Geographic Association Between Incidence of Acute Appendicitis and Socioeconomic Status

Abstract

Importance: Some studies based on proportions of patients with perforated appendicitis (PA) among all patients with acute appendicitis (AA) have found an association between socioeconomic status (SES) and risk of perforation. A potential limitation is their use of proportions, which assumes that incidence of AA is evenly distributed across populations at risk. This assumption may be invalid, and SES may have a more complex association with both AA and PA.

Objective: To generate population-based incidences of AA and PA and to examine geographic patterns of incidence alongside geographic patterns of SES.

Design, setting, and participants: Retrospective study of data from Washington's Comprehensive Hospital Abstract Reporting System and the 2010 US census. Geographic methods were used to identify patterns of age- and sex-standardized incidence in Washington State between 2008 and 2012. The study included all patients discharged with International Classification of Diseases, Ninth Revision codes for AA or PA. Data were analyzed between November 2016 and December 2018.

Exposures: Location of primary residence.

Main outcomes and measures: Age- and sex-standardized incidence for AA and PA was generated for each census tract (CT). Global spatial autocorrelation was examined using Moran index (0.0 = completely random incidence; 1.0 = fully dependent on location). Clusters of low-incidence CTs (cold spots) and high-incidence CTs (hot spots) were identified for AA. Census-based SES data were aggregated for hot spots and cold spots and then compared.

Results: Statewide, over the 5-year study period, there were 35 730 patients with AA (including 9780 cases of PA), of whom 16 574 were women (46.4%). Median age of the cohort was 29 years (IQR, 16-48 years). Statewide incidence of AA and PA was 106 and 29 per 100 000 person-years (PY), respectively. Crude incidence was higher within the male population and peaked at age 10 to 19 years. Age- and sex-standardized incidence of AA demonstrated significant positive spatial autocorrelation (Moran index, 0.30; P < .001), but autocorrelation for PA was only half as strong (0.16; P < .001). Median incidence of AA was 118.1 per 100 000 PY among hot spots vs 86.2 per 100 000 PY among cold spots (P < .001). Socioeconomic status was higher in cold spots vs hot spots: mean proportion of college-educated adults was 56% vs 26% (P < .001), and mean per capita income was $44 691 vs $30 027 (P < .001).

Conclusions and relevance: Age- and sex-standardized incidence of appendicitis is not randomly distributed across geographic subunits, and geographic clustering of AA is twice as strong as PA. Socioeconomic advantages, such as higher income and secondary education, are strongly associated with lower incidence of AA. These findings challenge conventional views that AA occurs randomly and has no predisposing characteristics beyond age/sex. Socioeconomic status, and likely other geographically circumscribed factors, are associated with incidence of AA.

Figure 1.. Study Flowchart of Inclusion and Exclusion Criteria

This figure illustrates how the initial data set of 45 436 patients was narrowed to a study cohort of 35 735 patients, as well as their designation as cases of perforated appendicitis or acute appendicitis. ICD-9 indicates International Classification of Diseases, Ninth Revision (ICD-9) and LOS, length of stay.

Figure 2.. Illustration of Moran Index for Spatial Autocorrelation

This figure illustrates the range of possible results for Moran index for Spatial Autocorrelation and how these values (from −1 to 1) are interpreted.

Figure 3.. Incidence of Acute Appendicitis (AA) and Perforated Appendicitis (PA) in the Tricounty Puget Sound Region

This figure depicts the population-based incidence of AA and of PA in the study area. The Moran index of 0.30 (P < .001) for AA and of 0.16 (P < .001) for PA indicate a statistically significant nonrandom geographic distribution of incidence for both conditions. The strength of the association between geographic location and incidence is twice is strong for AA as for PA.

Figure 4.. Getis-Ord Gi* Clustering Analysis of Hot Spots and Cold Spots of Incidence of Acute Appendicitis (AA) and Perforated Appendicitis (PA)

Getis-Ord Gi* is a regression-based geographic method that identifies clusters of spatially associated geographic subunits such as census tracts at 90, 95, and 99 percent confidence levels. A, Analysis for AA and identifies hot and cold spots for age- and sex-standardized incidence of AA. B, Analysis for PA and identifies hot and cold spots for age- and sex-standardized incidence of PA.