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Magnified 20,000 times, this colorized scanning electron micrograph depicts a grouping of methicillin-resistant Staphylococcus aureus (S. aureus) (MRSA) bacteria. These S. aureus bacteria are from one of the first isolates in the United States that showed increased resistance to vancomycin as well. Note the increase in cell wall material seen as clumps on the organisms’ surface.

AAOS Now

Published 9/1/2018
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Terry Stanton

Study Indicates Swabbing for MRSA Does Not Lower SSI Rates in Pediatric Spine Patients

Presurgical testing for methicillin-resistant Staphylococcus aureus (S. aureus) (MRSA) colonization with intranasal swabs is performed commonly, but a study published in the Sept. 1 issue of the Journal of the AAOS involving pediatric spinal fusion (PSF) patients found that MRSA swab results did not change treatment or outcome.

The study comprised 1,200 patients who underwent PSF. Of the participants, 64.3 percent (n = 772) were female, and the average age was 13.8 years (range, 0–21 years). Among the cohort, 51.3 percent (n = 615) received vancomycin perioperatively and 48.8 percent (n = 585) received cefazolin (Ancef) and ceftazidime; the latter regimen reflects a change in the antibiotic protocol that occurred at the institution during the study period.

Of the total patient population, 2.3 percent (n = 28) had positive MRSA swab results and 3.1 percent (n = 37) had a surgical site infection (SSI), with 43 organisms identified. Of note, 83.8 percent (n = 31) of the SSIs were early infections (< 90 days after surgery) and 16.2 percent (n = 6) were late infections (> 90 days after surgery). Of the patients who had a positive MRSA swab, just one experienced an SSI, which was MRSA. MRSA was the causative organism in 8.1 percent (n = 3) of all SSIs. Methicillin-sensitive S. aureus (MSSA) alone was the causative organism in 13.5 percent (n = 5), and MSSA and another organism were the causative organisms in 8.1 percent (n = 3). The most frequent causative organism was E. coli (24.3 percent, n = 9).

Thus, the rate of SSI in patients who had positive MRSA swabs was 3.6 percent, or one in 28, which was not significantly different from what was observed in patients who had negative swabs (3.1 percent, n = 36/1,172, P = 0.88). Surgery for nonidiopathic scoliosis was associated with a higher rate of SSI (idiopathic: n = 2/570, 0.3 percent; nonidiopathic: n = 35/630, 5.6 percent,
P < 0.001), but patient diagnosis was not correlated with MRSA swab results (P = 0.49).

Of the patients administered vancomycin for prophylaxis, 2.6 percent (n = 16) had a positive MRSA swab, and 4.4 percent developed an SSI (n = 27). Of the patients administered cefazolin and ceftazidime for prophylaxis, 2.1 percent (n = 12) had a positive MRSA swab, and 1.7 percent developed an SSI (n = 10). There was no significant difference in the rate of positive MRSA swabs between the two groups (P = 0.53), but those who received cefazolin and ceftazidime had a significantly lower rate of SSI (P = 0.007).



source: the Centers for Disease Control

The big picture

Overall, the rate of MRSA nares colonization is approximately 0.8 percent in the U.S. population (n = 1/125). Many studies have shown a correlation between S. aureus nasal colonization and subsequent S. aureus infections, including SSIs. Traditionally, patients with positive nasal swabs for S. aureus have been treated with intranasal mupirocin ointment and/or chlorhexidine baths to reduce colonization before surgical procedures, and research has shown that this intervention reduces rates of SSI for a variety of surgical procedures. One double-blind, randomized, placebo-controlled study on the effects of preoperative intranasal mupirocin in 3,864 patients undergoing cardiothoracic, general, oncologic, gynecologic, or neurologic surgical procedures found that rates of nosocomial S. aureus SSI were markedly reduced in the group treated with mupirocin, especially among patients with nasal carriage of S. aureus. Another study analyzing the development of SSI in orthopaedic surgical patients receiving preoperative chlorhexidine baths and intranasal mupirocin ointment found a 100 percent reduction in S. aureus SSI and an approximately 50 percent reduction in the overall rate of SSI in the intervention group.

“These large, prospective studies provide strong statistical evidence in support of preoperative nasal decolonization with mupirocin,” the authors wrote. “Clinically, mupirocin and chlorhexidine are frequently featured in SSI prevention bundles for orthopaedic surgery, and the World Health Organization recently recommended decolonization with mupirocin before orthopaedic surgical procedures in adults colonized with MRSA; no recommendations could be made for the pediatric population.”

However, several studies have questioned the effectiveness of mupirocin in reducing SSIs. A study that found a reduction in nosocomial S. aureus infection with its use also found no difference in the overall SSI rate between the treated and placebo groups. That finding was reproduced in another double-blind, randomized, placebo-controlled study evaluating the effect of mupirocin nasal ointment on SSI rates in 614 patients undergoing orthopaedic surgery. The overall SSI rate was not markedly different between the treated and control groups; there was a reduction in SSIs caused by endogenous S. aureus in the treated group, but it was not statistically significant.

Despite mixed outcomes for the overall SSI rate, the studies all demonstrated successful reduction of endogenous S. aureus SSI in MRSA-colonized patients treated with intranasal mupirocin. “Thus, a preoperative MRSA nasal swab theoretically allows physicians to prophylactically treat colonized patients with mupirocin or other interventions, reducing rates of endogenous S. aureus SSI in this high-risk population,” the authors wrote. However, they noted, “our clinical experience suggests that the use of MRSA screening has little effect on the treatment course of surgical patients at our institution [Children’s Hospital of Los Angeles].”

At their institution, a swab is collected within 24 hours of admission, typically on the day of surgery, as mandated by the California Health and Safety Code. Consequently, the results are not received until well after the procedure is completed. “Thus, the colonization status of the patient cannot influence the preoperative or intraoperative antibiotic choice at our institution,” they wrote.

The authors also noted that the SSI rate in their patients decreased when the perioperative antibiotic regimen was switched from vancomycin, which covers MRSA, to ceftazidime and cefazolin, which do not. Although patients were no longer being treated as though they were colonized with MRSA, rates of infection decreased. “In part, this trend may be due to the high rate of E. coli SSI found in our population, which is covered by ceftazidime,” the authors surmised.

This study has several limitations. First is its retrospective nature. Second, the study design detected only cases of infection. Third, the use of a single nasal swab for MRSA, although standard, may not have identified patients who were intermittent colonizers.

The authors concluded, “Our study adds to the literature on MRSA SSI by raising questions about the standard protocol of preoperative MRSA nasal swabs. Although many studies have shown benefit from intranasal mupirocin for MRSA-colonized patients, certain randomized, prospective studies have not demonstrated this effect. The results of this study add to this doubt. MRSA colonization status seems to have no correlation with future SSI in this population, so the efficacy of decolonization procedure in reducing SSIs is debatable.”

The authors of “Methicillin-resistant Staphylococcus Swab Results Did Not Change Treatment or Outcome in Pediatric Spinal Fusion Patients” are Ena Nielsen, BA; Lindsay M. Andras, MD; Liam R. Harris, BA; and David L. Skaggs, MD, MMM.

Terry Stanton is the senior science writer for AAOS Now. He can be reached at tstanton@aaos.org.

References:

  1. J Am Acad Orthop Surg 2018;00;xx-xx.