An appendectomy increases the risk of rheumatoid arthritis: a five-year follow-up study

Beyond a vestigial organ: effects of the appendix on gut microbiome and colorectal cancer

The role of appendectomy in the pathogenesis of colorectal cancer (CRC) is a recent topic of contention. Given that appendectomy remains one of the most commonly performed operations and a first-line management strategy of acute appendicitis, it is inherently crucial to elucidate the association between prior appendectomy and subsequent development of CRC, as there may be long-term health repercussions. In this review, we summarize the data behind the relationship of CRC in post-appendectomy patients, discuss the role of the microbiome in relation to appendectomy and CRC pathogenesis, and provide an appraisal of our current understanding of the function of the appendix. We seek to piece together the current landscape surrounding the microbiome and immunological changes in the colon post-appendectomy and suggest a direction for future research involving molecular, transcriptomic, and immunologic analysis to complement our current understanding of the alterations in gut microbiome.

Appendicitis is associated with an increased risk of systemic lupus erythematosus: a nationwide, population-based, case-control study

OBJECTIVE

To examine the association between appendicitis and the risk of systemic lupus erythematosus (SLE).

METHODS

Using claims data from the 2003-2013 Taiwanese National Health Insurance Research Database, we selected 6054 patients with newly diagnosed SLE from 2007 to 2012 and 36,324 age-, sex- and year of SLE diagnosis date-matched (1:6) non-SLE controls. After controlling for potential confounders, a multivariable conditional logistic regression model was used to calculate the adjusted odds ratio (aOR) with 95% confidence interval (CI) for the association of appendicitis history with SLE. Sensitivity analyses were conducted using various definitions of appendicitis. Subgroup analyses were conducted to examine possible modification effects by age, gender, level of urbanization, income and Charlson Comorbidity Index (CCI).

RESULTS

The average age of patients was 38 years old in both groups. The proportion of females was 86.5%. 75 (1.2%) of SLE cases and 205 (0.6%) of non-SLE controls had appendicitis history before the index date. After adjusting for potential confounding factors, appendicitis was associated with a higher risk of SLE (aOR, 1.84; 95% CI, 1.34-2.52), and such association remained robust after variation of appendicitis definition. No significant modification effects were found for the association between appendicitis and SLE by age, gender, urbanization level, income and CCI.

CONCLUSION

This nationwide, population-based case-control study demonstrates an association between appendicitis and incident SLE. Lack of individual smoking status is a major limitation. Key Points • Appendicitis was significantly associated with an increased risk of SLE. • Such association remained robust using various definitions of appendicitis.

Appendectomy as part of Ladd's procedure: a systematic review and survey analysis

BACKGROUND

Ladd's Procedure has been the surgical intervention of choice in the management of congenital intestinal malrotation for the past century. Historically, the procedure included performing an appendectomy to prevent future misdiagnosis of appendicitis, since the location of the appendix will be shifted to the left side of the abdomen. This study consists of two parts. A review of the available literature on appendectomy as part of Ladd's procedure and then a survey sent to pediatric surgeons about their approach (to remove the appendix or not) while performing a Ladd's procedure and the clinical reasoning behind their approach.

METHODS

The study consists of 2 parts: (1) a systematic review was performed to extract articles that fulfill the inclusion criteria; (2) a short online survey was designed and sent by email to 168 pediatric surgeons. The questions in the survey were centered on whether a surgeon performs an appendectomy as part of the Ladd's procedure or not, as well as their reasoning behind either choice.

RESULTS

The literature search yielded five articles, the data from the available literature are inconsistent with performing appendectomy as part of Ladd's procedure. The challenge of leaving the appendix in place has been briefly described with minimal to no focus on the clinical reasoning. The survey demonstrated that 102 responses were received (60% response rate). Ninety pediatric surgeons stated performing an appendectomy as part of the procedure (88%). Only 12% of pediatric surgeons are not performing appendectomy during Ladd's procedure.

