Identifying preoperative risk factors for surgical wound infections in clean cases

JUNE 10, 2007

Alexander J. Greenstein, MD
Resident
Department of Surgery

Celia M. Divino, MD
Chief, Division of General Surgery
Department of Surgery

Kaare J. Weber, MD
Assistant Professor of Surgery
Department of Surgery
Mount Sinai School of Medicine
New York, NY

Clean operative procedures, such as thyroidectomy, rarely result in wound infections; thus, there are no definitive guidelines or recommendations regarding antibiotic prophylaxis for such procedures. At the authors' institution, it is common practice not to administer antibiotic prophylaxis for clean cases. The authors report their experience with a patient who had multiple comorbidities and developed a surgical site infection after undergoing thyroidectomy. The authors review the current literature regarding antibiotic prophylaxis for patients undergoing such clean procedures and find that risk factors for wound infection should be considered before a decision is made.

Wound infections after thyroidectomy are uncommon, with reports in the literature documenting an incidence that varies from 0.1% to 2%.1-3 Clean wounds are defined as operative wounds that have been closed primarily and in which no inflammation has been encountered. By definition, a viscus (respiratory, alimentary, and genital or urinary tract) is not entered during a clean procedure, and such wounds have an infection rate that ranges from 1.3% to 2.9%.4,5 Whether clean or not, the risk of infection increases when various patient-related risk factors are present, such as immunosuppression, obesity, and diabetes. Guidelines and recommendations regarding antibiotic prophylaxis for clean cases vary since no randomized trials have provided definitive answers. We report a case of a surgical site infection after thyroidectomy in a morbidly obese woman on immunosuppressive therapy.

Case report

A 51-year-old morbidly obese woman with a history of hepatitis C, hypertension, gout, and end-stage renal disease presented with a right neck mass, which she had first noticed approximately 1 year earlier. She was asymptomatic, reporting no pain or effect on breathing, speaking, or swallowing. She had no symptoms of hyperthyroidism or hypothyroidism. She also noted no previous exposure to radiation. She was on mycophenolate and prednisone therapy because of a cadaveric renal transplant. Her family history was significant for a daughter who had a thyroidectomy for a benign nodule.

On physical examination, the patient had a cushingoid appearance, no exophthalmos, and a firm, 3-cm mass in the right lobe of the thyroid with no palpable lymphadenopathy. Her thyroid-stimulating hormone was within the normal range at 3.5 mIU/mL (normal, 0.5-5.0 mIU/mL), and ultrasonography revealed a solid hypoechoic mass measuring 3.5 x 4.5 x 3.0 cm. A fine needle aspiration of the thyroid mass was suspicious for papillary carcinoma.

The patient subsequently underwent a total thyroidectomy, which revealed a 4.5-cm right thyroid mass. Extensive involvement of her paratracheal lymph nodes was observed and mandated a central lymph node dissection. Pathological investigation confirmed a 4.5-cm papillary carcinoma with lymphocytic thyroiditis of her left lobe. Eight of ten lymph nodes were positive for cancer.

The patient's postoperative course was complicated by transient symptomatic hypocalcemia due to hypoparathyroidism. She was treated with calcium repletion and responded well. Two weeks after the operation, the patient presented with an incision that was well healed but marked by erythema and induration without drainage. After a computed tomography (CT) scan of the neck was performed (Figure), a 20-gauge needle was inserted through the incision, and 30 cc of pus was drained. The patient was started on intravenous vancomycin and piperacillin/tazobactam. She was brought back to the operating room for a washout. Approximately 50 cc of pus was drained and the wound was packed and left open. Cultures revealed methicillin-sensitive Staphylococcus aureus, and the patient's antibiotic therapy was changed to cefazolin. She was discharged on a short course of cephalexin and received home nursing care. The patient's recovery was uncomplicated, and the wound healed well.

