During the four and one-half-year time period of the study, a total of 63 patients were diagnosed with 72 SSIs within one year of their breast surgery. During this time period 1298 admissions for breast reduction, mastectomy, or breast reconstruction were identified, resulting in an SSI rate of 4.9% within one year of surgery. Sixty-one SSIs in 57 patients involved the breast incision(s) (5 bilateral), while 10 patients had SSIs involving the donor site incision (TRAM or latissimus dorsi flap donor site). Four patients were diagnosed with both breast and donor site SSIs. 20 (27.8%) SSIs were classified as deep infections (involving the muscle or fascial layers). The deep SSIs in 17 patients involved breast implants (permanent or tissue expanders), and involved abdominal mesh in 2 patients following TRAM reconstruction. One patient had a deep breast SSI without an implant following delayed reconstruction with a pedicled TRAM flap. The remaining 52 (72.2%) SSIs in 44 patients were classified as superficial.
Four patients (6.3%) were diagnosed with SSIs during their original surgical admission, four patients were diagnosed as outpatients (6.3%), and the remaining 55 patients (87.3%) were diagnosed at readmission (inpatient or outpatient surgical) to the hospital. 41 of the 57 patients with SSIs involving the breast incision were diagnosed with infection ≤ 30 days after surgery (71.9%). Breast SSIs in 7 of the 16 patients (43.8%) diagnosed > 30 days after surgery required removal of an implant, while 10/41 (24.4%) of the breast SSIs diagnosed ≤ 30 days after surgery required implant removal. Five of the 10 SSIs (50%) involving the donor site incision were diagnosed > 30 days after surgery, and infection in 2 of the 5 patients required abdominal mesh removal (compared to 0/5 for donor site SSIs diagnosed ≤ 30 days after surgery).
The microbiology of breast and donor site SSI is shown in . Staphylococcus aureus (sensitive or methicillin-resistant) was isolated from 58% (25/43) of the patients with breast SSIs in whom cultures were obtained and in one patient with a donor site SSI. Pseudomonas aeruginosa was isolated in 19% of the breast SSI cases and from 3 of the 8 patients with donor site SSIs in which cultures were performed. Mixed cultures containing three or more organisms were obtained from 16% of patients with breast SSIs and 50% (4/8) of patients with donor site SSIs whose wounds were cultured. Coagulase-negative Staphylococci were isolated in pure culture from only 7% of patients with breast SSI.
Microbiology of SSI Associated with Breast Surgery
The comprehensive list of potential risk factors included in the case-control study is shown in , and selected results of the univariate analysis for categorical risk factors are shown in . Factors significantly associated with breast SSI in univariate analysis included ASA score greater than 2, obesity (BMI > 30) or morbid obesity (BMI > 35), mastectomy with 0–2 lymph nodes removed, mastectomy with 3 or more lymph nodes removed, diagnosis of current breast cancer, diagnosis of breast cancer at any time (current or past), diagnosis of carcinoma in situ, breast implant placed during surgery (tissue expander or permanent implant), reduction surgery only (decreased risk), and local anesthetic infiltration (decreased risk). Local anesthetic was used significantly more often in breast reduction surgery than in other types of breast surgical procedures (32/71 reductions vs. 40/254 non-reduction breast surgeries, p < .001). Local anesthetic was associated with significantly decreased risk of SSI only in the subset of patients who did not undergo mastectomy (p = .025), and not in the patients who had mastectomy (alone or in combination with immediate reconstruction, p = .734).
Potential Risk Factors for Surgical Site Infection After Breast Surgery Included in Study
Univariate Comparisons of Risk Factors in Patients With and Without Breast Incisional SSI
Receipt of a sub-optimal dose of prophylactic antibiotic was associated with increased risk of breast SSI. Patients included in this category included individuals with a BMI greater than 30 who received 1 gram of cefazolin instead of the recommended 2 grams, and patients with no prophylactic antibiotic recorded in the anesthesiologists’ or nurses’ notes. We included the 7 subjects without documented prophylactic antibiotic in the sub-optimal prophylactic antibiotic dose and sub-optimal prophylactic antibiotic timing, despite the fact that none of them developed an SSI. There was no association between sub-optimal timing of prophylactic antibiotic, defined as no antibiotic given or antibiotic given > 60 minutes before surgery or after incision, and SSI risk (p = .844). There was also no association between sub-optimal prophylactic antibiotic administration, including timing of the 1st dose and either lack of receipt of a 2nd dose for surgeries with duration > 6 hours, or administration of the second dose > 6 hours after incision in long surgeries (p = .743).
Among the continuous variables, only BMI was associated with significantly increased risk of breast SSI (p = .007, student’s T-test). There was a trend towards increased risk of breast SSI with increasing breast weight (p = .095; Mann-Whitney U test), although breast weight was missing in over half of the pathology reports. There was no association between breast SSI and number of lymph nodes removed, age, estimated blood loss, or duration of surgery.
There was no difference in preoperative blood glucose values between tested patients with SSI and control uninfected patients (p = .992). Postoperative blood glucose levels within 5 days after surgery were significantly higher in patients with SSI compared to uninfected control patients (mean value 223 vs. 174 mg/dL, p = .025, Mann-Whitney U test) although only 89 (27%) patients had postoperative glucose testing performed.
Multivariate logistic regression was used to identify independent risk factors for breast incisional SSI. The independent factors significantly associated with breast SSI included mastectomy, having a breast implant/expander placed during the surgery, sub-optimal dosing of the prophylactic antibiotic, receiving one or more units of transfused packed red blood cells or platelets during or after surgery, and a history of chest irradiation prior to surgery. Current or recent smoking was associated with marginally increased risk of breast SSI, while local infiltration of an anesthetic agent at surgery was associated with significantly decreased risk of breast SSI. No significant interactions between variables were identified. The c-statistic for the model was .782, indicating acceptable discrimination of the model.
Case subjects with breast SSIs caused by gram-negative organisms (n = 12) were also compared to the cases with breast SSI caused by only gram-positive organisms (n = 24), to determine if there were unique risk factors associated with gram-negative bacterial infections. Morbid obesity (BMI > 35) was more common in patients with gram-negative SSIs (any gram-negative bacteria in the cultures) compared to patients with SSIs caused solely by gram-positive bacteria (6/10 morbidly obese patients had gram-negative SSIs vs. 6/26 patients with BMI < 35, p = .053, Fisher’s exact test). Current smoking was more frequent in patients with gram-negative SSIs than in those with gram-positive SSIs (5/7 current smokers had gram-negative SSIs, vs. 2/7 with a history of previous smoking, and 5/22 in patients who never smoked, p for linear trend = .028). Patients with previous chest irradiation were more likely to have gram-negative SSIs (3/3 patients with previous chest irradiation had gram-negative SSIs vs. 9/33 with no history of chest irradiation, p = .031, Fisher’s exact test), as were patients who had undergone TRAM reconstruction (4/5 TRAM patients had gram-negative SSIs vs. 8/31 patients who did not have TRAM reconstruction, p = .034, Fisher’s exact test). There were no other significant differences between SSI case-patients with infection caused by gram-negative bacteria compared to those with gram-positive bacterial infections.
Half of the SSI case-patients with positive cultures of their breast wound were infected with bacteria susceptible to cefazolin (18/36). In addition, 10/19 (56%) obese patients with suboptimal dosing of prophylactic antibiotic were infected with bacteria susceptible to cefazolin.