Capacity, Consent, and Selection Bias in a Study of Delirium
Journal of Medical Ethics 2005 March; 31(3): 137-143
OBJECTIVES: To investigate whether different methods of obtaining informed consent affected recruitment to a study of delirium in older, medically ill hospital inpatients. DESIGN: Open randomised study. SETTING: Acute medical service for older people in an inner city teaching hospital. PARTICIPANTS: Patients 70 years or older admitted to the unit within three days of hospital admission randomised into two groups. INTERVENTION: Attempted recruitment of subjects to a study of the natural history of delirium. This was done by either (a) a formal test of capacity, followed by either a request for consent or an attempt at obtaining assent from a proxy, or (b) a combined informal capacity/consent process. MAIN OUTCOME MEASURES: Prevalence and severity of delirium, and, as case mix measures, length of hospital stay and destination on discharge. RESULTS: Recruitment of subjects through establishing formal capacity and then informed consent was less successful (43.9% v 74% of those approached) and, compared with those recruited through the usual combined capacity/consent approach, yielded a sample with less cognitive impairment, lower severity of delirium, lower probability of case note diagnosis of delirium and lower rate of entering a care home. CONCLUSIONS: Methods of obtaining informed consent may significantly influence the case mix of subjects recruited to a study of delirium. Stringent testing of capacity may exclude patients with delirium from studies, thus rendering findings less generalizable. A different method is necessary to achieve an ethical balance between respecting autonomy through obtaining adequate informed consent and avoiding sample bias.
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