Discussion

Surgical wound infections are the second most common type of nosocomial infection. In one survey that excluded asymptomatic bacteriuria as a cause of nosocomial infection, surgical site infections were the most frequent infection documented accounting for 38% of infections.6 Wound infections are estimated to develop in 2% to 5% of the 16 million patients undergoing surgical procedures annually7 Approximately 25 years ago, the National Academy of Sciences and the National Research Council developed a wound classification scheme based upon the degree of expected microbial contamination during surgery.8 This scheme stratified wounds as clean, clean-contaminated contaminated or dirty.

While the efficacy of antibacterial prophylaxis against postoperative infections has been established for clean-contaminated procedures, the utility of prophylaxis for clean procedures has yet to be resolved. Most studies suggest that the infection rate for clean cases is less than 5%, but other studies show that the rate can be as high as 10% when patients are subject to 6 weeks of intense follow-up and the definition of wound infection is expanded.9 However, because of the low control rates of infection in these studies, large studies of more than 1,000 clean operative procedures must be undertaken to detect significant reductions in infection rates reliably.

In 1990, Platt and colleagues conducted a randomized double-blind study evaluating the benefit of prophylaxis in 1,218 patients undergoing herniorrhaphy or surgery involving the breast.10 Patients were given a 1-g dose of cefonicid 30 minutes before surgery and were followed up for 4 to 6 weeks postoperatively. Patients who received prophylaxis had 48% fewer infections than those who did not, with wound infection rates decreasing from 12.2% to 6.6% in breast surgery patients and from 4.2% to 2.3% in herniorrhaphy patients.

In 1992, Platt and colleagues conducted a second study investigating the effects of antibiotic prophylaxis on definite wound infection in patients undergoing herniorrhaphy or breast surgery.11 This study included 3,202 patients who were identified preoperatively and monitored for 4 weeks or longer postoperatively. Prophylaxis was administered at the discretion of the surgeon and typically given to those considered to be at higher risk for developing an infection. In total, 34% received prophylaxis, and the results showed that these patients had 41% fewer definite wound infections after adjustment for duration of surgery and type of procedure. Further adjustments for age, body mass index, diabetes, presence of drains, and corticosteroid exposure did not change the magnitude of this finding. Both the 1990 and 1992 studies, however, have been criticized because of the higher than expected incidence of wound and urinary tract infections in the control group. The 1990 study was also criticized for its small sample size.

The literature is littered with conflicting results regarding preoperative prophylaxis. Taylor and associates conducted a randomized double-blind prospective study comparing antibiotic prophylaxis versus no prophylaxis in 619 open hernia repairs that showed no benefit for the antibiotic group.12 In addition, Gilbert and colleagues' prospective study of 2,493 inguinal hernia repairs showed no difference in wound infection rates between patients who underwent repairs with mesh versus those who had repairs without mesh, regardless of whether or not they had received antibiotics.13

Surgical site infections develop as a result of complex interactions between the local and systemic defense mechanisms of the patient and the contaminating organism. In addition to pathogenic factors, such as the virulence and amount of the organism involved a variety of patient and perioperative factors influence the development of surgical site infections. Perioperative factors that increase the risk for such infections are operating room ventilation, preoperative showering and hair removal, duration of surgical scrub, length of operation, use of surgical drains, presence of foreign material in the surgical site, trauma to tissue, and surgical site surveillance. Patient-related factors include age, obesity, diabetes mellitus, smoking, malnutrition, altered immune response, duration of previous hospitalization, and presence of an infection at a remote body site.

Several studies examining the patient-related factors of obesity, diabetes, and immunosuppression have been published. A prospective study of 6,336 patients by Dindo and associates showed a significantly increased incidence of wound infection after open surgery in patients who were obese.14 The authors proposed that wound infections in obese patients are related to the presence of excessive fat tissue, which has low regional oxygen tension and therefore predisposes them to impaired wound healing. It also has been shown that obese patients have an underlying immune impairment in responsiveness of lymphocytes, which may contribute to their higher rates of wound infection.15 While it has not been possible to completely separate the effects of diabetes from hyperglycemia as primary risk factors for wound infection, it has been proven in multiple settings that elevated postoperative glucose levels are associated with higher wound infection rates16 and that tight glucose control reduces morbidity and mortality among critically ill patients in the surgical intensive care unit.17

