RESEARCH ETHICS

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

D Adamis¹, F C Martin², A Treloar³ and A J D Macdonald³

¹ Academic Unit, Ladywell House, London, UK
² Department of Ageing and Health, St Thomas’ Hospital, London, UK
³ Department of Old Age Psychiatry, Guy’s, King’s, and St Thomas’ Schools of Medicine, Dentistry and Biomedical Sciences, London, UK

Correspondence to:
Correspondence to:
Dr D Adamis
Academic Unit, Ladywell House, 330 Lewisham High Street, London SE13 6JZ, UK; dimaadamis{at}yahoo.com

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 generalisable. A different method is necessary to achieve an ethical balance between respecting autonomy through obtaining adequate informed consent and avoiding sample bias.

Keywords: capacity; consent; formal assessment; delirium; geriatrics

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