Background Multimorbidity in older adults is the norm, typically resulting in a complex illness trajectory pervaded by uncertainty. We aimed to identify and synthesise evidence of the uncertainties experienced by older people with advanced multimorbidity, their informal caregivers, and health and social care professionals. We focused on differences in experience between groups.

Methods Systematic review and thematic synthesis of published and grey literature from nine databases. Inclusion criteria: people over 65 with advanced multimorbidity (defined as the presence of two life-limiting conditions, or one life-limiting condition with comorbidities/functional impairment/acute healthcare use); informal carers/healthcare professionals caring for this group; qualitative research with exploration of uncertainty. We used Gough’s Weight of Evidence to assess quality. We extracted the results section of included articles and conducted thematic synthesis, using an existing taxonomy to classify the sources, issues, and loci of uncertainty.

Results From 4427 search results, we included 43 articles from 11 countries. 22 focused on patient experiences of uncertainty (n = 460 participants), 14 carer experience (n=185), and 19 health professional experience (n = 490). Patients experience uncertainty due to unpredictable and constantly changing health. They may be uncertain about the cause of symptoms, how to access care, or how to balance multiple problems, particularly if provided with ambiguous information. As a result, many disengage from care planning and focus on day-to-day life. Carer uncertainties tend to link to more practical concerns: ‘what do I do now?’ Conversely, healthcare professional uncertainties arise as they attempt to disentangle complex combinations of illness; they also experience uncertainty about care processes when multiple professionals are involved.

Conclusion The differing experiences of patients, professionals and carers point to an ‘uncertainty experience gap’ in advanced multimorbidity. Communication interventions to address uncertainty may help to focus care-planning conversations and facilitate delivery of person centred care.