The first national study of the prevalence of IBD in Australia has found rates to be twice as a high as previously estimated in smaller regional studies.

Based on lifetime IBD diagnosis data from more than 2.4 million primary care patients, the national study also found that there were slightly higher rates of ulcerative colitis than Crohn’s Disease.

The investigation, conducted by researchers from NPS MedicineWise, Sydney, looked for mentions of IBD in electronic medical records of patients from 481 GP practices via the MedicineInsight program between 2017 and 2019.

It found that the unadjusted prevalence of IBD, Crohn’s Disease and ulcerative colitis was 653 per 100,000, 306 and 334, respectively.

These estimates were considerably higher than those from a 2010–11 study from Victoria which showed the crude prevalence of IBD to be 344.6 per 100,000 (197.3 for Crohn’s Disease and 136.0 per 100,000 for ulcerative colitis, the study authors noted.

However the rates were on a par with the prevalence of IBD recently reported in European countries, they said.

“The higher prevalence in our study may reflect the increasing incidence of IBD, improved life expectancies of patients with IBD; variations in the source population  … and differences in data collection,” they wrote.

The use of lifetime prevalence may also give rise to higher rates than single point (current) prevalence estimates if patients who are in long term remission were included, they added.

Consistent with other Australian studies they found that the prevalence of Crohn’s Disease was greater among females than males but the prevalence of ulcerative colitis was similar for both sexes.

Indigenous people also had a lower risk for developing IBD, consistent with previous Australian reports.

Smoking was independently associated with a greater risk of Crohn’s Disease (Odds Ratio 1.13), but with a lower risk of ulcerative colitis (OR 0.52).

This paradoxical observation had no obvious biological mechanism but might be due to smoking-induced epigenetic alterations, disruption in intestinal microbiota, integrity of the intestinal epithelium and the immune system, the study authors said

There were also regional variations in prevalence of IBD, with people in Tasmania and South Australia showing higher rates than other states.

The study investigators said that there appeared to be no current Australian national prevalence estimate for IBD, and therefore the results from the national primary care database would be useful for planning health services and assessing economic costs.

The results are published in PLOS One.