A counterculture movement: Characterizing the prognostic utility of obtaining wound cultures for incisional surgical site infections

Graham McLaren; Jesse Chou; Robert G Sawyer

doi:10.4103/wjsi.wjsi_16_22

ORIGINAL ARTICLE

Year : 2022 | Volume : 1 | Issue : 2 | Page : 62-66

A counterculture movement: Characterizing the prognostic utility of obtaining wound cultures for incisional surgical site infections

Graham McLaren¹, Jesse Chou², Robert G Sawyer¹
¹ Department of Surgery, Western Michigan University Homer Stryker MD School of Medicine, Michigan, USA
² Department of Surgery, Western Michigan University Homer Stryker MD School of Medicine, Michigan; Department of Plastic and Maxillofacial Surgery, University of Virginia, Virginia, USA

Date of Submission	24-Dec-2022
Date of Decision	04-Jan-2023
Date of Acceptance	06-Jan-2023
Date of Web Publication	15-Feb-2023

Correspondence Address:
Robert G Sawyer
1000 Oakland Drive Kalamazoo, MI 49008
USA

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/wjsi.wjsi_16_22

Abstract

Background: Surgical site infections (SSIs) account for 15% of all healthcare-associated infections, yet, the utility of cultures remains controversial. We hypothesized that obtaining cultures would not affect outcomes from incisional SSI.
Methods: All incisional SSI from general surgery patients treated as inpatients at a single institution between 1997 and 2017 were included. Patient variables were compared by Student's t-test and Chi-square analysis. Predictors of in-hospital mortality, duration of therapy, and hospital length of stay, including the acquisition of wound cultures, were determined by multivariate (MV) logistic regression analysis.
Results: In total, 2054 SSIs were identified: 1077 (52.4%) with cultures and 977 (47.6%) without. Obtaining cultures were associated with higher severity of illness/Acute Physiology and Chronic Health Evaluation (APACHE-II) score (12.4 ± 0.2 vs. 8.8 ± 0.2; P < 0.0001) and multiple comorbidities, as well as a longer antimicrobial course (13.8 ± 0.3 days vs. 9.1 ± 0.2 days; P < 0.0001), length of stay (17.4 ± 0.8 days vs. 9.7 ± 0.5 days; P < 0.0001), and mortality (8.6% vs. 4.2%; P < 0.0001). Factors independently predicting mortality included age in years (odds ratio [OR] 1.03 [95% confidence interval [CI] 1.02–1.05], P < 0.0001), APACHE-II (OR 1.17 [95% CI 1.14–1.21], P < 0.0001), days from operation to diagnosis (OR 1.01 [95% CI 1.01–1.02], P < 0.0001), and diagnosis of SSI after discharge (OR 4.98 [95% CI 2.18–11.35], P < 0.0001). Obtaining cultures (OR 1.04 [95% CI 0.65–1.64], P = 0.88) were not associated with mortality. Acquisition of cultures was independently associated with longer antimicrobial duration and length of stay.
Conclusion: Routinely obtaining cultures from infected surgical incisions does not change the overall outcome and should be reserved for special circumstances, such as critical illness.

Keywords: Culture, surgical site infection, wound

How to cite this article:
McLaren G, Chou J, Sawyer RG. A counterculture movement: Characterizing the prognostic utility of obtaining wound cultures for incisional surgical site infections. World J Surg Infect 2022;1:62-6

How to cite this URL:
McLaren G, Chou J, Sawyer RG. A counterculture movement: Characterizing the prognostic utility of obtaining wound cultures for incisional surgical site infections. World J Surg Infect [serial online] 2022 [cited 2023 Mar 30];1:62-6. Available from: https://www.worldsurginfect.com/text.asp?2022/1/2/62/369707

Introduction

Surgical site infections (SSIs) make up a significant portion of healthcare-associated infections. Previous studies report that 14%–38% of all nosocomial infections are SSIs.^{[1],[2],[3],[4],[5]} It is also estimated that at least 1%–2% of all operations are complicated by an SSI.^[2] The most commonly applied definition of an SSI is an infection of the surgical site (including incisional and organ space) within 30 days of the procedure or up to 1 year if an implant is present.^[1] The most likely causative organisms of SSI are native flora of the patient's skin including aerobic Gram-positive cocci such as Staphylococcus species,^[6] but are often polymicrobial.^[7] Infections involving flora from colonized organs such as the gastrointestinal tract are also important. After a skin incision interrupts epithelial integrity, the subcutaneous tissue is then exposed to pathogens. Infection occurs due to an imbalance between the number and virulence of bacteria and host defense mechanisms.^[8] A wound infection is suspected in the setting of erythematous and edematous skin that may be accompanied by a fever, purulent discharge, malodor, or other signs.^[9]

