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Gut. 2007 July; 56(7): 1033.
PMCID: PMC1994349

Authors' response

As Fernández‐Banñares et al point out, it is difficult to compare crude incidence or prevalence rates across populations with different demographic structures. They suggest that we report our population pyramid to aid in interpreting the difference in microscopic colitis incidence observed in Minnesota compared with Spain. However, this would not be appropriate as there is no simple way to integrate the population structure with the various age‐ and sex‐specific incidence rates and arrive at any kind of coherent conclusion. Instead, as they suggest, one should standardise the local incidence rates to an external reference population, which we, in fact, did in our paper.1 As we standardised our rates to the 2000 US white population, it would be the demographic “pyramid” to consider. Standardising their age‐ and sex‐specific incidence rates to this same reference population to enable a direct comparison would be even better. Unfortunately, the age‐ and sex‐specific incidence rates with which to accomplish this comparison are not reported in their paper.

The remaining alternative is to carry out an indirect standardisation, whereby we apply our own age‐ and sex‐specific incidence rates to their population structure. Thus, if the age‐ and sex‐specific incidence rates for microscopic colitis in Olmsted County, Minnesota, USA, in 1985–20011 are applied to the comparable age‐ and sex‐specific denominator populations in the Hospital Mútua de Terrassa health district of Terrassa, Spain in 1993–7, then one would expect to see 73 cases of microscopic colitis instead of the 60 cases that were actually observed. Thus, the Olmsted County rates are still higher even after accounting for any demographic differences between the two populations.

Therefore, explanations other than differences in the underlying age structure in our respective populations must be considered. One possible explanation, as discussed in our paper, is different exposure rates in the two populations to certain environmental factors, such as medications, associated with microscopic colitis.2 Higher incidence rates could also be expected if a higher percentage of our population underwent sigmoidoscopy or colonoscopy with mucosal biopsies for the evaluation of diarrhoea—that is, differential application of the diagnostic test.

Footnotes

Competing interests: None.

References

1. Pardi D S, Loftus E V, Smyrk T C. et al The epidemiology of microscopic colitis: a population‐based study in Olmsted County, Minnesota. Gut 2007. 56504–508.508 [PMC free article] [PubMed]
2. Beaugerie L, Pardi D S. Drug‐induced microscopic colitis: a proposal for a scoring system and review of the literature. Aliment Pharmacol Ther 2005. 22277–284.284 [PubMed]

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