Using twin studies to label disease as genetic or environmental is inappropriate

doi:10.1136/bmj.324.7345.1100/b

BMJ 2002;324:1100 ( 4 May )

Letters

Using twin studies to label disease as genetic or environmental is inappropriate

The first 150 words of the full text of this article appear below.

EDITOR $---$ The cover of the BMJ on 2 February poses the question "Rheumatoid arthritis: is it genetic?" and answers "Probablynot." Neither the question nor the response isvalid.

The Danish twin study on which this conclusion is based identified no concordant monozygotic twin pairs and only two concordantdizygotic pairs from a sample of 37 338.¹ By modelling thereported numbers of confirmed cases of rheumatoid arthritis, wecalculate that the study had 80% power to detect a heritabilityof 65% and 90% power to detect a heritability of 75%. Thus therewas insufficient power to detect a genetic influence on rheumatoidarthritis equivalent to the 60% heritability estimated in thetwo most recent twin studies of the disease.²

The point estimates of concordance are also cause for concern. The prevalence of rheumatoid arthritis at 0.15% seems low,indicating possible deficiencies in screening. The absence ofthe middle aged birth . . . [Full text of this article]

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