Study population

Adults 18 years of age or older at mBC diagnosis were identified via diagnosis codes: International Classification of Diseases (ICD), Ninth Revision, Clinical Modification (ICD-9-CM; 174.0–174.9 and 175.0–175.9), ICD-Tenth Revision, Clinical Modification (ICD-10-CM; C50.xxx) or mBC de novo metastatic criteria (i.e. presence of Stage IV and/or a secondary malignant neoplasm code such as C78–C79) using the Systematized Nomenclature of Medicine Clinical Terms code system. mBC diagnosis must have occurred between 01 January 2010 and 31 December 2018, although patients may have been diagnosed with non-mBC at an earlier date. Patients had to have a HER2-negative biomarker test result (fluorescence in situ hybridization [FISH] negative/not-amplified, immunohistochemistry negative [0, 1 or 2] or negative not otherwise specified) and no indicators for HER2-positive disease (FISH positive/amplified, immunohistochemistry positive [3 +] or positive not otherwise specified), within 30 days of mBC diagnosis. Patients without at least one EHR entry for a patient encounter, treatment laboratory test or related intervention after their mBC diagnosis date or patients with any other primary cancer (except non-metastatic, non-melanoma skin cancer) before or on the date of mBC diagnosis were excluded. Comorbidities recorded in EHRs during the 12-month period before or on the date of mBC diagnosis were used to calculate weighted Charlson Comorbidity Index (CCI) scores [30, 31]; patients with a missing CCI score were recorded as having a score of 0.

Patients were assigned to one of three cohorts according to their BRCA status (recorded at any time before or after the date of mBC diagnosis): BRCAm, BRCA wild type (BRCAwt) or unknown BRCA status (BRCAu). Patients were also stratified by HR status (HR-positive or TNBC).