CONCLUSION

It is difficult to implement a modification in a successful procedure like Ladd's procedure. The majority of pediatric surgeons perform an appendectomy as part of its original description. This study has identified gaps in the literature pertaining to analyze the outcomes of performing Ladd's procedure without an appendectomy which should be explored in future research.

Appendectomy Is Associated With Alteration of Human Gut Bacterial and Fungal Communities

Recent research has revealed the importance of the appendix in regulating the intestinal microbiota and mucosal immunity. However, the changes that occur in human gut microbial communities after appendectomy have never been analyzed. We assessed the alterations in gut bacterial and fungal populations associated with a history of appendectomy. In this cross-sectional study, we investigated the association between appendectomy and the gut microbiome using 16S and ITS2 sequencing on fecal samples from 30 healthy individuals with prior appendectomy (HwA) and 30 healthy individuals without appendectomy (HwoA). Analysis showed that the gut bacterial composition of samples from HwA was less diverse than that of samples from HwoA and had a lower abundance of Roseburia, Barnesiella, Butyricicoccus, Odoribacter, and Butyricimonas species, most of which were short-chain fatty acids-producing microbes. The HwA subgroup analysis indicated a trend toward restoration of the HwoA bacterial microbiome over time after appendectomy. HwA had higher gut fungi composition and diversity than HwoA, even 5 years after appendectomy. Compared with those in samples from HwoA, the abundance correlation networks in samples from HwA displayed more complex fungal–fungal and fungal–bacterial community interactions. This study revealed a marked impact of appendectomy on gut bacteria and fungi, which was particularly durable for fungi.

A link between appendectomy and gastrointestinal cancers: a large-scale population-based cohort study in Korea

An association between appendectomy and subsequent gastrointestinal (GI) cancer development has been postulated, although the evidence is limited and inconsistent. To provide clarification, we investigated the link between appendectomy and GI cancers in a large nationwide appendectomy cohort. This cohort was derived from the claims database of the National Health Insurance Service in South Korea and comprised 158,101 patients who had undergone appendectomy between 2007 and 2014. A comparison cohort of 474,303 subjects without appendectomy was selected after 1:3 matching by age and sex. The incidence of GI cancers after appendectomy was observed, and risk factors for GI cancers were determined by using a multivariable-adjusted proportional hazards model. Appendectomy did not significantly increase the incidence of GI cancers in the overall population (1.529 and 1557 per 1000 person-years in the non-appendectomy and appendectomy cohorts, respectively). However, appendectomy significantly increased the incidence of GI cancers in subgroups consisting of elderly (≥ 60 years) patients (adjusted HR, 1.102; 95% confidence interval, 1.011-1.201; p = 0.028) or women (adjusted HR, 1.180; 95% confidence interval, 1.066-1.306; p = 0.001).

Incidence and risk of sepsis following appendectomy: a nationwide population-based cohort study

Appendectomy is a frequently performed surgical procedure; however, long-term consequences have not been fully explored. We used a nationwide population-based cohort to determine whether patients undergoing appendectomy are at an increased risk of sepsis. Overall, 252,688 patients undergoing appendectomy and 252,472 matched controls were identified from the National Health Insurance Research Database in Taiwan. A propensity score analysis was used for matching age, sex, index year and comorbidities at a ratio of 1:1. Multiple Cox regression and stratified analyses were used to estimate the adjusted hazard ratio (aHR) of developing sepsis. Patients undergoing appendectomy had a 1.29 times (aHR: 1.29; 95% confidence interval [CI], 1.26-1.33) higher risk of developing sepsis than those not undergoing. Patients aged 20-49 years had a 1.58-fold higher risk of sepsis in the appendectomy cohort (aHR; 95% CI, 1.50-1.68). Also, having undergone appendectomy, patients had a higher likelihood of sepsis, regardless of sex and with or without comorbidities. Patients with <1 year follow-up showed a 1.98-fold risk of sepsis in the appendectomy cohort. Patients with 1-4 and ≥5 years follow-up showed a 1.29 and 1.11-fold risk of sepsis, respectively. Future research is required to elucidate the possible immuno-pathological mechanisms of these associations.

