1084 Background: In recent years novel therapies have been approved for patients (pts) with mBC and ESR1 , AKT1 , PTEN , PIK3CA , and gBRCA alterations. Given limited evidence on which patients are receiving standard of care, this study assessed racial and ethnic inequities in biomarker testing and the role of social determinants of health (SDOH) in explaining potential inequities. Methods: This study leveraged the US nationwide Flatiron Health electronic health record (EHR)-derived, deidentified database of > 750 000 pts with BC. Adult female pts diagnosed (dx) with HR+ HER2- mBC between 1/1/2011, and 4/30/2024, with a geocodeable address were included. Testing rates for alterations in ESR1 , PIK3CA , AKT1 , PTEN , and gBRCA were measured over time from mBC dx using variables extracted from unstructured clinician documentation in the EHR using machine learning. Fine and Grey models accounting for competing risks were used to estimate subdistribution hazard ratios (HR) and 95% confidence intervals (CI) for biomarker access. Models were adjusted for covariates including age, stage, ECOG status, and dx year, followed by practice setting and area-level SDOH factors (ie, English language proficiency, residential segregation, vehicle ownership, urbanicity, and residence in medically underserved areas). Results: The cohort included 36 316 pts (61.5% non-Latinx [NL]-White, 6.1% Latinx, 9.7% NL-Black, 1.9% NL-Asian, and 20.8% NL-Other/Unknown). Overall, Asian, Black, and Latinx pts were less likely than White pts to undergo biomarker testing (adjusted HR [95% CI]: Latinx, 0.88 [0.82-0.95]; NL-Black, 0.87 [0.82-0.93]; NL-Asian, 0.87 [0.76-0.98]). Racial/ethnic inequities in overall biomarker testing were partially explained by SDOH factors. Specifically, the White-Latinx inequity in testing was mediated by residential segregation ie, association attenuated towards the null (mediated HR [95% CI], 0.94 [0.87-1.02]), limited English proficiency (0.92 [0.85-1.00]), and lack of vehicle ownership (0.91 [0.84-0.98]). Compared with White pts, NL-Black pts were less likely to be tested for ESR1 (HR [95% CI], 0.86 [0.77-0.95]) and PIK3CA (0.86 [0.80-0.92]). Latinx pts were less likely to be tested for PIK3CA (0.87 [0.80-0.95]) and this inequity was mediated by residential segregation (0.96 [0.87-1.05]) and limited English proficiency (0.92 [ 0.84-1.00]). Conclusions: Asian, Black, and Latinx pts were generally less likely than their White counterparts to receive biomarker testing after a mBC dx, especially for PIK3CA and ESR1 . SDOH factors explained some of these biomarker testing inequities. Equitable access to biomarker testing should be prioritized to ensure patients have access to the most effective therapies. Future research should examine whether racial/ethnic inequities in biomarker testing are associated with inequities in treatment and outcomes.
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