Variables

For our main outcome variable, we evaluated the difference in the 6-month rate of PCP visits during a 24-month baseline period (the 6–30 mo before diagnosis) and the 6-month treatment period (6 mo from the start of adjuvant chemotherapy). Visits that took place in the emergency department or inpatient locations were excluded. Diagnostic codes were noted. Visits were considered cancer-related if the diagnostic code was listed as female or male breast neoplasm, other malignant neoplasm, breast carcinoma in situ or adverse drug effect.

Our main predictor variables were baseline physical comorbidity and mental health history (MHH). We determined physical comorbidity level using the Johns Hopkins Aggregated Diagnosis Groups (ADGs),¹⁵ which group similar conditions based on characteristics such as duration, severity and specialty care involvement.¹⁶ Each of the roughly 25 000 possible International Classification of Diseases, Ninth Revision and International Classification of Diseases, Tenth Revision diagnosis codes associated with baseline visits were categorized into 1 of 32 ADGs (e.g., time limited: minor; likely to recur: progressive; chronic medical: stable).¹⁷ We excluded psychosocial ADGs and categorized physical comorbidity into low (0–5 physical ADGs), medium (6–9 physical ADGs) and high (≥10 physical ADGs) levels, similar to a previous CanIMPACT study.¹⁸ We determined the presence of MHH based on whether a patient had any PCP visits during the baseline period associated with previously validated mental health diagnostic codes (Appendix 1A).¹⁹

Variables that we considered potential confounders, possibly affecting both physical comorbidity and MHH as well as PCP visits, were chosen a priori based on clinical insight and previously reviewed literature. These confounders included age at diagnosis,^20–24 immigration status (nonimmigrants were classified as Canadian-born citizens or immigrants arriving to Canada before 1985),^25–27 income quintile based on neighbourhood income,^20,24 rurality,^28,29 regional health district (1 of 14 Local Health Integration Networks in Ontario), primary care continuity^30–32 and primary care practice type.³³

Primary care continuity was measured using the Usual Provider of Care index:³⁴ the proportion of visits to the most-often–visited PCP during the 24-month baseline interval for patients with at least 3 visits to any PCP during that interval. As such, continuity of primary care was divided into the following categories: 0 PCP visits, 1–2 PCP visits, low continuity (usual provider of care index ≤ 0.75) and high continuity (usual provider of care index > 0.75). Primary care practice type was determined by enrolment in a particular funding model at the time of diagnosis (“team-based capitation” for interprofessional teams with physicians paid primarily by capitation, “enhanced fee-for-service [FFS]” for physicians paid primarily by FFS with some capitation, “capitation,” “straight FFS” for physicians not enrolled in a primary care model, and “other”).³⁵