F. T. Drake1, R. Golz3, C. Donovan3, X. Liu3, D. R. Flum2, S. E. Sanchez1 3San Francisco State University,Geography And Environment,San Francisco, CA, USA 1Boston Medical Center,Boston, MA, USA 2University Of Washington,Seattle, WA, USA
Introduction: Acute appendicitis (AA) has long been considered an ideal model for studying surgical disparities because AA is thought to be random in onset and to inevitably progress towards perforated appendicitis (PA) without treatment. However, these assumptions may be inaccurate. Most studies of risk factors for perforation rely on proportions of PA among cases of AA, but this depends on an equivalent baseline risk of AA across groups being compared. We studied geographic patterns in population-based incidences of AA and PA and evaluated these patterns for associations with socioeconomic status (SES).
Methods: We queried a statewide administrative database for adults with appendicitis treated between 2008-2012. Population estimates were based on the 2010 US Census. We generated age/sex-standardized incidences for AA and PA at the census tract level. The Tri-county Puget Sound was our analytic area (>50% WA population). Geographic correlation of incidence rates (“clustering”) was examined using Moran’s Index for spatial dependency.
Results: Overall annual incidence of AA and PA was 106/100,000 and 29/100,000. Incidence was strongly associated with male sex and peaked at 10-19 years. Age/sex-standardized incidence of AA showed strong geographic clustering (Moran’s Index 0.30, p<0.001), meaning it is not randomly distributed across the region. PA was also clustered (0.16, p<0.001), but the geographic association was only half as strong. Areas of low-incidence (“cold spots”) and high-incidence (“hot spots”) were identified using the Getis-Ord GI* statistic [Figure]. One low-incidence region, the Seattle area, had an AA incidence of 88/100,000/year. A comparable hot spot, the Tacoma area, had an AA incidence of 120/100,000/year. However, the rate ratio of PA to AA was similar: 0.29 for low-incidence regions and 0.26 for high incidence regions. SES markers such as college education and income were dramatically higher in low incidence regions compared to high incidence regions. Other SES markers were similar, including high school education, employment, public assistance, and race.
Conclusion: Incidence of appendicitis is not randomly distributed across geographic space, and AA is twice as clustered as PA. These findings challenge the conventional view that AA occurs randomly and has no predisposing characteristics or circumstances. Rate-ratios are similar between high incidence and low incidence regions. Interestingly, major markers of advantaged-SES are strongly associated with low-incidence regions of PA and AA. Relationships between SES and both AA/PA are more complex than previously understood, and proportion of PA is an inadequate measure of surgical disparities.
