Saturday, February 17, 2018

Surgically prepared skin is not sterile

Preoperative Skin-Surface Cultures Can Help to Predict the Presence of Propionibacterium in Shoulder Arthroplasty Wounds

These authors point out that propionibacterium species are commonly cultured from specimens harvested at the time of revision shoulder arthroplasty. These bacteria reside in normal sebaceous glands, out of reach of surgical skin preparation. The arthroplasty incision transects these structures, which allows Propionibacterium to inoculate the wound and to potentially lead to the formation of a biofilm on the inserted implant.

To help identify patients who are at increased risk for wound inoculation, they investigated whether preoperative cultures of the specimens from the unprepared skin surface were predictive of the results of intraoperative cultures of dermal wound-edge specimens obtained immediately after incision of he surgically prepared skin.

Sixty-six patients (mean age, 66.1 ± 9.4 years [range, 37 to 82 years]; 73% male) undergoing primary shoulder arthroplasty had preoperative cultures of the unprepared skin surface and intraoperative cultures of the freshly incised dermis using special culture swabs.

For the first 50 patients, a control swab was opened to air during the same time that the dermal specimen was obtained.

The results for female and male patients were characterized as the Specimen Propionibacterium Value (SpPV) (see this link). Here are some examples of the semiquantitative laboratory reports they used in the SpPV determination.

They then determined the degree to which the results of cultures of the skin surface specimens were predictive of the results of culture of the dermal specimens.

The skin-surface SpPV was ≥ 1 in 3 (17%) of the 18 female patients and 34 (71%) of the 48 male patients (p <0.001). The dermal SpPV was  ≥ 1 in 0 (0%) of the 18 female patients and 19 (40%) of the 48 male patients (p < 0.001).

None of the control samples had an SpPV of  ≥ 1  The predictive characteristics of a skin-surface SpPV of  ≥ 1 for a dermal SpPV of  ≥ 1 were as follows: sensitivity, 1.00 (95% confidence interval [CI], 0.82 to 1.00); specificity, 0.62 (95% CI, 0.46 to 0.75); positive predictive value, 0.51 (95% CI, 0.34 to 0.68); and negative predictive value, 1.00 (95% CI, 0.88 to 1.00).

A preoperative culture of the unprepared skin surface can help to predict whether the freshly incised dermal edge is likely to be positive for Propionibacterium. This test may help to identify patients who may merit more aggressive topical and systemic antibiotic prophylaxis.

This study shows that surgeons have the opportunity to use preoperative skin cultures to determine the likelihood that the shoulder arthroplasty wound will be culture-positive for Propionibacterium.

Comment: This study is important for at least four reasons: (1) when the skin is incised for a shoulder arthroplasty, the freshly cut dermal edge is often culture positive for Propionibacterium in spite of IV antibiotics and surgical skin preparation, (2) it is important that each surgeon know his/her rate of positive control cultures to better inform the interpretation of deep wound cultures, (3) the semiquantitative results of cultures appear to be more useful than simply reporting a culture as 'positive or negative', and (4) cultures of the unprepared skin surface can be predictive of the results of cultures of the freshly incised dermis.

The results of preoperative cultures of specimens from the unprepared skin surface may be helpful for anticipating the risk of positive intraoperative dermal wound-edge cultures that may, in turn, have a bearing on the risk of prosthetic bacterial colonization. This simple test may help to identify patients who may or may not merit more aggressive topical and systemic antibiotic prophylaxis
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