Conventional treatment of SSIs may involve obtaining a culture of the wound, antimicrobial therapy, and local wound care. However, clinical signs of inflammation are not objective or definitive in diagnosing surgical incision wound infection. Wound cultures may be sensitive but not accurate for distinguishing between colonization and acute infection. In addition, it remains poorly understood whether incisions suspected of being infected should be sampled and whether this would yield meaningful data that alters clinical treatment.

The major argument against culturing incisions is that when wounds are opened and appropriate wound care is applied, the causative pathogens are predictable and antimicrobials can be chosen without culture data. In fact, culture results often return after a very brief course of antimicrobials (0 to 72 h) have been completed. We hypothesized that the acquisition of wound cultures does not improve outcomes in general surgery patients diagnosed with SSI.

Methods

A prospectively collected dataset was analyzed to identify all incisional SSIs at a single institution for a 20-year period between 1997 and 2017. Subjects included those managed on the general surgery services, including those diagnosed during the hospitalization for their index operation or readmitted after discharge for the management of SSI diagnosed as an outpatient. Patients with SSIs treated exclusively on an outpatient basis were not included because of the difficulty identifying these patients as well as unreliable treatment data. Superficial and deep incisional SSIs were included, but organ space infections were excluded due to different source control and antimicrobial treatment paradigms. All SSIs identified were then divided into two groups: those with cultures and those without cultures.

Demographics and outcomes were compared by univariate analysis using Student's t-test and Chi-square analysis. Results are presented either as mean ± standard error of the mean or as a percentage of the total referent group. Independent predictors of mortality, prolonged hospital length of stay (>7 days after diagnosis of SSI), and prolonged antimicrobial treatment course (>10 days of therapy) were determined by multivariate (MV) logistic regression analysis, with the variable of wound culture added. Hosmer and Lemeshow testing was used for calibration and the receiver operating characteristic (ROC) curve C-statistic was calculated to evaluate precision.

Results

In total, 2054 SSIs were identified: 977 (47.6%) with cultures and 1077 (52.4%) without. Demographics are shown in [Table 1]. Notably, obtaining cultures was associated with a higher severity of illness/APACHE-II score, black race, and multiple comorbidities. Obtaining cultures was less common among subjects with malignancy.

Table 1: Baseline demographics and clinical characteristics*

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The most common pathogens isolated were Staphylococcus aureus (158, 16.2%) including 82 methicillin-resistant isolates, Enterococcus faecalis (122, 12.5%), Candida albicans (109, 11.2%), Escherichia ^{More Details} coli (106, 10.8%), Pseudomonas aeruginosa (102, 10.4%), Enterococcus faecium (80, 8.2%) including 56 vancomycin-resistant isolates, and Streptococcus species (60, 6.1%). Overall, Gram-positive isolates were slightly more common than Gram-negative isolates: 562 versus 516 subjects.

By univariate analysis, obtaining cultures was associated with the rate of antimicrobial use, longer antimicrobial course, greater length of stay after diagnosis, and increased crude mortality [Table 2]. A greater percentage of patients who had cultures had a duration of antimicrobial therapy of more than 10 days (594/977 = 60.8% vs. 366/1077 = 34.0%, P < 0.0001) and a hospital length of stay after diagnosis of more than 7 days (542/977 = 55.5% vs. 346/1077 = 32.1%, P < 0.0001)

Table 2: Outcomes*

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Three logistic regression models were constructed to evaluate factors independently associated with all-cause, in-hospital mortality, prolonged duration of antimicrobial therapy of more than 10 days, and prolonged hospital length of stay of more than 7 days after diagnosis of SSI. Variables included were age in years, sex, APACHE-II score at the time of diagnosis of SSI, days from index operation to the diagnosis of SSI, the presence of chronic immunosuppression (including solid organ transplantation), diagnosis after discharge from hospitalization including the index operation, and availability of wound cultures. Variables associated with mortality are given in [Table 3] and included increasing age, APACHE-II score, days from operation to diagnosis, and initial diagnosis of SSI after discharge from index operation; the acquisition of wound cultures was not independently associated with mortality (OR 1.04, 95% CI 0.65–1.64, P = 0.88).