Risk of irritable bowel syndrome in patients who underwent appendectomy: A nationwide population-based cohort study

BACKGROUND

Appendectomy is one of the most common surgical procedures; however, the possible long-term consequences have not been fully explored. The appendix has been associated with microflora of the gut and immune functions. However, literature examining the relationship between prior appendectomy and the risk of irritable bowel syndrome (IBS) is lacking. The aim of this study was to evaluate the risk of irritable bowel syndrome for patients who underwent appendectomy by using a nationwide longitudinal population-based cohort.

METHODS

Data from this study was collected from Taiwan's National Health Insurance Research Database (NHIRD), a population-based database. We identified 12,760 patients who underwent appendectomy between January 1, 2000 and December 31, 2012. A total of 9236 patients who had appendectomy (case group) were randomly matched with 9236 patients who had not undergone appendectomy (control group) in a ratio of 1:1 by means of propensity scores. The hazard ratio (HR) of IBS was calculated by multiple Cox regression. Furthermore, sensitivity test and stratified analysis were performed.

FINDINGS

The incidence rate of IBS was 51.30 per 10,000 person-years in patients having appendectomy, more than the 35.28 per 10,000 person-years in patients not having appendectomy. Patients who underwent appendectomy had 1.46-fold risk of IBS compared to patients not having appendectomy (HR, 1.46; 95% CI, 1.24-1.72). Stratified analysis revealed that the higher HR of 1.55 (95% CI, 1.18-2.04) in patients <40 years old, and particularly within the first 5 years follow-up period of undergoing appendectomy. In addition, patients diagnosed with fibromyalgia had a greater risk of suffering IBS after appendectomy (HR, 1.41; 95% CI, 1.04-1.92).

INTERPRETATION

Patients with appendectomy have a higher incidental risk of IBS than the control population. The risk is higher for patients under 40 years old and those who received appendectomy within 5 years. Physicians could take this into consideration for treatment plans of patients who have underwent this surgery. Further research on the pathogenesis of this association is required.

FUNDING

This work was supported by grants from the Ministry of Health and Welfare, Taiwan (MOHW108-TDU-B-212-133004), China Medical University Hospital, Academia Sinica Stroke Biosignature Project (BM10701010021), MOST Clinical Trial Consortium for Stroke (MOST 108-2321-B-039-003-), Tseng-Lien Lin Foundation, Taichung, Taiwan, and Katsuzo and Kiyo Aoshima Memorial Funds, Japan.

The gallbladder and vermiform appendix influence the assemblage of intestinal microorganisms

Surgical procedures for the symptomatic removal of the gallbladder and the vermiform appendix have been posited to adversely shift the assemblage of the intestinal microbiome increasing the risk of disease. The associated mechanisms have been linked with dysbiosis of the gut microbiota. Cholecystectomy causes changes of bile acid compositions and bile secretion patterns as bile acids interact with the intestinal microbiota in a bidirectional capacity. An appendectomy precludes the further recolonization of the proximal colon with a commensal biofilm that could maintain a stable intestinal microbiome. Epidemiological studies indicate that there is an increased risk of disease rather than causality following a cholecystectomy and appendectomy. This narrative review summarizes studies that report on the role that bile salts and the appendix, contribute to the assemblage of the intestinal microbiome in health and disease.

