Secular Trends in Hospitalization for IBD: The Ecological Niche

One of the most common designs in epidemiology research is the ecological study, in which the units of analysis are populations or groups of people, rather than individuals. In addition to their ease and low-cost, these studies can also provide very valuable information when individual-level data are difficult to obtain or when the primary interest is in the ecologic inference, such as in public policy or program legislation. The article by Dr. Sonnenberg and colleagues in this month’s issue adds to a growing body of ecological studies examining the secular trends (trends that are not cyclical or seasonal but exist over a relatively long period of time) of hospitalizations for inflammatory bowel disease (IBD) in this country [1]. Prior studies have demonstrated stable to increasing hospitalization rates for ulcerative colitis (UC) and increasing hospitalization rates for Crohn’s disease (CD) in the US [2, 3]. Dr. Sonnenberg’s work extends this analysis to the US military veteran’s population, increasing the body of data on ageand race-specific results. The authors found rising hospitalization rates until 1989 when rates of hospitalizations decreased overall. However, when examining these trends by race, they found increasing rates of hospitalization in non-whites, most significantly in the oldest (65?) age group, for both UC and CD throughout the study period. Prior studies have shown race-specific differences in utilization of surgery as well as disease manifestations and complications in IBD patients [4–8]. Age-specific changes in hospitalizations were not found in one prior study [2], but were found to peak during earlier years in CD patients in another [3]. Taken together, these ecological studies suggest that inpatient care for IBD may represent an increasingly large proportion of our healthcare resources. There are several reasons why one might observe an increase in IBD hospitalizations over time. The simplest is that there are more people being hospitalized, the same number of people are being hospitalized more often, or a combination of the two. Likewise, if there is an increase in the actual number of patients with IBD, the rate of hospitalization per individual could be less, but the rate of hospitalization could appear to be rising solely because the disease is becoming more common. However, ecological studies are prone to certain biases that can result in an artifactual appearance of long-term trends. Like all studies, the data used to examine secular trends needs to be accurately measured to avoid misclassification bias, in which errors occur in classifying populations exposure or disease status. In this article by Sonnenberg, the investigators have measured incidence of hospitalization in each year relative to the total population of the United States. The potential problem with this design is that the total USA population is not eligible to be hospitalized in Veteran Affairs hospitals. Thus, observed trends could reflect disproportionate changes in the demographics and size of the population eligible to use the Veteran Affairs resources rather than a true change in the rates of hospitalizations among eligible Veterans. To address this, Sonnenberg et al. also used the population of patients hospitalized in the Veteran Affairs healthcare system for any reason as an alternative denominator for defining hospitalization rates. This comes closer to defining the truly eligible population, but remains subject to bias from secular trends in hospitalization for other conditions. Reassuringly, results using this alternative denominator were relatively similar to the primary results in their study. Ecological studies may also suffer from confounding by group, where the apparent association between a group and M. Bewtra (&) J. D. Lewis University of Pennsylvania, Philadelphia, Pennsylvania, USA e-mail: Meenakshi.Bewtra@uphs.upenn.edu