Capacity, consent, and selection bias in a study of delirium

To investigate whether different methods of obtaining informed consent affected recruitment to a study of delirium in older, medically ill hospital inpatients.

Open randomised study.

Acute medical service for older people in an inner city teaching hospital.

Patients 70 years or older admitted to the unit within three days of hospital admission randomised into two groups.

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.

Prevalence and severity of delirium, and, as case mix measures, length of hospital stay and destination on discharge.

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.

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.