Appendectomy and rheumatoid arthritis: A longitudinal follow-up study using a national sample cohort

The present study evaluated the association between appendectomy and rheumatoid arthritis (RA) using a national sample cohort of the Korean population. In this cohort study, the Korean National Health Insurance Service-National Sample Cohort of individuals ≥20 years old was collected from 2002 to 2013. A total of 14,995 appendectomy participants were 1:4 matched with 59,980 control subjects for age, group, sex, income group, region of residence, hypertension, diabetes, and dyslipidemia. We analyzed the occurrence of RA in both the appendectomy and control groups. Appendectomies were identified using operation codes for appendicitis only. RA was defined by International Classification of Disease-10 codes (M05 or M06) and medication histories. Crude and adjusted hazard ratios (HRs) were analyzed using a stratified Cox proportional hazard model. Subgroup analyses were performed on groups stratified by age and sex. The adjusted HR for RA was 1.02 (95% confidence interval = 0.76-1.38) in the appendectomy group (P = .883). In all of the subgroup analyses according to age and sex, the adjusted HRs for RA were not higher in the appendectomy group than those in the control group. We could not identify any significant relationship between appendectomy and RA.

Association between tonsillitis and newly diagnosed ankylosing spondylitis: A nationwide, population-based, case-control study

Objectives

To investigate the association between tonsillitis and the risk of newly diagnosed ankylosing spondylitis (AS).

Methods

We used 2003–2012 data from Taiwan’s National Health Insurance Research Database to conduct this nationwide, population-based, case-control study. We identified AS patients newly diagnosed between 2005 to 2012 as the study group and selected age, sex and index-year matched (1:6) non-AS individuals as controls. The association between tonsillitis and risk of newly diagnosed AS was determined by calculating odds ratios (ORs) with 95% confidence intervals (CIs) using conditional logistic regression analysis.

Results

We identified 37,002 newly diagnosed AS cases and 222,012 matched non-AS controls. Patients with AS were more likely to have tonsillitis (aOR 1.46, 95% CI 1.43–1.50), appendicitis (aOR 1.29, 95% CI 1.13–1.48) and periodontitis (aOR 1.35, 95% CI 1.31–1.38) than non-AS control subjects. The association between tonsillitis and AS was consistent using varying definitions for tonsillitis, and we further found that a high frequency of visits for tonsillitis, a high medical cost for tonsillitis and a long interval between diagnosis were associated with newly diagnosed AS in a dose-response manner. Furthermore, the association between tonsillitis and AS appeared to be stronger in females (aOR 1.59, 95% CI 1.53–1.65) than those in males (aOR 1.39, 95% CI 1.35–1.44).

Conclusions

The present study revealed an association between AS risk and prior tonsillitis and indicates the need for vigilance of AS-associated symptoms in patients who had been diagnosed with tonsillitis, particularly in females.

Mucosa-Environment Interactions in the Pathogenesis of Rheumatoid Arthritis

Mucosal surfaces play a central role in the pathogenesis of rheumatoid arthritis (RA). Several risk factors, such as cigarette smoking, environmental pollution, and periodontitis interact with the host at the mucosal level, triggering immune system activation. Moreover, the alteration of microbiota homeostasis is gaining increased attention for its involvement in the disease pathogenesis, modulating the immune cell response at a local and subsequently at a systemic level. Currently, the onset of the clinical manifest arthritis is thought to be the last step of a series of pathogenic events lasting years. The positivity for anti-citrullinated protein antibodies (ACPAs) and rheumatoid factor (RF), in absence of symptoms, characterizes a preclinical phase of RA-namely systemic autoimmune phase- which is at high risk for disease progression. Several immune abnormalities, such as local ACPA production, increased T cell polarization towards a pro-inflammatory phenotype, and innate immune cell activation can be documented in at-risk subjects. Many of these abnormalities are direct consequences of the interaction between the environment and the host, which takes place at the mucosal level. The purpose of this review is to describe the humoral and cellular immune abnormalities detected in subjects at risk of RA, highlighting their origin from the mucosa-environment interaction.

