Breast cancer is the leading cause of cancer mortality among women. The majority of cases and resulting deaths occur in low-resource settings. Effective breast cancer treatment requires a detailed assessment of the tumor. Unfortunately, the lack of clinical infrastructure in low-resource settings prohibits the use of traditional breast cancer pathological methods. The D4 assay is a miniaturized, self-contained assay that can measure protein biomarkers from complex biological milieu with high sensitivity and specificity without the need for equipment (other than a smartphone) and can be performed with minimal user training. Previously, a D4 assay for HER2 detection from fine needle aspirate (FNA) samples was developed. However, due to variation in FNA sampling, it is important to normalize HER2 concentration in clinical settings. GADPH, a housekeeping protein, provides a normalization standard. Thus, I constructed a multiplexed assay that simultaneously detects HER2 and GADPH. The assay was tested for cross-reactivity and optimal antibody concentrations. We found that there is minimal cross reactivity between HER2 and GADPH. Furthermore, multiplexing does not compromise the analytical performance of the test. The ability to multiplex HER2 and GADPH will make the D4 assay a more accurate diagnostic tool to enable effective breast cancer treatment.