Possible patient overlap in studies

I read with great interest the article by Sandra Dial and colleagues, in which they reported an increased risk of Clostridium difficile-associated disease with exposure to proton pump inhibitors.1 The cases in this study were recorded in the United Kingdom General Practice Research Database (GPRD) between Jan. 1, 1994, and Dec. 31, 2004. The authors claim that there was no overlap between the cases in this study and those in a study they published previously.2 Interestingly, in the previous study conducted by Dial and colleagues, data were derived from the same database over the same time period.2 The cases included in that study were instances of a first occurrence of C. difficile-associated disease defined on the basis of a positive C. difficile toxin assay or a clinical diagnosis by a general practitioner. How is it possible that the patients in the current study are not the same ones who were previously studied? It seems very likely that any of the patients treated with oral vancomycin who were included in the current study either would have had a positive C. difficile toxin result or would have received a clinical diagnosis of C. difficile-associated disease during the study period.