Background: Opportunities have increased for diagnostic test results to affect treatment choice in tumor types with homologous recombination deficiencies. According to NCCN Guidelines, biomarker testing has the potential to identify patients eligible for targeted treatment such as PARP inhibitors. Methods: We conducted a noninterventional, mixed-methods cohort study to evaluate biomarker testing concordant with NCCN guidelines in 2018-19. Starting with an abstraction of structured and unstructured data from electronic health records, a cohort of 300 patients newly diagnosed with advanced ovarian cancer (aOC), HER2-negative metastatic breast cancer (MBC), metastatic pancreatic cancer (mPaC), and metastatic prostate cancer (mPC) was selected in reverse chronological order of diagnosis date, proportionately distributed from the NCCA (National Cancer Care Alliance, LLC) network. Outcomes included: proportion of patients who completed biomarker testing (defined as at least BRCA1/2 testing), time from diagnosis to test order, and time from test order to results. For patients that did not receive a biomarker test, the treating physicians were sent questionnaires to capture reasons for not ordering or for non-completion of biomarker testing. Results: Patients were identified at 10 practices from 8 states (CA, ME, NM, NY, OH, TX, UT, VT) in 2018 (N=86) and 2019 (N=214). The most commonly used tests were germline only (47%-66%), followed by tissue for multiple genes (18%-40%). Ovarian cancer had the highest completion of biomarker testing (Table). For HER2-negative MBC, the completion rate of BRCA testing was 85% for triple-negative disease and 55% for hormone receptor-positive disease. All questionnaires were completed (N=85); the most common reasons identified as barriers to testing were no perceived need or clinical benefit (42%), biomarker tests considered for a later date depending on patient’s response to treatment (14%), lack of a standard practice or guidance for biomarker testing at the practice (14%), and reimbursement for genetic counseling (12%). Conclusions: Biomarker workup was completed for the majority of patients. Given the first FDA approval of a PARP inhibitor was for aOC in 2014, biomarker testing rates and timing may naturally improve for more recent approvals such as mPC (2020). Evaluation of long-term trends for adherence to NCCN biomarker testing recommendations on the impact to patient outcomes is warranted.