer size by ensuring that those with larger ulcers are more likely to undergo bone biopsy, whereas those with smaller ulcers will either not be included in the data or will be presumed, perhaps falsely, to be disease negative. The main result of this bias will be incorrect elevation of the tests sensitivity and specificity.

To eliminate this work-up or verification bias all patients with diabetic foot ulcers regardless of its size would need to be biopsied for the presence of osteomyelitis. This would be both expensive and invasive making it making researchers less likely to obtain a bone biopsy.

BEEM Bottom Line: First thing to do when trying to diagnose osteomyelitis of the lower extremity is determining whether or not the patient is diabetic. An ulcer size of >2cm and a positive bone-to-probe test each significantly increases the LR of a DM osteomyelitis. Clinical gestalt was almost a useful as these two things. An ESR>70 strongly suggests the diagnosis in the correct clinical setting. An abnormal plain film can increase the probability, only MRI substantially reduces the LR. No single physical exam finding or test reliably excludes the diagnosis of osteomyelitis in a diabetic patient.

Dr. Jo-Ann Talbot

Case Resolution: You order standard blood work on this diabetic man including an ESR which comes back elevated at 77. Plain films are also performed showing some focal loss of trabecular bone and periosteal reaction. You make a diagnosis of osteomyelitis and start the man on appropriate antibiotics and consult orthopaedics.

KEENER KONTEST: No winner last week:(

Dr. Suneel Upadhye

Be sure to listen to the podcast to hear this weeks Keener Kontest question. Email your answer to TheSGEM@gmail.com. Use “Keener Kontest” in the subject line. First one to email me the correct answer will win a cool skeptical prize:)