Diagnostic misclassification reduces the ability to detect linkage in inflammatory bowel disease genetic studies

Background-Linkage data have now identified several inflammatory bowel dise ase (IBD) susceptibility loci but these data have not been consistently rep licated in independent studies. One potential explanation for this is the p ossibility that patients enrolled in such studies may have been erroneously classified with respect to their diagnosis. Aims-To determine the rate and type of misclassification in a large populat ion of individuals referred for participation in an IBD genetics study and to examine the effect of diagnostic misclassification on the power to detec t linkage. Methods-The medical records of 1096 patients entered into an IBD genetics p rogramme were reviewed using standardised diagnostic criteria. The original patient reported diagnoses were changed, if necessary, based on review, an d the reasons for the change in diagnosis were recorded. To evaluate the ef fect of misclassification on linkage results, simulations were created with Gensim and analysed using Genehunter to evaluate a model for IBD inheritan ce. Results-Sixty eight of 1096 (6.2%) individuals had a change in diagnosis fr om that originally reported. The majority of changes were patients with eit her Crohn's disease or ulcerative colitis who were determined not to have I BD at all. The principal reasons for changes to the original diagnosis were discordance between the patients' subjective reports of diagnosis and actu al clinical history, endoscopic, or pathological results; a change in disea se pattern over time; and insufficient information available to confirm the original diagnosis. A 10% misclassification rate resulted in 28.4% and 40. 2% loss of power to detect a true linkage when using a statistical model fo r a presumed IBD locus with lambda (s) values of 1.8 and 1.3, respectively. Conclusions-Diagnostic misclassification occurs in patients enrolled in IBD genetic studies and frequently involves assigning the diagnosis of IBD to nonaffected individuals. Even low rates of diagnostic misclassification can lead to significant loss of power to detect a true linkage, particularly f or loci with modest effects as are likely to be found in IBD.