Background: Breast cancer with low human epidermal growth factor receptor (HER2) expression is increasingly considered as a distinct subtype which consists of types of HER2 immunohistochemistry (IHC) 1+ and HER2 IHC 2+/in-situ hybridization (ISH)-negative. We aim to assess the survival difference between HER2 IHC 1+ and HER2 IHC 2+/ISH-negative breast cancer patients with metastasis at presentation and construct a prognostic nomogram for HER2-low patients.

Method: Patients diagnosed with de novo metastatic HER2-low breast cancer from 2010 to 2015 were included and analyzed using the National Cancer Database (NCDB). Cox proportional hazards regression model and Kaplan–Meier (KM) method were used for survival analysis. Nomograms were built to predict survival.

Result: A total of 7897 patients were included in the final analysis, among which 5458 (69.1%) patients were HER2 IHC 1+ and 2439 (30.9%) were HER2 IHC 2+/ISH-negative. Although the Kaplan–Meier survival analysis showed difference in survival, this survival difference was lost in the multivariate Cox analysis (multivariate: HR (hazard ratio) = 0.97; 95% CI (confidence interval) [0.92–1.03]). A prognostic nomogram was successfully constructed for individually predicting the long-term survival rate of HER2-low patients, which exhibited an acceptable predictive capability in training (C index: 0.719) and validation cohort (C index: 0.706). This nomogram could easily divide patients into high and low-risk subgroups with distinct prognoses.

Conclusions: Our data suggest no statistical survival differences between HER2 1+ and HER2 2+ breast cancer. Additionally, a nomogram was constructed with an acceptable capacity to individually predict the long-term outcome of HER2-low metastatic breast cancer patients.