Merchant et al bacteria, such as Clostridium difficile, will increase among patients who have undergone prior appendectomy as com- pared with patients who have an intact appendix. We there- fore hypothesized that the rate of prior appendectomy would be higher among patients who test positive for C. difficile as compared with the appendectomy rate among those testing negative. Materials and Methods We expected the prevalence of C. difficile infection to be relatively low among our tested population and therefore elected to employ case-control methodology to expedite our work and preserve efficiency. We also wanted to examine ap- pendectomy rates among a “normal” population. These con- siderations required us to identify three separate populations; our case series, our controls, and our “normals.” We identified our case and control populations by re- viewing pathology results from our institution and identify- ing all patients over 18 years of age who underwent clinical testing for C. difficile using Toxin A & B EIA tests between July 2006 and December 2008. We classified patients as positive for C. difficile infection if test results on their initial specimen were positive. These patients formed our case series. We considered patients to be free of C. difficile infection (negative for disease) if their specimens tested negative. We constructed our control popu- lation from among patients with negative results by selecting individuals matching the age (within two years), sex of each patient in our case series. We constructed our “normal” population from patients who presented to our institutional ambulatory care center with non-gastrointestinal complaints and who did not under- go C. difficile testing. From among this group we enrolled two individuals matching the age and sex of each patient in our case and control series. We surveyed all participants and asked them whether or not they had undergone an appendectomy. We used these responses to calculate the proportion of patients who had undergone prior appendectomy in each of our three popu- lations. To test our primary hypothesis, we calculated the confidence interval for the difference in prior appendectomy rates between the case (patients testing positive for C. dif- ficile) and control (patients testing negative for C. difficile) populations. We considered our hypothesis to be confirmed if the lower confidence interval for this difference was posi- tive. We designed our study to have an 80% power to detect a 15% absolute increased prevalence of in appendectomy rates under one-sided testing of a 95% confidence interval. We used statpages.org for our statistical calculations [8]. The study was reviewed and approved by our institutional review board. J Clin Med Res • 2011;4(1):17-19 Results We enrolled 853 patients in our study, including 17 patients who could not recall whether they had undergone prior ap- pendectomy. After excluding these individuals from further analysis, our final population of 836 patients included 136 patients who tested positive for C. difficile (case popula- tion), 121 patients testing negative (control population), and a “normal” population of 569 patients. Patients in our case series ranged in age from 20 to 99 years (median 59; interquartile range: 49 to 72), and includ- ed 68 males (50.0%) and 68 females (50.0%). Our control group ranged from 18 to 96 (median 61; interquartile range: 47 to 72) with 57 males (47.1%) and 64 females (52.9%). Our “normal” population ranged in age from 18 to 102 years (median 62; interquartile range: 48 to 76) with 265 males (46.6%) and 304 females (53.4%). Among the 136 cases (patients who tested positive for C difficile), 27 had prior appendectomies (19.9%), while among the 121 controls (patients who tested negative for C difficile), 38 had a prior appendectomy (31.4%), yielding a difference in appendectomy rates of -11.6% (95% C.I.: -21.6% to -0.9%). In our “normal” population, 105 patients had prior appendectomies (18.5%). Discussion Our results show that the rate of prior appendectomy was actually lower among patients who tested positive for C. dif- ficile as compared with patients who tested negative. This result contradicts our primary hypothesis, and indicates that patients who tested positive for C. difficile were more likely to have an intact appendix than those who tested negative. These results probably reflect some naivete on our part regarding the true function of the appendix. While our study cannot show causality, a possible interpretation of our find- ings would be that the appendix is indeed a microbial reser- voir, and that C. difficile is among the harbored organisms. Under the selective pressure of antimicrobials, C. difficile is able to emerge and colonize the lower gut. Conversely, the higher rates of prior appendectomy we observed among pa- tients with negative C. difficile tests may be due to the lack of an appendix to harbor C. difficile and promote its inocula- tion into the gut. Thus it appears that rather than providing protection against CDAD, an intact appendix may promote C. difficile acquisition, carriage, and disease. Our study provides additional support for the concept of an appendiceal microbial reservoir. We observed the high- est rate of appendectomy, 31.4%, among our control popu- lation (patients who had negative test results). The rate of appendectomy among this population exceeded the rates for our case population, as well as the 18.5% appendectomy rate among our “normal” population. This high rate of ap- 18 Articles © The authors | Journal compilation © J Clin Med Res and Elmer Press™ | www.jocmr.org