Abstract Introduction: Biomarker testing identifies actionable mutations to inform targeted tx for personalized healthcare. This study examined biomarker testing and tx patterns ≤ 90 days (d) after advanced (adv) or metastatic (met) diagnosis (Dx) at the OneOncology (OneOnc) network. Methods: A retrospective observational study using the nationwide Flatiron Health electronic health record-derived de-identified database from selected OneOnc practices included patients (pts) ≥ 18 y, who had Dx between 1/1/18 - 4/30/21 with adv non-small cell lung cancer (aNSCLC), met breast cancer (mBC), met colorectal cancer (mCRC) or adv melanoma (aMel), with ≥ 1 visit ≤ 90 d after adv or met Dx, and who had ≥ 90 d follow-up. Descriptive analyses were conducted. Results: Of the pts who met the inclusion criteria, 91% of 2,391 aNSCLC, 100% of 1,043 mBC, 92% of 1,344 mCRC and 84% of 358 aMel had ≥ 1 biomarker test at any time, where 78% aNSCLC, 82% mBC, 70% mCRC and 67% aMel were tested ≤ 90 d after adv or met Dx. Testing rates varied by biomarker (Table). Across indications, ≥ 70% commercial health insured vs. ≥ 50% Medicaid insured pts were tested. Commercial labs were used in 91% aNSCLC, 86% mBC, 80% mCRC and 93% aMel. Amongst treated pts, 14% of 1,693 aNSCLC, 2% of 725 mBC, 22% of 881 mCRC and 25% of 169 aMel received tx before test results were available. NGS was tested in 69% aNSCLC, 29% mBC, 70% mCRC, and 57% aMel. The median turnaround time (TAT) from adv or met Dx to 1st test result from NGS vs other biomarker tests was 26 vs 13 d in aNSCLC, 14 vs 5 d in mBC, 35 vs 10 d in mCRC and 44 vs 21 d in aMel. Conclusions: The majority of testing occurred ≤ 90 d after adv or met Dx, while 18% - 33% of pts were not tested. The varying testing rates across indications reflect the intended use of biomarker tests to guide 1st line therapies. More than 75% pts treated after testing result available signal community oncologist recognizing the importance of biomarker testing in guiding tx decisions. Improvements in TAT for NGS may reduce the issue of tx decisions made prior to test results. Table. Biomarker testing rate by indication amongst pts tested ≤ 90 d after adv or met Dx aNSCLCn=1,858 mBCn=859 mCRCn=936 aMeln=239 EGFR 89% ALK 87% BRAF 81% 80% 98% KRAS 67% 82% ROS1 86% PD-L1 85% 18% 27% ER 93% PR 93% HER2 92% BRCA 20% PIK3CA 28% MMR/MSI 93% NRAS 77% 49% KIT 50% ER, estrogen receptor; MMR, mismatch repair; MSI, microsatellite instability; PR, progesterone receptor. Citation Format: Esprit Ma, Elaine Yu, Tania Szado, Richard Price, Craig S. Meyer, Anuj Shah, Baiyu Yang, Daniel Vaena, Davey Daniel, Dennis Slater, Harry Staszewski, Bruno Fang, Lasika Seneviratne, Lee Schwartzberg. Biomarker testing and treatment (tx) patterns in a large community oncology network [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2267.