Review of a Decade of IHC Quality – Expectations, Disappointments and Challenges in Commonly Used Assays Based on NordiQC Experience

Immunohistochemistry (IHC) has undergone substantial expansion during the past decades, with an increasing number of assays becoming essential to routine diagnostic, prognostic, and predictive pathology. Parallel to this development, external quality assessment (EQA) programs have accumulated extensive experience regarding the real-world performance of commonly used IHC assays across laboratories. This review summarises observations derived from the Nordic Immunohistochemical Quality Control Program and its EQA activities over the last 10 years, focusing on expectations, recurring disappointments, and emerging challenges in IHC testing. Particular attention is given to the analytical and diagnostic accuracy and quality of IHC assays being impacted by protocol variability and the need to align IHC methods to evolving clinical purposes. The accumulated experience demonstrates that while overall pass rates are acceptable for many markers, significant and recurrent limitations persist, particularly when assays are repurposed or when analytical sensitivity requirements change. These findings underline the continued need for critical assay validation, identification and application of appropriate controls, and evidence-based standardisation of IHC in the era of precision diagnostics.