Natural history studies provide invaluable data on the disease course. Rungoe et al1 report changes in medical treatment and surgery rates over the past three decades (1979–2011) using a large Danish population-based cohort of 48 467 patients with IBD issued from a Danish health administrative database. Interestingly, the authors found a decrease in surgery rates in both Crohn's disease (CD) and UC. Changes in surgery rates were accompanied by a significant decrease in the use of five aminosalicylic acids (5-ASA) and corticosteroids. The use of 5-ASA significantly decreased from 51% in 1995–2002 to 19% in 2003–2011 in CD; surprisingly, it also decreased in UC from 63% to 51%, while in UC 5-ASA remain the backbone therapy for mild-to-moderate UC according to recent international guidelines.2 As expected, there was a dramatic increase between 1995–2002 and 2003–2011 in the use of thiopurines and anti-tumor necrosis factor (TNF) 10 years after diagnosis: from 32% to 39% for azathioprine and from 10% to 23% for anti-TNF in CD. In UC, this increase was smaller than in CD and did not reach statistical significance. These rates are much lower than those reported in a French referral centre where the cumulative probabilities of receiving thiopurines and anti-TNF 5 years after CD diagnosis were 70% and 65%, respectively.3 Such discrepancy between referral centres and population-based studies demonstrates the importance to conduct cohort studies at the population level to avoid selection bias from referral centres in which patients with more severe disease are usually treated.

Overall, …