Review for 10.1186/1471-230X-14-140 (2014)

Reviewer's report
Title:Decreasing Incidence of Inflammatory Bowel Disease in Eastern Canada: A
Population database study
Version:2Date:17 April 2014
Reviewer: Sander Veldhuyzen van Zanten
Reviewer's report:
This is a carefully-conducted population-based study addressing the question
whether the incidence of inflammatory bowel disease, specifically Crohn’s
disease and ulcerative colitis, has changed in Nova Scotia.
Nova Scotia is an Atlantic province in Canada which has a relatively stable
population of close to 1 million people. Previously used and validated case
definitions were used for Crohn’s disease, ulcerative colitis and colitis
undetermined. A washout period of five years was used to avoid including
patients with inflammatory bowel disease who may have had a relatively benign
course of their disease, not warranting at least a visit once a year to a physician.
In order to fit the diagnostic criteria, patients needed to have at least one
hospitalization for IBD or four or five physician outpatient visits for IBD over a
two-year period. Using this case definition, they showed a significant decrease in
age-standardized incidence in both Crohn’s disease and ulcerative colitis for
virtually all age groups. Somewhat of a weakness is the high percentage of 16%
of undetermined inflammatory bowel disease. The latter number is certainly
higher than what one would encounter in clinical practice. It is acknowledged that
this may be related to the way the data were coded at the time that the billing for
the patient took place. I think the discussion should be expanded somewhat to
further delve into this issue. For example, if one assumed that the
mis-clarification rate among Crohn’s disease, ulcerative colitis and undetermined
colitis remained the same throughout the period of study why then would the
incidence in the undetermined cases not change. It would also be interesting to
see whether observed if the years 1991 and 1995 showed the same trend if they
expanded the analysis to include those years.
The reviewer recommends that the diagnostic criteria for each disease are listed
in the table so that readers do not have to go back to the references, if they really
are interested in knowing more about this.
I think the discussion also needs to be expanded to look at the frequency with
which colonoscopy and perhaps CT are performed in the province. Certainly, in
the 1980s and 1990s, there has been an increase in the rate of endoscopic
procedures. It is therefore possible that during the early part of the cohort
individuals still were subject to detection bias because of the increased rate of
endoscopic procedures followed by a subsequent leveling off. If data on
colonoscopy rates were available over the same time period in Nova Scotia, that
would be helpful and interesting. The reviewer understands that this may not be
easily available but then this point should at least be addressed in the discussion.
Overall, this is an interesting study that would provide new data to the literature.
The total number of cases is relatively small as a result of the relatively small
population in the province. Clearly, data from larger cohorts need to also address
this question to see whether the changes in trends can be replicated elsewhere.
Level of interest: An article of outstanding merit and interest in its field
Quality of written English: Acceptable
Statistical review: No, the manuscript does not need to be seen by a statistician.
Declaration of competing interests:
no financial COI
I am a good friend of one of the authors