The UK National External Quality Assessment Scheme for Immunocytochemistry and In-Situ Hybridisation (UK NEQAS ICC & ISH) HER2-low EQA programme: a review of results and factors influencing successful staining

An EQA specifically designed for the assessment of immunohistochemical stain quality in the area of HER2-low testing was established by UK National Quality Assessment Scheme for Immunocytochemistry and In-Situ Hybridsiation (UK NEQAS ICC & ISH) at the beginning of 2023. We report on the results produced during the first 18-months of its operation (7 assessment runs, 527 submissions). At the first run 54.5% of participants’ submissions failed. The principal reason for failure was weak staining at this and at all subsequent assessments where submissions failed. The fail-rate continued to be above 50% for the first three assessment runs; beginning at run 4 and continuing in all following runs, the fail-rate declined such that it was around 20% at run 7. This change was not associated with any methodological parameter on which data was available (primary antibody, antigen retrieval, detection system or automation platform). Overall, 2258 core samples were available, 99.2% of all samples expected to be negative (HER2 0) showed staining concordant with that category; 71.% of those expected to stain as HER2 1+ and 60.5% of those expected to stain as HER2 2+ were assessed as showing the correct level of staining. Core samples showing weaker than expected staining totalled 531 (23.5%) while 52 (2.3%) showed stronger staining than expected. The 4B5 clone (Ventana) was the most commonly used primary antibody with 391 (87.3%) of submissions using this clone; of which, 275 (70.3%) passed. In contrast none of the 25 (5.6%) submissions that used the CB11 clone (Oracle, Leica) achieved a pass.