Background: HER2 status in breast cancer (BC) is routinely assessed by immunohistochemistry (IHC) and in situ hybridization (ISH) using current guidelines that binarily classifies patients as HER2 positive(+ve) (IHC3+, IHC2+ & ISH+) or HER2 negative(-ve) (IHC0, IHC1+, IHC2+ & ISH-). Based on the clinical data, current anti-HER2 therapies are primarily prescribed in HER2+ve patients. However, recent data suggested fam-trastuzumab deruxtecan-nxki may also have activity in metastatic BC (mBC) patients with low HER2 expression (IHC1+, IHC2+ & ISH-). This study aims to estimate the prevalence of patients with low HER2 expression (HER2 low) among HER2-ve mBC patients in US clinical practice. Methods: Physicians in the Cardinal Health Oncology Provider Extended Network (OPEN) who had treated ≥1 BC patient in the past 12 months (n=504) were invited to participate in a retrospective chart review study. Participating physicians submitted data for up to 5 HER2-ve mBC patients who had received any systemic therapy between Apr2018-May2020 and had ≥1 IHC result available. Most recent IHC and ISH tests were examined to identify patients with low HER2 expression. Data were summarized descriptively. Results: 68 physicians participated (82% were in private community practices) and provided data on 325 HER2-ve mBC patients. Based on the most recent test, 49%(n=160) met HER2 low criteria, 50%(n=161) had IHC0 and 1% had undetermined (n=3) or HER2+ve(n=1) results. Among HER2 low patients [median age=65 years], the most recent IHC test result was obtained at initial BC diagnosis for 57% and at mBC diagnosis for 41% of patients. Most HER2 low patients (76%) had IHC1+ and 24% had IHC2+ & ISH- results. Only 33% of HER2 low patients (n=52) had more than one test conducted, and 2 HER2 low patients had prior test results meeting HER2+ve criteria. A total of 157 HER2 low patients were receiving treatment at the time of the survey, of which combination of hormonal therapy and CDK 4/6 inhibitor was the most common regimen (51%), followed by single-agent chemotherapy (16%) and hormonal therapy alone (12%); only 2% received anti-HER2 drugs. Conclusions: About one in two HER2-ve mBC patients had HER2 test results meeting HER2 low criteria. HER2 low mBC patients were primarily utilizing regimens that do not target HER2 and many were tested only at the time of initial BC diagnosis. Further research is needed for in-depth understanding of treatment patterns and outcomes of HER2 low mBC patients.