While cutting-edge technologies like Next-Generation Sequencing (NGS) capture headlines, traditional methods like Immunohistochemistry (IHC) remain a fundamental cornerstone of companion diagnostics, especially for the detection of critical protein biomarkers. IHC uses antibodies to visualize the presence and location of specific proteins within a tissue sample, providing not only quantitative data but also essential spatial context, which is vital for many oncology treatment decisions.

IHC is the standard CDx method for determining HER2 protein overexpression in breast and gastric cancers, a test that dictates eligibility for anti-HER2 targeted therapies. Similarly, the IHC-based assessment of PD-L1 expression on tumor cells and immune cells is the primary companion diagnostic for selecting patients for immunotherapy using checkpoint inhibitors in non-small cell lung cancer (NSCLC) and other malignancies. Its widespread availability, relative cost-effectiveness, and ease of integration into existing pathology labs ensure its continued high volume use.

The IHC technology in companion diagnostics remains a dominant technology segment, securing a substantial market share. Even with the emergence of sequencing, the IHC segment, valued at $1.8 billion in 2024, is projected to maintain strong performance. Its simplicity and long-established clinical utility for high-prevalence cancers like breast cancer (which accounts for a significant portion of the oncology CDx market) underscore its foundational role in delivering precision medicine and contributing to the overall market's growth trajectory.

Innovation in IHC is now focused on standardization and quantification. Digital pathology platforms are being paired with advanced image analysis software, often utilizing AI, to provide more objective and reproducible scoring of IHC slides, especially for complex markers like PD-L1. This blending of traditional histopathology with modern computation ensures that IHC remains a rapid, essential, and highly reliable tool in the expanding arsenal of companion diagnostics.