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Evidence for interaction between disease severity and comorbidity in rheumatoid arthritis? comment on the article by Navarro-Cano et alTo the Editor:Navarro-Cano et al are to be congratulated on their elegant study demonstrating the independent effects of disease severity and comorbidity on rheumatoid arthritis (RA) mortality (Navarro-Cano G, del Rincón I, Pogosian S, Roldàn JF, Escalante A. Association of mortality with disease severity in rheumatoid arthritis, independent of comorbidity. Arthritis Rheum 2003;48:2425-33). I am only curious about one aspect they do not report: was there any evidence for interaction between these 2 factors?It is tempting to speculate that the excess mortality in RA could be, in part, explained by such an interaction, especially as regards cardiovascular comorbidity. We should, of course, be aware that detecting main effects is always easier than detecting interaction, but it would be interesting to learn the results of such an analysis in this data set.