The vermiform appendix: an immunological organ sustaining a microbiome inoculum

The hominoid vermiform appendix has been characterized as a diverticulum of the caecum and describes an entity at the juxtaposition of the colon in the confluence of tanias. The independent development of the lymphoid follicle centres of the appendix is progressed at birth in the presence of the intestinal commensal microbiome, an obligatory prompt for the diversification of intestinal and extra-intestinal mucosal immunological tissue. In the vermiform appendix, this activity is centred on further developing the inventory of primary antibodies and the maturation of T- and B-lymphocyte cells in the follicles within the lymphoid tissue. Furthermore, the columnar epithelia, enterocytes and goblet cells comprise the complement of cells that occupy the lamina propria and muscularis mucosae of the vermiform appendix's mucosa, while macrophages and an abundance of immunoglobulin A and immunoglobulin G generating plasma cells seed the lamina propria Intraepithelial immune cells consisting predominantly of specific CD8⁺ T regulatory lymphocytes occupy sites in the appendix analogous to those present in the intestinal epithelia of the caecal colon. The complement of bacterial genera concealed in the vermiform appendix is posited extant as a biofilm inoculum of the intestinal commensal microbiome. This facilitates re-inoculation of the proximal colon and to a lesser degree the terminal ilium post an intestinal perturbation such as occurs with daily lifestyle stressors, dietary choices and the short-term administration of antibiotics rather than an infectious fulminant colitis. A plausible appreciation results of the importance of multiple immunological aspects of a healthy vermiform appendix and the provision of a commensal biofilm to the gut that repairs a dysbiotic microbiome contributing to balancing intestinal pro- and anti-inflammatory activity for maintaining homeostasis in the gut. Since the composition of the gut microbiome can vary over the short-term and long-term, it is plausible that the appendix inoculum may be instrumental in maintaining the intestinal microbiome.

Impact of age at appendectomy on development of type 2 diabetes: A population-based cohort study

Aim

Diabetes is a complex metabolic disease characterized by chronic low-grade inflammation in which genetic and environmental factors are involved. Growing evidence implicates that alterations of the gut microbiota potentially contribute to the emergence of metabolic diseases. The human appendix has more recently been recognized as a microbial reservoir for repopulating the gastrointestinal tract and an important part of the immune system. Thus, appendectomy may influence microbial ecology and immune function. This study investigated the association between appendectomy and type 2 diabetes risk.

Methods

We analyzed a cohort of 10954 patients who underwent appendectomy between 1998 and 2013 based on the Taiwan National Health Insurance Program database. A comparison cohort of 43815 persons without appendectomy was selected randomly and matched by sex, age, comorbidities, and index year. To ensure reliability of the results, a sensitivity analysis using a propensity score–matched study was performed. We observed the subsequent development of type 2 diabetes in both cohorts.

Results

Although the overall incidence of type 2 diabetes in the appendectomy patients was 7.9% higher than that in the non-appendectomy patients, it was not statistically significant (95% confidence interval [CI], 0.997–1.168) after the adjustment of confounding factors. Multivariate regression analysis revealed that the adjusted hazard ratio (HR) of type 2 diabetes was 1.347 for appendectomy patients < 30 years of age (95% CI, 1.009–1.798) compared to non-appendectomy patients. The incidence of type 2 diabetes was higher within 3 years of post-appendectomy follow-up than for non-appendectomy patients (HR, 2.017; 95% CI, 1.07–3.802). Age impacted the association between appendectomy and type 2 diabetes risk (P_interaction = 0.002); in contrast, sex did not affect the association between appendectomy and type 2 diabetes risk (P_interaction = 0.88).

Conclusions

Our study results suggest that appendectomy increases type 2 diabetes risk, particularly when performed prior to middle age.

