The effect on risk estimates of excluding cases from a case-control study of ischemic stroke

There is limited information about the effect on stroke risk estimates of excluding cases who are unable to respond to interviews. A case-control study of ischemic stroke between 1988 and 1990 in Shreveport, La., USA, provided a basis for studying this question. Of 197 consecutively admitted cases of ischemic stroke, 77 were excluded due to dementia, aphasia or impaired consciousness. Information about risk factors and stroke characteristics was obtained from hospital records. Excluded cases had more left hemispheric (52 versus 19%) and fewer vertebrobasilar (12 versus 29%) and lacunar (5 versus 13%) infarcts than included cases. Excluded cases were also older (p < 0.01), and they had larger infarcts (p < 0.01), multiple strokes (p < 0.01) and congestive heart failure (p < 0.01) more often than included cases. Cases were matched to hospital controls by age, sex, race, and date of admission. Odds ratios (ORs) were higher for excluded cases for 5 of 6 exposures with a significantly higher OR for congestive heart failure (p < 0.01). The ORs changed by as much as 63% when excluded cases were added. These results emphasize the importance of acquiring information about excluded cases and considering selection bias when interpreting stroke studies that exclude cases who are unable to respond to interviews.