We conducted a retrospective cohort study using electronic health record and billing data from one academic and one community hospital. We identified mastectomy admissions among women aged ≥18 years from 7/1/2010 to 6/30/2015 using International Classification of Diseases, 9^th Revision procedure codes (Supplement Table 1) and verified mastectomy by surgeon description.

Potential SSIs were identified by selecting records to review based on performance of a microbiology culture or diagnosis or procedure code suggestive of a wound complication within 180 days after mastectomy (Supplement Table 1). Infections were verified by review of outpatient and hospital records for signs/symptoms of infection, intervening procedures, expansion history, and microbiology data.

We defined clinically apparent infections as infections that met the pre-2013 NHSN definition or documented signs consistent with SSI (e.g., cellulitis necessitating implant removal). 180-day SSI rates were calculated using the pre-2013, 2020 NHSN, and clinical definitions (Supplement Table 2). Fisher’s exact tests were used to compare implant removal rates and physician documentation of infection in SAS version 9.4 (SAS Institute Inc., Cary, NC). The study was approved by the Washington University Human Research Protection Offices with a waiver of informed consent.

A total of 1,902 women underwent mastectomy, of whom 148 developed clinically apparent infection at the surgical site within 180 days after operation. Infections in 69 (46.6%) women met the 2020 NHSN SSI criteria. Infections in 102 women (68.9%) met the pre-2013 NHSN criteria, due to 33 infections after implant reconstruction meeting the earlier SSI criteria (Table 1).

Of the 148 women with clinically apparent infections, 23 (15.5%) underwent mastectomy only and 125 (84.5%) underwent immediate reconstruction. One hundred (67.6%) women had placement of tissue expander(s), 10 (6.8%) had permanent implant(s), and 15 (10.1%) underwent autologous flap reconstruction.

The reasons for exclusion of clinically apparent infections based on 2020 NHSN criteria (not mutually exclusive) included diagnosis of a superficial incisional SSI > 30 days post-mastectomy (n=22), diagnosis of a deep incisional or organ-space SSI > 90 days (n=19; 2 deep incisional, 17 organ-space), manipulation of the surgical site after mastectomy in the absence of signs/symptoms of infection (i.e., surgical debridement (n=18), needle aspiration of seroma (n=9), tissue expander access (n=40)), and negative intraoperative cultures (n=14) (Table 2). Methicillin-sensitive Staphylococcus aureus was the most commonly isolated etiologic organism, regardless of the onset timing of infection (Supplement Table 3).

Tissue expanders were accessed in 40 women before the onset of clinically apparent infection, a median two times (range: 1–7), resulting in exclusion of 13 infections (32.5%) using the 2020 NSHN criteria. Expanders were last accessed a median 16 days (range 5–132 days) before infection onset. Nine women had 1–3 documented needle aspirations of a seroma, with the last aspiration a median of 11 days (range 1–48 days) before infection onset. Eighteen women had debridement of necrotic skin flaps a median of 13.5 days (range 3–111 days) before infection onset.

Of the 14 women who failed to meet the NHSN criteria for SSI due to a negative intraoperative culture, all had been treated with antibiotics (median 3 days) in the two weeks prior to implant removal. Prior antibiotics before the negative intraoperative cultures included intravenous vancomycin, piperacillin/tazobactam, and/or clindamycin (n=8 women), or oral clindamycin, doxycycline, cephalexin, and/or sulfamethoxazole-trimethoprim (n=6).

Of the 33 women with infection after implant reconstruction that met the pre-2013 but not the 2020 NHSN SSI definition, 13 had an organ-space SSI > 90 days, and 7 met the pre-2013 definition of deep incisional or organ space SSI (3 deep incisional, 4 organ-space) solely due to surgeon diagnosis at the time of implant removal.

Ninety-four (85.5%) of the 110 women who had expander or permanent implant reconstruction had an infection at the surgical site diagnosed by a surgeon and/or infectious diseases physician. The percentage of women with physician-documented infection was the same in women whose infection met the 2020 NHSN SSI criteria (86.8%; (46/53)) or whose infection did not meet 2020 criteria (84.2% (48/57); p=0.790).

Overall, 92.7% (102/110) of women who developed clinically apparent infection following immediate implant reconstruction had their implants removed ; this was 92.5% (49/53) vs. 93.0% (53/57) among those who met vs. didn’t meet the 2020 NHSN SSI criteria (p=1.0).

We reviewed records of women post-mastectomy with or without reconstruction and found that 53% of women with clinical infection did not meet the 2020 NHSN SSI definition. The number of infections after implant reconstruction that met NHSN criteria decreased substantially after the criteria changes, which restricted the surveillance period in 2013 and definitions of deep incisional and organ space SSIs in 2014, and excluded infections after access of tissue expanders in 2017. The differential impact of the definition changes on implant reconstruction SSIs is important, as immediate reconstruction has steadily increased during the past two decades and tissue expanders account for the majority of reconstruction procedures.⁴ The SSI rate post-mastectomy is typically at least twice as high with vs. without reconstruction,⁵ which is obfuscated by changes to the surveillance definition that have differential impact on post-implant infections.

Manipulation of the surgical site post-mastectomy may contribute to the risk of infection, depending on the frequency and adherence to asepsis during manipulation. This is especially true for breast tissue expander reconstruction, since the port is accessed frequently for saline injection during expansion. Surveillance with feedback of all infections involving the surgical site post-mastectomy may alert surgeons and infection prevention specialists of sub-optimal practices, particularly regarding tissue expansion, which could be targeted for infection prevention.^6,7

In our cohort, systemic antibiotic therapy was administered in women with implant reconstruction who presented with cellulitis, in hopes of salvaging the implant.⁸ This likely led to negative intraoperative cultures at the time of implant removal in the 14 women who met the clinical but not 2020 NHSN SSI definition. Most of these infections met the pre-2013 NHSN definition due to physician diagnosis. Re-evaluation of physician diagnosis to define SSI may need to be considered given the frequent empiric antibiotic treatment in this population.

We demonstrated that implant loss was virtually the same in women with clinically diagnosed SSIs, regardless of whether the 2020 NHSN definition was met or not. Implant loss is important from a patient perspective, since it results in additional morbidity, procedures, hospital costs, and delays in completion of adjuvant therapy.⁹

Our study of two academically-affiliated hospitals may not reflect all community practices. The substantial changes in the NHSN SSI definitions in the past decade,^2,3,10 result in underreporting of potentially preventable infections and underestimation of infectious morbidity post-mastectomy. Comprehensive infection surveillance, particularly after breast implant reconstruction, is essential to provide women with accurate information about complication risks and determine the need for additional infection prevention strategies.