Women who had appendectomy have increased risk of systemic lupus erythematosus: a nationwide cohort study

The appendix is involved in immune function, and an appendectomy may alter the immune system. Studies evaluating the relationship between previous appendectomy and the risk of systemic lupus erythematosus (SLE) are lacking. This nationwide cohort study investigated the incidence and risk of SLE in patients who underwent appendectomy. Patients aged > 20 years who received appendectomy from 2000 to 2011 were identified from the National Health Insurance Research Database and assigned to the appendectomy cohort. Patients without appendectomy were randomly selected from the NHIRD and assigned to the control cohort; they were frequency matched to each study patient at a 4:1 ratio by sex, age, and index year. All patients were followed until SLE diagnosis, withdrawal from the National Health Insurance program, or the end of 2011. We used Cox models to estimate the hazard ratio (HR) and 95% confidence interval (CI) to compare the risk of SLE between the appendectomy and control cohorts. From 23.74 million people in the cohort, 80,582 patients undergoing appendectomy and 323,850 patients without appendectomy were followed for 723,438 and 2,931,737 person-years, respectively. The appendectomy cohort had a 2.04-fold higher risk of SLE than the control cohort (adjusted HR = 2.04, 95% CI = 1.52-2.76). Women aged ≤ 49 years who underwent appendectomy had a 2.27-fold higher risk of SLE than the corresponding controls (adjusted HR = 2.27, 95% CI = 1.62-3.19). Women aged ≤ 49 years who underwent appendectomy have a significantly higher risk of SLE.

Increased chronic kidney disease development and progression in diabetic patients after appendectomy: a population-based cohort study

Background

The vermiform appendix serves as a "safe house" for maintaining normal gut bacteria and appendectomy may impair the intestinal microbiota. Appendectomy is expected to profoundly alter the immune system and modulate the pathogenic inflammatory immune responses of the gut. Recent studies have shown that a dysbiotic gut increases the risk of cardiovascular disease and chronic kidney disease (CKD). Therefore, we hypothesized that appendectomy would increase the risk of CKD.

Methods

This nationwide, population-based, propensity-score-matched cohort study included 10,383 patients who underwent appendectomy and 41,532 propensity-score-matched controls. Data were collected by the National Health Insurance Research Database of Taiwan from 2000 to 2013. We examined the associations between appendectomy and CKD and end-stage renal disease (ESRD). The major outcome was a new diagnosis of CKD based on an outpatient diagnosis made at least three times or hospital discharge diagnosis made once during the follow-up period. ESRD was defined as undergoing dialysis therapy for at least 90 days, as in previous studies.

Results

The incidence rates of CKD and ESRD were higher in the appendectomy group than in the control cohort (CKD: 6.52 vs. 5.93 per 1,000 person-years, respectively; ESRD: 0.49 vs. 0.31 per 1,000 person-years, respectively). Appendectomy patients also had a higher risk of developing CKD (adjusted hazard ratio [aHR] 1.13; 95% CI [1.01-1.26]; P = 0.037) and ESRD (aHR 1.59; 95% CI [1.06-2.37]; P = 0.024) than control group patients. Subgroup analysis showed that appendectomy patients with concomitant diabetes mellitus (aHR 2.08; P = 0.004) were at higher risk of incident ESRD than those without diabetes mellitus. The interaction effects of appendectomy and diabetes mellitus were significant for ESRD risk (P = 0.022); no interaction effect was found for CKD risk (P = 0.555).

Conclusions

Appendectomy increases the risk of developing CKD and ESRD, especially in diabetic patients. Physicians should pay close attention to renal function prognosis in appendectomy patients.

Appendectomy and the Risk of Colectomy in Ulcerative Colitis: A National Cohort Study

OBJECTIVES

Ulcerative colitis (UC) is a chronic inflammatory disease usually responding well to anti-inflammatory drugs but many patients will still need colectomy. Appendectomy is associated with a lower risk of later developing UC. We aimed to assess the longitudinal relationship between appendectomy, appendicitis, and disease course in UC patients.

