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580 Background: Significant challenges to breast cancer control in low- and middle-income countries (LMICs) include late-stage disease presentation because of few/no early detection programs, inadequately staffed and equipped pathology laboratories, and constrained treatment options. Estrogen receptor (ER) expression is critical to determining candidacy for cost efficient and accessible hormonal agents in LMICs; however, access to standard immunohistochemistry (IHC)-based ER analysis is grossly limited/nonexistent due to cost and technical requirements. We have identified Nw-hydroxy-L-Arginine (NOHA) as a low cost and accessible blood-based biomarker to distinguish estrogen-receptor negative (ER–) from estrogen-receptor positive (ER+) breast cancer, differentiate ER– high grade versus low grade tumors, and correlate ER– molecular phenotype with ethnic variation. Our studies with US patients suggest the NOHA threshold of <4nM as a reliable indicator of ER– versus ER+ disease (Table 1). Here we examine the clinical utility of NOHA as an alternative to IHC in distinguishing ER– from ER+ breast tumors in Tanzanian patients. Methods: Following informed consent, 70 newly diagnosed breast cancer patients were recruited at Kilimanjaro Christian Medical Center (KCMC; Moshi, Tanzania). Prior to any treatment, a needle prick amount of blood was collected from each patient on a Noviplex plasma card (Shimadsu, U.S.) and stored at -80˚C. Plasma cards and unstained tumor pathology slides were shipped at 2-3 months intervals to US labs for NOHA and immunohistochemistry (IHC) ER testing. Statistical difference was set at p<0.01, with NOHA and IHC assay operators blinded to patient clinical status. Results: Our early data show correlation between NOHA levels and ER IHC results, providing a means to distinguish ER– from ER+ breast cancer in the low-resource setting. Plasma cards stored at -80˚C for up to 3 months retained NOHA stability in assays involving a proprietary antibody-based ELISA, and by LC-MS. Conclusions: This study suggests the clinical utility of NOHA as a cost-effective, accessible replacement for standard IHC testing in determining ER status among breast cancer patients in LMICs, promising to extend access to cost efficient and available hormonal agents and improving outcomes and quality of life. The present study provides foundational knowledge for broader studies of NOHA utility in global breast cancer control, as well as in ongoing development of NOHA rapid-testing technologies. [Table: see text]