SLR - June 2014 - Ciara Nicholson

Risk Factors for Methicillin-Resistant Staphylococcus Aureus in Diabetic Foot Infections

Reference: Lavery, L. La Fontaine, J. Bhavan, K. Kim, P. Williams, J. Hunt, N. Risk factors for methicillin-resistant Staphylococcus aureus in diabetic foot infections. Diabetic Foot & Ankle. March 2014; 5: 256-258.

Scientific Literature Review

Reviewed By: Ciara Nicholson, DPM
Residency Program: Saint Vincent Hospital, Worcester, MA

Podiatric Relevance: An average of one-in-four diabetic patients will develop an ulcer during their lifetime, half of which will become infected. The efficacy of antibiotic treatment and precautionary measures ultimately depend upon early diagnosis of infecion.  Staphylococcus aureus is the most common organism isolated in diabetic foot infections, and because 46 percent of S. aureus isolates are methicillin-resistant (MRSA), identifying risk factors for MRSA could improve overall treatment and prevention. 

Methods: The clinicians’ aim in this retrospective, cohort study was to evaluate the risk factors for methicillin-resistant Staphylococcus aureus (MRSA) in patients hospitalized for diabetic foot infections. A total of 57 consecutive patients were evaluated. Patient data abstracted for analysis included nasal swabs at the time of admission, and anaerobic and aerobic wound cultures from deep tissue specimens collected in the operating room. The diagnosis of infection was based upon criteria from the Infectious Disease Society of America and the International Working Group on the Diabetic Foot. The parameters evaluated included: age, gender, diabetes type, glycated hemoglobin, amputation, neurovascular status, tobacco use, previous hospitalization, nursing home residence, and antibiotic therapy in the 12 months prior to admission. Positive predictive value (PPV), negative predictive value (NPV), sensitivity, and specificity of nasal swabs, history of MRSA diabetic foot infections, and multidrug-resistant organisms were then calculated to diagnose MRSA foot infection. 

Results: Of the 57 patients evaluated, the prevalence of S. aureus in diabetic foot infections was 42.1 percent . Seventy percent of these isolates were methicillin resistant, resulting in an overall prevalence of 29.8 percent  in the population studied. In relation to MRSA versus non-MRSA diabetic foot infections, there was no reported difference in the length of hospitalization, bone infection, or amputation. Three risk factors associated with MRSA infection were identified. These included the presence of multidrug-resistant organisms, history of MRSA diabetic foot infection, and a positive MRSA nasal culture (p < 0.05). Positive predictive value (PPV) and negative predictive value (NPV) of nasal colonization with MRSA in relation to MRSA diabetic foot infections were 66.7 percent  and 80.0 percent , respectively (sensitivity 41 percent , specificity, 90 percent ). The PPV and NPV for a previous MRSA foot infection were 100 percent  and 75.5 percent  (sensitivity 23.5 percent , specificity 47.5 percent). The positive likelihood ratio were reported as follows: positive nasal swab was 4.11, previous history of MRSA infection was 0.23, and multidrug resistance was 1.73. Negative likelihood ratios were reported as: positive nasal swab was 0.65, previous history of MRSA infection was 0.76, and multidrug resistance was 0.33.

Conclusions: The authors of this study report a positive likelihood ratio of 4.11, indicating that a positive MRSA nasal swab is four times more likely to occur in patients with a diabetic foot infection involving MRSA. The authors advocate for the use of nasal swabs as an easy test to identify those patients with diabetic foot infections in which MRSA may be involved.  

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