METHODS

A national cohort of UC patients with a diagnosis in 1964-2010 was identified from the Swedish National Patient Register that also provided information regarding appendicitis and/or appendectomy before or after the UC diagnosis. The risk for colectomy and UC-related hospital admissions was evaluated.

RESULTS

Among 63,711 UC patients, 2,143 had appendectomy and 7,690 underwent colectomy. Appendectomy for appendicitis before 20 years of age and for non-appendicitis at all ages before UC diagnosis was associated with a lower risk of colectomy (hazard ratio (HR) 0.44, 0.27-0.72 and HR 0.62, 0.43-0.90, respectively), and fewer hospital admissions (incidence rate ratio (IRR) 0.68, 95% confidence interval (CI) 0.64-0.73 and IRR 0.54, 0.47-0.63, respectively). Appendectomy for appendicitis after the UC diagnosis was associated with a higher risk of colectomy (HR 1.56, 1.20-2.03), whereas no such association was found for other pathology (HR 1.40, 0.79-2.47).

CONCLUSIONS

Appendectomy early in life and before developing UC is associated with a lower risk of colectomy as well as UC-related hospital admissions. Appendectomy for appendicitis after established UC appears associated with a worse disease course, with an increased rate of subsequent colectomy.

Diabetes increases the risk of an appendectomy in patients with antibiotic treatment of noncomplicated appendicitis

BACKGROUND

This retrospective cohort study examined whether diabetic patients have a higher risk for recurrent appendicitis during a 1-year follow-up period after successful antibiotic treatment for patients with acute uncomplicated appendicitis than nondiabetic patients using a population-based database.

METHODS

We included 541 appendicitis patients who received antibiotic treatment for acute appendicitis. We individually tracked each patient for a 1-year period to identify those who subsequently underwent an appendectomy during the follow-up period.

RESULTS

Cox proportional hazard regressions suggested that the adjusted hazard ratio of an appendectomy during the 1-year follow-up period was 1.75 for appendicitis patients with diabetes than appendicitis patients without diabetes. We found that among females, the adjusted hazard ratio of an appendectomy was 2.18 for acute appendicitis patients with diabetes than their counterparts without diabetes. However, we failed to observe this relationship in males.

CONCLUSIONS

We demonstrated a relationship between diabetes and a subsequent appendectomy in females who underwent antibiotic treatment for noncomplicated appendicitis.

Appendectomy correlates with increased risk of pyogenic liver abscess: A population-based cohort study in Taiwan

Little is known on the association between appendectomy and pyogenic liver abscess. The objective of this study was to investigate the association between appendectomy and the risk of pyogenic liver abscess in Taiwan.This population-based retrospective cohort study was conducted using the hospitalization dataset of the Taiwan National Health Insurance Program. There were 212,530 subjects age 20 to 84 years with newly diagnosed appendectomy as the appendectomy group since 1998 to 2010, and 850,099 randomly selected subjects without appendectomy as the nonappendectomy group. Both appendectomy and nonappendectomy groups were matched with sex, age, comorbidities, and index year of diagnosing appendectomy. The incidence of pyogenic liver abscess at the end of 2011 was estimated in both groups. The multivariable Cox proportional hazards regression model was applied to investigate the hazard ratio (HR) and 95% confidence interval (CI) for risk of pyogenic liver abscess associated with appendectomy and other comorbidities including alcoholism, biliary stone, chronic kidney disease, chronic liver diseases, and diabetes mellitus.The overall incidence of pyogenic liver abscess was 1.73-fold greater in the appendectomy group than that in the nonappendectomy group (3.85 vs 2.22 per 10,000 person-years, 95% CI 1.71, 1.76). The multivariable regression analysis disclosed that the adjusted HR of pyogenic liver abscess was 1.77 for the appendectomy group (95% CI 1.59, 1.97), when compared with the nonappendectomy group.Appendectomy is associated with increased hazard of pyogenic liver abscess. Further studies remain necessary to confirm our findings.