A systematic review «Pereira Gray DJ, Sidaway-Lee K, White E ym. Contin...»1 conducted in 2018 included 22 studies (out of 726 articles identified in searches) which fulfilled the eligibility criteria. Using the Newcastle-Ottawa Scale, all 22 studies were rated as high quality, with nine 10 studies (40.9%) gaining maximum scores from both reviewers independently. No study was scored less than 7 out of 9 by any reviewer. All of the reports were published since 2010.
The number of patients in the studies varied between 287 and 396 838. The studies were all cohort or cross-sectional and most adjusted for multiple potential confounding factors. These studies came from nine countries with very different cultures and health systems. It was not possible to combine the results of studies, due to the heterogeneity of continuity and mortality measurement methods and time frames. Nine of the studied researched general practitioners' patients, three only specialists' patients, and 10 included doctors of any kind.
The most common measure for continuity of care was the Usual Provider of Care (UPC) index which was used in 10 studies (45.5%). Six studies used more than one measure, some only for sensitivity analysis. The length of time over which continuity was measured (when not a survey response or hospital visit indicating a relationship) varied greatly between studies, from a single weekend in hospital up to 17 years. The median length of continuity measurement was 2 years. Most of the studies (20, 90.9%) reported all-cause mortality as a primary outcome.
18 (81.8%) high-quality studies reported statistically significant reductions in mortality follow up time ranging from 6 months to 12 years, with increased continuity of care, 16 of these were with all-cause mortality. These significant protective effects occurred with both generalist and specialist doctors. Three others showed no statistically significant association and one demonstrated mixed results. I.e. in three Canadian studies, the proportion of people dying was significantly lower in high-continuity groups, proportions varying between 1.4-9% in high-continuity groups, and 1.9-18% in low-continuity groups, respectively (p < 0.05 for all).
All the studies were observational, and the majority were high-quality cohort studies including three prospective cohort studies. The issue of reverse causality applies to all the evidence presented here. This could bias an association between continuity of care and mortality in either direction. As patient health worsens when approaching death, continuity of care may deteriorate for many reasons, for example, patients moving areas to accommodate increased health needs, the need to see more specialists or a loss of ability to obtain and attend appointments.