P29. Gram Negative Vs Gram Positive Cultures: A Comparison of Gram Type on Washouts & Antibiotic Duration

BACKGROUND CONTEXT Infections are uncommon but serve as a significant source of morbidity after spinal surgery. While the virulence of bacterial strains has been explored in previous literature, their effects on surgical complications after spine surgery have yet to be elucidated. PURPOSE To evaluate differences in readmission rates, number of surgical washouts, and length of antibiotic therapy between gram positive and gram-negative infections after spinal fusion. STUDY DESIGN/SETTING Retrospective cohort analysis. PATIENT SAMPLE Patients of all ages undergoing lumbar fusion with a diagnosis of surgical site infection. Patients with a history of cancer, trauma, autoimmune disease on suppressive therapy, or non-compliance with medical therapy were excluded. OUTCOME MEASURES Number of washouts, duration of antibiotic use, biomarker data such as white blood cell (WBC), erythrocyte sedimentation rate (ESR), c-reactive protein (CRP), mean platelet volume (MPV), platelet number, MPV/platelet ratio, days in ICU, and use of epinephrine & norepinephrine. METHODS Patients undergoing lumbar fusion with a diagnosis of surgical site infection between 2013–2019 were retrospectively identified and a review of the electronic medical records was conducted to obtain the aforementioned outcome measures. Demographics, Charlson comorbidity index, microbiology data, serology, biomarkers, follow-up time, surgical management, antibiotic treatment, and washout duration was compared between patients with gram positive, gram negative, and mixed infections. RESULTS A total of 195 patients were included, with 94 in the gram-positive (G+) group, 65 in the gram-negative (G-) group, and 36 in the mixed (M) group. There were significant differences between the three groups in age, body mass index (BMI), and spinal levels fused (p<0.05). Patients in the G- group were older on average compared to the M and G+ cohorts (66 v 61.6 v 60.4 years p=.012). Body Mass Index (BMI) was highest in the M group when compared to the G- and G+ cohorts (37.5 v 34.0 v 32 p=.01). Patients in the M and G- cohort had a higher CCI as compared to G+ cohorts (4.36 v 4.39 v 3.35 p=.032). Spinal levels were significantly higher in the mixed group as compared to G- and G+ groups (3.29 v 2.45 v 2.45 p=.035). Significant differences were found between infectious cohorts in number of washouts (G+: 1.35 v G-: 1.68 v M: 2.16) (p<0.05). The mixed infections group had a significantly longer duration of antibiotic treatment with a mean of 55.5 days, compared to 41.9 days and 46.8 days for G- and G+, respectively. (p<0.05). No significant differences were found between groups with regard to surgical approach, number of spinal levels operated, or readmission window. CONCLUSIONS Postoperative infections after spinal fusion caused by mixed bacterial flora are more virulent as compared to gram-negative or gram-positive infections in isolation. A mixed bacterial infection resulted in an increased number of washouts needed to resolve the infection and longer duration of antibiotic use. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs. Infections are uncommon but serve as a significant source of morbidity after spinal surgery. While the virulence of bacterial strains has been explored in previous literature, their effects on surgical complications after spine surgery have yet to be elucidated. To evaluate differences in readmission rates, number of surgical washouts, and length of antibiotic therapy between gram positive and gram-negative infections after spinal fusion. Retrospective cohort analysis. Patients of all ages undergoing lumbar fusion with a diagnosis of surgical site infection. Patients with a history of cancer, trauma, autoimmune disease on suppressive therapy, or non-compliance with medical therapy were excluded. Number of washouts, duration of antibiotic use, biomarker data such as white blood cell (WBC), erythrocyte sedimentation rate (ESR), c-reactive protein (CRP), mean platelet volume (MPV), platelet number, MPV/platelet ratio, days in ICU, and use of epinephrine & norepinephrine. Patients undergoing lumbar fusion with a diagnosis of surgical site infection between 2013–2019 were retrospectively identified and a review of the electronic medical records was conducted to obtain the aforementioned outcome measures. Demographics, Charlson comorbidity index, microbiology data, serology, biomarkers, follow-up time, surgical management, antibiotic treatment, and washout duration was compared between patients with gram positive, gram negative, and mixed infections. A total of 195 patients were included, with 94 in the gram-positive (G+) group, 65 in the gram-negative (G-) group, and 36 in the mixed (M) group. There were significant differences between the three groups in age, body mass index (BMI), and spinal levels fused (p<0.05). Patients in the G- group were older on average compared to the M and G+ cohorts (66 v 61.6 v 60.4 years p=.012). Body Mass Index (BMI) was highest in the M group when compared to the G- and G+ cohorts (37.5 v 34.0 v 32 p=.01). Patients in the M and G- cohort had a higher CCI as compared to G+ cohorts (4.36 v 4.39 v 3.35 p=.032). Spinal levels were significantly higher in the mixed group as compared to G- and G+ groups (3.29 v 2.45 v 2.45 p=.035). Significant differences were found between infectious cohorts in number of washouts (G+: 1.35 v G-: 1.68 v M: 2.16) (p<0.05). The mixed infections group had a significantly longer duration of antibiotic treatment with a mean of 55.5 days, compared to 41.9 days and 46.8 days for G- and G+, respectively. (p<0.05). No significant differences were found between groups with regard to surgical approach, number of spinal levels operated, or readmission window. Postoperative infections after spinal fusion caused by mixed bacterial flora are more virulent as compared to gram-negative or gram-positive infections in isolation. A mixed bacterial infection resulted in an increased number of washouts needed to resolve the infection and longer duration of antibiotic use.