Surgical site infections (SSIs) are among the most common healthcare-associated infections in the United States.¹ The most recent Centers for Disease Control and Prevention (CDC) guidelines for the prevention of SSI recommend against administration of prophylactic antibiotics in clean surgeries after the surgical incision is closed, even in the presence of surgical drains,² due to lack of data showing benefit for this practice. Other organizations recommend discontinuing prophylactic antibiotics at the completion of surgery.^3-5 In practice, compliance with antibiotic discontinuation within 24 hours of surgery was found to vary from 58–100% in a study of National Surgical Quality Improvement Program hospitals.^6,7

In a recent systematic review, the pooled incidence of SSI after spine surgery was 3.1% (range 0.2%–16.1%). Variation in SSI rates is associated with the indication for surgery (i.e., scoliosis), instrumentation, duration of surgery, and surgical approach.⁸ There is limited information in the literature regarding post-operative antibiotic prophylaxis^9-16 and very sparse information on post-discharge antibiotic prophylaxis¹⁷ and its impact on SSI rates after spinal fusion procedures. Most studies analyzing the impact of post-operative/post-discharge antibiotics report no difference in SSI rates between short versus prolonged antibiotic prophylaxis after spine procedures.^9-15,17 Overall, high-quality data are lacking, as these studies in general were underpowered to detect a difference in SSI rates given small sample sizes and low incidence of SSI.

A concern with the use of post-operative antibiotic prophylaxis is exposure of patients to unnecessary antibiotics. Unnecessary antibiotics may result in additional costs, adverse drug events, selection of antibiotic-resistant bacteria,¹⁸ and the development of Clostridioides difficile infection.^19-22 The goal of our study was to determine the prevalence of post-discharge prophylactic antibiotic use and identify patient, operative, and surgeon factors associated with use in a cohort of adults undergoing spinal fusion at three academically-affiliated hospital study sites.

The initial study cohort included 9,502 spinal fusion admissions. A total of 850 admissions were excluded for the following: infection coded during the fusion admission (n = 599), spinal fusion was not performed based upon further review (n = 235), or patient was discharged on IV antibiotics (n = 16). The final study cohort included 8,652 spinal fusion admissions: 4,263 (49.3%) at study site 1, 1,589 (18.4%) at site 2, and 2,800 (32.4%) at site 3. None of the sites performed routine detection of intranasal Staphylococcus aureus colonization or decolonization, and only site 2 recommended preoperative bathing with chlorhexidine for spinal fusion patients during the study period.

In the final cohort of fusion admissions, the median age of patients was 58 years (interquartile range 49–67); 4,717 (54.5%) were female; 676 (7.8%) had trauma (i.e., sustained a fracture/dislocation and/or fall); and 301 (3.5%) had underlying cancer (Table 2).

We determined the prevalence, variation, and factors associated with post-discharge prophylactic antibiotic use after spinal fusion at three academic medical center study sites. We found a low prevalence of post-discharge prophylactic antibiotic use, but variation by study site ranging from 2.2–5.1% and by surgeon ranging from 0.5–9.8% among surgeons who performed at least 50 spinal fusions per year. Patient and operative factors associated with post-discharge antibiotic use included ASA class of 3 or higher, lymphoma, solid tumor, morbid obesity, paralysis, hematoma/seroma, type of intraoperative antibiotic, surgery on the thoracic spine, and longer length of stay.

Of the patient-level factors associated with increased use of post-discharge prophylactic antibiotics, morbid obesity,^26,27 cancer,^28,29 higher ASA score,^26,29 and paralysis³⁰ are known to be associated with increased risk of SSI after spinal fusion. Thoracic surgery was associated with increased antibiotic use, consistent with invasiveness of spine surgery³¹ and correspondingly with increased risk of SSI.³² The association of these factors suggest that surgeons may be assessing infection risk in patients and selectively prescribing continuation of prophylaxis for higher risk patients in an attempt to prevent SSI. Interestingly, there was more consistency in post-discharge use of first-generation cephalosporin/penicillin in persons who received intraoperative cefazolin than for use of an anti-MRSA antibiotic in persons who received intraoperative vancomycin or clindamycin prophylaxis. This suggests that other factors outweighed simple continuation of surgical prophylaxis at hospital discharge.

