Observational study design II

Researchers investigated whether the use of oral or transdermal hormone replacement therapy was a risk factor for stroke. Data were taken from the General Practice Research Database, a computerised database of anonymised longitudinal medical records collected prospectively in primary care. A cohort of women was identified, aged 50-79 years with records between 1 January 1987 and 31 October 2006, and without a diagnosis of stroke on the date of registration with their general practice. A woman was identified as a case if she experienced a stroke during follow-up, and up to four controls were randomly selected from the cohort. Controls were matched to cases on age (within one year) at the date of diagnosis of the stroke, general practice where registered, and the year of joining the practice. The risk of stroke was increased with oral hormone replacement therapy of any dose but only with transdermal patches containing high doses of oestrogen.1

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