Table 3: Multivariate analysis for in-hospital mortality, all surgical site infection

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Prolonged antimicrobial duration was independently associated with younger age (OR 0.98, 95% CI 0.98–0.99, P < 0.001), APACHE-II score (OR 1.04, 95% CI 1.02–1.06, P < 0.001), and acquisition of wound cultures (OR 2.55, 95% CI 2.09–3.11, P < 0.001). Calibration was acceptable with a Hosmer–Lemeshow test P of 0.68 and precision was fair with a ROC area under the curve C-statistic of 0.67. Prolonged length of hospital stay after diagnosis of SSI was independently associated with younger age (OR 0.99, 95% CI 0.98–0.99, P < 0.012), APACHE-II score (OR 1.15, 95% CI 1.12–1.17, P < 0.001), diagnosis after discharge after index procedure (OR 4.15, 95% CI 3.23–5.33, P < 0.001), and acquisition of wound cultures (OR 2.23, 95% CI 1.78–2.79, P < 0.001). Model performance characteristics included Hosmer–Lemeshow test P = 0.06 and C-statistic = 0.80.

Discussion

This study presents an analysis of postoperative SSIs based upon wound culture and its effect on surgical outcomes. A trend toward wound sampling was observed with patients that were more critically ill as indicated by APACHE-II scores, multiple medical comorbidities, prior blood transfusion, and management in the intensive care unit. These relationships are certainly related to the association between the acquisition of cultures and higher in-hospital mortality, longer antimicrobial duration, and longer hospital stay. Bernard et al. queried the National Surgical Quality Improvement Program, a national database of mixed nontrauma, noncardiothoracic surgical outcomes, and found that transfusion of even 1 U-packed red blood cell was associated with increased postoperative morbidity including SSI and sepsis.^[10] Transfusion of blood products can be an essential intervention in surgical patients but also induces an inflammatory response that can alter immunity and increase the risk of infection.^[11],[12] Transfusion of blood products alone has been associated with not only increased morbidity but increased length of hospital stay.^[13]

Previous studies in other surgical disciplines have found no clinical benefit to obtaining postoperative surgical site cultures.^[14] Part of the difficulty lies in the fact that there is no consensus method of sampling and analyzing surgical wounds.^{[15],[16],[17],[18],[19]} Upon MV analysis of our institution's data, obtaining SSI cultures did not have a significant effect on patient mortality, and was independently associated with a longer treatment course and hospital stay. These data suggest that obtaining a wound culture may not improve clinical outcomes under normal circumstances. One potential explanation is that local wound management in the care of SSI, including opening the wound and wound care, may minimize the role of antimicrobials other than in the presence of invasive fascial infection.

Our study can be seen in the context of several limitations. First, as an observational study without randomization, it is not surprising that the patients with cultures had several characteristics suggesting they were more ill than patients without cultures (APACHE-II score, immunosuppression status, etc.). Second, at least half of SSI are diagnosed as outpatients, and since we only included patients treated as inpatients, we cannot comment on those managed outside the hospital. Third, in-hospital mortality is a crude outcome measure, and the role of SSI in the deaths of those subjects was not assessed and may have been minor. Fourth, even within one institution, there may be surgeon-to-surgeon variability in terms of which wounds get sampled and the sampling technique in the context of dressings, the incision itself, and timing with antimicrobial therapy.

Conclusion

We found no evidence that the availability of cultures altered mortality or other relevant surgical outcomes following the treatment of SSIs. Although we attempted to control for variables affecting outcomes, only a randomized study of culture versus no culture will clearly answer the question of culture utility. Our findings suggest cultures are generally not necessary when treating SSI. Routinely obtaining cultures from infected surgical incisions seems unnecessary and should be reserved for special circumstances, such as significant prior antimicrobial exposure, unusually invasive infection, or critical illness.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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