BACKGROUND: delirium and dementia co-exist commonly in hospital. Older people with delirium have high rates of undiagnosed dementia, but delirium affects the use of cognitive testing in dementia diagnosis. Novel methods to detect dementia in delirium are needed. The purpose of the study was to investigate the diagnostic test accuracy of informant tools to detect dementia in hospitalised older people with delirium. METHODS: the presence of dementia on admission was assessed using the short form of the Informant Questionnaire of Cognitive Decline in the Elderly (IQCODE-SF) and Alzheimer's Disease 8 (AD8) in people over 70 years old with delirium. Reference standard diagnosis was made using Diagnostic and Statistical Manual of Mental Disorders fourth edition (DSM-IV) criteria at 3 months. The main outcome measures were the diagnostic test accuracy of the IQCODE-SF and the AD8 in diagnosing DSM-IV dementia. RESULTS: dementia prevalence at 3 months was 61%. The area under the receiver operating characteristic curve (AUROC) was 0.93 (P < 0.0005) for admission IQCODE-SF and 0.91 (P < 0.0005) for admission AD8. An IQCODE-SF test result of >3.82 on admission had a sensitivity of 0.91 (0.79-0.97) and specificity of 0.93 (0.76-0.99) for detecting dementia. An AD8 of >6 had a sensitivity of 0.83 (0.69-0.92) and specificity of 0.90 (0.72-0.97) for detecting dementia. CONCLUSION: the IQCODE-SF and AD8 are sensitive and specific tools to detect prior dementia in older people with delirium. The routine use of either tool in practice could have important clinical impact, by improving the recognition and hence management of those with dementia.