To determine if the choice of methodological elements affects the results in continuity of care studies.
This is a retrospective cohort study. The association between continuity of care and clinical outcome was investigated using the Continuity of Care Index. The association was explored in 12 scenarios based on four definitions of the relative timing of continuity and outcome measurements in three populations (three s × four s).
National Health Insurance claims from all primary and secondary care facilities in South Korea between 2007 and 2015.
Participants were patients diagnosed with dyslipidaemia, made ≥2 ambulatory visits and were newly prescribed with ≥1 antihyperlipidaemic agent at an ambulatory setting in 2008. Three study populations were defined based on the number of ambulatory visits: 10 084 patients in population 1 (P1), 8454 in population 2 (P2) and 4754 in population 3 (P3).
Hospitalisation related to one of the four atherosclerotic cardiovascular diseases, including myocardial infarction, stable or unstable angina, ischaemic stroke and transient ischaemic attack.
Concurrent measure of continuity and outcome (T1) showed a significantly higher risk of hospitalisation (adjusted HRs: 2.73-3.07, p<0.0001) in the low continuity of care group, whereas T2, which measured continuity until the outcome occurred, showed no risk difference between the continuity of care groups. T3, which measured continuity as a time-varying variable, had adjusted HRs of 1.31-1.55 (p<0.05), and T4, measuring continuity for a predefined period and measuring outcomes in the remaining period, had adjusted HRs of 1.34-1.46 (p<0.05) in the low continuity of care. Within each temporal relationship, the effect estimates became more substantial as the inclusion criteria became stricter.
The study design in continuity of care studies should be planned carefully because the results are sensitive to the temporal relationship between continuity and outcome and the population selection criteria.

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