While surgeons may be targeting high risk patients for post-discharge antibiotics, continuing antibiotics after incision closure is not recommended by the CDC, European Centre for Disease Prevention and Control, or the World Health Organization.^2-4 The North American Spine Society does make an exception for complex procedures including trauma, diabetes, obesity, and multilevel instrumented surgery, but does not specify a recommended duration or if prophylaxis should be discontinued before discharge.⁵ There is little evidence that use of prolonged prophylaxis among fusion patients is associated with benefit.^9-17 A single-institution study found significantly reduced SSI rates in patients undergoing instrumented spine surgery who were given antibiotic prophylaxis for 72 hours (2009–2014) compared to a single perioperative dose (2003–2008), but this finding has not been replicated in other cohorts and the investigators did not assess utilization of post-discharge antibiotics.¹⁶ Inabathula and colleagues reported a significant decrease in infections following hip and knee arthroplasty after implementing oral antibiotic prophylaxis for 7 days post-discharge among patients at high risk for infection at one academic medical center.³³ In this study, however, 60% and 70% of the hip and knee cohorts, respectively, were considered high risk, calling into question the benefit of stratification. In addition to lack of evidence for benefit among spinal fusion patients, there is clear potential for harm with unnecessary use of antibiotics, including selection of antibiotic-resistant bacteria,¹⁸ C. difficile infection,^19-22 and acute kidney injury.²⁰ In our study, the incidence of C. difficile did not differ by discharge antibiotic use, however our study was not adequately powered for this comparison.

Inclusion of surgeon factors in the hierarchical model did not significantly improve the model fit, although in this model surgeons who performed 50–99 spinal fusions per year were 1.34 times more likely to prescribe prophylactic antibiotics at discharge compared to higher volume surgeons (≥ 100 fusions per year). Low volume surgeons (<50 fusions per year) had a trend towards increased utilization of post-discharge prophylactic antibiotics, albeit not significant (odds ratio 1.20; 95% confidence interval 0.80, 1.80), likely due to the smaller number of fusions by low volume surgeons. Given the variation in discharge antibiotic prescribing at the level of the individual physician, surgeon education would be important to improve the success of hospital antibiotic stewardship programs.³⁴

Variation existed by study site with respect to post-discharge antibiotic use and choice of antibiotic. Use of prophylactic discharge antibiotics was highest at site 2 (5.1%), followed by site 3 (4.1%), and lowest at site 1 (2.2%). While site 2 had the highest proportion of fusion admissions with spinal trauma (10.1%), the surgeries were generally less complex and invasive (e.g., fewer multi-level and thoracic procedures, smaller percentage involving both anterior and posterior approaches), and thus carried a lower risk of infection.³²

Ciprofloxacin, trimethoprim/sulfamethoxazole, and cephalexin were the top three antibiotic choices for post-discharge prescriptions at sites 1 and 3. More than 25% of antibiotic prescriptions at sites 1 and 3 were for trimethoprim/sulfamethoxazole, suggesting that surgeons at these sites were concerned about prophylaxis against methicillin-resistant S. aureus. Site 2 used a broader mix of post-discharge antibiotics than the other two sites, with cephalexin and doxycycline most commonly prescribed. During the study period the antibiotic stewardship program at site 2 restricted use of fluoroquinolones during inpatient stays, which may have carried over to the prescribing pattern of discharge antibiotics. In contrast, use of doxycycline or levofloxacin was rare at sites 1 and 3 (cumulative 2.1% and 9.3% by site, respectively). Antibiotic heterogeneity was not explained by the number of surgeons, as site 2 had the most variation in antibiotics, but only 7 spine surgeons who prescribed post-discharge antibiotics. Sites 1 and 3 had less heterogeneity in antibiotic choice, but more surgeons who prescribed post-discharge antibiotics (16 and 19, respectively).

Our study has several limitations. We included only three study sites, so our findings might not reflect practices in the community, particularly community hospitals that are not associated with a teaching hospital. We did not collect information on continuation of prophylactic antibiotics after incision closure or use of intraoperative vancomycin powder before incision closure. There was a possibility of misclassification of a therapeutic antibiotic as prophylactic if an infectious diagnosis was not recorded during the fusion admission and/or if a continued therapeutic antibiotic was only coded at discharge and not at hospital admission. Because of low post-discharge antibiotic use in our study cohort, we did not have enough power to detect differences in SSI or C. difficile infection rates by post-discharge antibiotic use or by individual surgeon. Notably, in our hierarchical model, study site was not significantly associated with variation in post-discharge antibiotic use, likely due to the small number of study sites with insufficient power to identify clustering. A future study with additional study sites would be of interest. Also of interest, although the surgeon volume variable did not significantly improve our hierarchical model, there was a trend towards more post-discharge antibiotic use by lower volume surgeons. Future studies examining surgeon-based factors for prescribing post-discharge antibiotics would be helpful given that guidelines recommend against prolonged use of postoperative prophylactic antibiotics.

In summary, we found that post-discharge prophylactic antibiotic use was uncommon after spinal fusion, but varied by study site. Patient characteristics associated with risk of SSI were associated with use of post-discharge prophylactic antibiotics. Stewardship efforts to discourage continuation of antibiotics after hospital discharge are needed to avoid further increases in antimicrobial resistance and adverse events.