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Diagnostics Companion diagnostics

Which Assay for Atezolizumab?

We read, with great interest, David Rimm’s comments regarding the FDA-approved Roche Ventana PD-L1 (SP142-based) assay, which is intended to determine triple-negative breast cancer patients’ eligibility for treatment with the immune checkpoint inhibitor drug atezolizumab. Rimm makes very serious allegations regarding this immunohistochemistry (IHC) assay and serious allegations regarding IHC assays in general. We would like to share our views on relevant general concepts regarding IHC methodology and quality assurance, as well our response to some specific comments made regarding the FDA-approved Roche Ventana PD-L1 (SP142) assay.

Rimm states, “It’s the (IHC) protocol – not the recipe – that leads to a high level of reproducibility.” Clearly, some IHC protocols may be more robust and easier to reproduce; however, it is not the protocol alone that leads to reproducibility. The IHC protocol is not the entirety of the analytical phase; the other important component is the readout, which is mostly done by pathologists with or without the assistance of image analysis (1). Such readouts may or may not be highly reproducible – an issue that applies not only to IHC scoring, but also all areas of pathology practice. Reproducibility depends on both the complexity of the task and the nature of the readout’s subject. Some readouts require more training and experience, whereas others are intuitive and simple, even for the novice. Nonetheless, it is a truism that more training and experience will lead to more reproducible results.

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About the Authors

Emina Torlakovic

Division Head of Hematopathology in the Department of Pathology, University of Saskatchewan and Saskatchewan Health Authority, Saskatoon, Saskatchewan, Canada.


Allen M. Gown

Founder and Senior Pathologist at PhenoPath Laboratories, Seattle, Washington, USA.

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