A prospective study by Penel and associates examined 212 clean head and neck procedures conducted without antibiotic prophylaxis and found previous anticancer chemotherapy to be a significant risk factor for wound infection.18 They reported an overall wound infection rate of 6.6%, and all but 1 of 15 patients who underwent chemotherapy previously developed a wound infection. Anticancer chemotherapy adversely affects the number and function of neutrophils, macrophages, and B and T cells, which results in diminished opsonizing activity, inadequate agglutination and lysis of bacteria, and deficient neutralization of bacterial toxins. Patients consequently develop defects in cellular and humoral immunity and antibody production and have increased susceptibility to pyogenic bacterial infections, even if they are not neutropenic. These adverse effects can persist for more than 6 months after chemotherapy has ended.19

While there are no good studies on the relationship between steroid use and wound infection in clean head and neck cases, there is evidence that chronic steroid use is a significant risk factor for wound infection. In a retrospective analysis by Finan and associates of 1,505 subjects undergoing ventral hernia repair, chronic steroid use was associated with a four-fold increase in wound infection rates.20 Other significant risk factors included smoking and increased operative time. Finally, an important retrospective study by Humar and associates of 2,013 kidney recipients found that 4.8% developed a superficial or deep wound infection.21 Significant risk factors were obesity (body mass index above 30 kg/m2; relative risk of 4.4), diabetes (relative risk of 1.63), and the use of mycophenolate mofetil (relative risk of 2.43).

The National Nosocomial Infection Surveillance (NNIS) index5 is most widely used to predict the risk of surgical site infections. The system is ranked by the type of operation performed and assigns points based on patient-related risk factors, the degree of microbial contamination of the incision, and the duration of the operation. Patient-related risk is determined by the American Association of Anesthesiologists (ASA) preoperative assessment score, and operative duration has been estimated for a number of different procedures. The NNIS index ranges from a score of 0 to 3, with one point each assigned for a contaminated or dirty wound an operative duration exceeding the 75th percentile allotted for that procedure, and an ASA score greater than 3. The predicted rate of infection increases as the NNIS score for each incision classification increases. For example, a clean incision with a score of 0 has a predicted infection rate of 1.0%, whereas one with a score of 2 has a predicted infection rate of 5.4%. While many agree that the NNIS index has some value in predicting infections, others point out that use of the system has not reduced the rate of infections.22

Despite the development of the NNIS index, surgical textbooks and the literature in general lack consensus on the utility of antibiotic prophylaxis for clean cases. When cost effectiveness is brought into the discussion, more questions are raised and conclusions become significantly more difficult to reach. While it will take many more studies to generate accurate conclusions regarding the use of prophylaxis in clean procedures for uncomplicated patients, it would be prudent in the meantime to consider a stratification process for using prophylaxis in these procedures, especially given the increasing complexity of today's patients. Our patient, for example, was morbidly obese, had hepatitis C, and was on immunosuppressive therapy At our institution, antibiotic prophylaxis is not administered for clean cases, especially when artificial materials are not being placed intraoperatively Given our patient's comorbidities and extended operative time, however, she would have scored a 2 on the NNIS index and therefore had a predicted 5.4% chance of developing an infection. Preoperatively, it would have been impossible to predict her extended operative time, but with an NNIS score of 1 and a multitude of known risk factors, she probably should have been considered for prophylactic antibiotics.

Conclusion

Although surgical site infections rarely result after clean operative procedures, there are many factors that can place patients at risk. Some of the most investigated patient-related risk factors include obesity, diabetes, and immunosuppressive therapy. Numerous perioperative factors, such as operative time, also play a role. With so many factors to consider, it is difficult to determine which patients will develop infections after a clean operative procedure, but the NNIS index offers some insight until more conclusive studies can be conducted. Based on the NNIS index, our patient was at risk for developing an infection. This case report illustrates that risk factors for wound infection should be considered before making a decision regarding the use of antibiotic prophylaxis for clean cases.

References

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