IHC p53 Performance in NordiQC: Recalibration of IHC Assays is Needed.

The TP53 gene is the most frequently mutated gene in human cancers, with alterations occurring in ∼50% of all malignancies. When properly optimized, p53 immunohistochemistry (IHC) can serve as a reliable surrogate for detecting TP53 mutations. However, many laboratories struggle to identify p53 IHC reaction patterns beyond overexpression, particularly the absence, cytoplasmic, and wild-type patterns. As the clinical relevance of these additional patterns became recognized, NordiQC updated its assessment in 2021 to include wild-type and absence patterns, thereby increasing the complexity of p53 IHC and revealing widespread difficulties across laboratories. In this study, we analyze data from 6 consecutive NordiQC external quality assessment rounds from 2007 to 2024, covering 1,796 submitted p53 IHC assays. Our findings show that while most laboratories can reliably detect high antigen expression of overexpression, many protocols fail to adequately demonstrate low-level p53 expression, limiting the validity of the IHC assay. Ready-To-Use products, particularly when used according to manufacturer recommendations, performed suboptimally. These kits are calibrated exclusively to detect overexpression, leading to false-negative or too-weak results for other mutation-associated patterns. Our data represents the most comprehensive evaluation of p53 IHC assay performance to date. The findings underscore an urgent need to recalibrate p53 IHC protocols, with a special focus on improving analytical sensitivity and ensuring consistent use of both internal and external controls. Aligning IHC assays calibrated to identify the full spectrum of TP53 mutations, including those that result in absent and wild-type patterns, can significantly enhance the diagnostic accuracy of p53 IHC.