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Aims Recently, human epidermal growth factor 2 (HER2)‐low (i.e. HER2 score 1+ or 2+ without amplification) breast cancer patients became eligible for trastuzumab–deruxtecan treatment. To improve assay standardisation and detection of HER2‐low in a quantitative manner, we conducted an external quality assessment‐like study in the Netherlands. Dynamic range cell lines and immunohistochemistry (IHC) calibrators were used to quantify HER2 expression and to assess interlaboratory variability. Methods and results Three blank slides with a dynamic range cell line and an IHC calibrator were stained with routine HER2 assays by 35 laboratories. Four different antibody clones were used: 19 (54.3%) 4B5, six (17.1%) A0485, five (14.3%) DG44 (HercepTest) and five (14.3%) SP3. Laboratories used two different detection kits for 4B5 assays: 14 (73.7%) ultraView and five (26.3%) OptiView. Variability of HER2 expression in cell lines, measured with artificial intelligence software, was median (min–max) = negative core 0.5% (0.0–57.0), 1+ core 4.3% (1.6–71.3), 2+ core 42.8% (30.4–92.6) and 3+ core 96.2% (91.8–98.8). The calibrators DG44 and 4B5 OptiView had the highest analytical sensitivity, closely followed by 4B5 ultraView. SP3 was the least sensitive. Calibrators of A0485 assays were not analysable due to background staining. Conclusions As assays were validated for detecting HER2‐amplified tumours, not all assays and antibodies proved suitable for HER2‐low detection. Some tests showed distinct expression in the negative cell line. Dynamic range cell line controls and quantitative analysis using calibrators demonstrated more interlaboratory variability than commonly appreciated. Revalidation of HER2 tests by laboratories is needed to ensure clinical applicable HER2‐low assays.
Hempenius et al. (Mon,) studied this question.