10592 Background: An individual diagnosed with a metachronous or synchronous cancer that arises independently, not due to recurrence or metastasis, has multiple primary cancers (MPC). Practice guidelines recommend germline genetic testing based on specific combinations of primary cancers and ages of diagnoses but do not recommend testing for all patients with MPC. In prior work (PubMed ID 37276540), we found that patients with breast cancer are more likely to receive testing than patients with other primary cancer types. Here, we evaluated whether a first breast cancer was more likely than another first cancer type to prompt testing referral. Methods: Using Oncoshare, a breast cancer research database that integrates data from electronic medical records and the California Cancer Registry, we identified female patients with MPC diagnosed between January 2000 and June 2023 with breast cancer as either the first or second cancer and seen at Stanford University. We analyzed testing rates after first vs. second cancer diagnosis and the yield of pathogenic variants (PV). We evaluated the association between the receipt of genetic testing and the timing of breast cancer (1st or 2nd), using both univariate and multivariable logistic regression that adjusted for age at first diagnosis, race/ethnicity, and time between the first and second diagnoses. Results: 1,069 patients met the study eligibility; 75% were non-Hispanic White, and 73% had breast as the first cancer. 342 (32%) patients had testing, and 113 (33%) had at least one PV. Most patients had testing after the first cancer (44%, n=150) or after the second cancer (48%, n=165) (Table). Those with breast cancer as their first primary tended to have a higher testing rate, (OR 1.62, 95% CI 0.9-3.0), however, the statistical significance of this association was attenuated in the multivariable model with more parameters (P=0.11), possibly due to small sample size. Overall PV yield (33%) was high, consistent with a tertiary referral center. Conclusions: In this academic clinic-based sample of patients with MPC, we observed a higher testing rate when breast vs. another cancer was the first primary although it did not reach statistical significance. Additional research is needed to validate this finding in a larger cohort and determine whether similar referral patterns are observed in community practice, where most U.S. patients receive cancer care. [Table: see text]