BackgroundThe Comprehensive Geriatric Assessment (CGA) is a multidisciplinary process that determines a frail older person’s medical, functional, psychological and social capability to ensure that they have a co-ordinated plan for treatment and follow-up.ObjectivesTo improve our understanding of the effectiveness, cost-effectiveness and implementation of the CGA across hospital and hospital-at-home settings.MethodsWe used a variety of methods. We updated a Cochrane review of randomised trials of the CGA in hospital for older people aged ≥ 65 years, conducted a national survey of community CGA, analysed data from three health boards using propensity score matching (PSM) and regression analysis, conducted a qualitative study and used a modified Delphi method.ResultsWe included 29 trials recruiting 13,766 participants in the Cochrane review of the CGA. Older people admitted to hospital who receive the CGA are more likely to be living at home at 3–12 months’ follow-up [relative risk (RR) 1.06, 95% confidence interval (CI) 1.01 to 1.10] (high certainty). The probability that the CGA would be cost-effective at a £20,000 ceiling ratio for quality-adjusted life-years (QALYs), life-years (LYs) and LYs living at home was 0.50, 0.89, and 0.47, respectively (low-certainty evidence). After PSM and regression analysis comparing CGA hospital with CGA hospital at home, we found that the health-care cost (from admission to 6 months after discharge) in site 1 was lower in hospital at home (ratio of means 0.82, 95% CI 0.76 to 0.89), in site 2 there was little difference (ratio of means 1.00, 95% CI 0.92 to 1.09) and in site 3 it was higher (ratio of means 1.15, 95% CI 0.99 to 1.33). Six months after discharge (excluding the index admission), the ratio of means cost in site 1 was 1.27 (95% CI 1.14 to 1.41), in site 2 was 1.09 (95% CI 0.95 to 1.24) and in site 3 was 1.70 (95% CI 1.40 to 2.07). At 6 months’ follow-up (excluding the index admission), there may be an increased risk of mortality (adjusted) in the three hospital-at-home cohorts (site 1: RR 1.09, 95% CI 1.00 to 1.19; site 2: RR 1.29, 95% CI 1.15 to 1.44; site 3: RR 1.27, 95% CI 1.06 to 1.54). The qualitative research indicates the importance of relational aspects of health care, incorporating caregivers’ knowledge in care planning, and a lack of clarity about the end of an episode of health care. Core components that should be included in CGA focus on functional, physical and mental well-being, medication review and a caregiver’s ability to care.LimitationsThe risk of residual confounding limits the certainty of the findings from the PSM analysis; a second major limitation is that the research plan did not include an investigation of social care or primary care.ConclusionsThe CGA is an effective way to organise health care for older people in hospital and may lead to a small increase in costs. There may be an increase in cost and the risk of mortality in the population who received the CGA hospital at home compared with those who received the CGA in hospital; randomised evidence is required to confirm or refute this. Caregiver involvement in the CGA process could be strengthened.FundingThe National Institute for Health Research Health Services and Delivery Research programme.