Companion diagnostic immunohistochemistry (IHC) tests are developed and performed without incorporating the tools and principles of laboratory metrology. Basic analytic assay parameters such as lower limit of detection (LOD) and dynamic range are unknown to both assay developers and end users. We solved this problem by developing completely new tools for IHC—calibrators with units of measure traceable to National Institute of Standards & Technology (NIST) Standard Reference Material (SRM) 1934. In this study, we demonstrate the clinical impact and opportunity for incorporating these changes into PD-L1 testing. Forty-one laboratories in North America and Europe were surveyed with newly-developed PD-L1 calibrators. The survey sampled a broad representation of commercial and laboratory-developed tests (LDTs). Using the PD-L1 calibrators, we quantified analytic test parameters that were previously only inferred indirectly after large clinical studies. The data show that the four FDA-cleared PD-L1 assays represent three different levels of analytic sensitivity. The new analytic sensitivity data explain why some patients’ tissue samples were positive by one assay and negative by another. The outcome depends on the assay’s lower LOD. Also, why previous attempts to harmonize certain PD-L1 assays were unsuccessful; the assays’ dynamic ranges were too disparate and did not overlap. PD-L1 assay calibration also clarifies the exact performance characteristics of LDTs relative to FDA-cleared commercial assays. Some LDTs’ analytic response curves are indistinguishable from their predicate FDA-cleared assay. IHC assay calibration represents an important transition for companion diagnostic testing. The new tools will improve patient treatment stratification, test harmonization, and foster accuracy as tests transition from clinical trials to broad clinical use.
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The authors also are grateful to the many laboratories in the United States, Canada, the Netherlands, and Belgium for their participation.
Research reported in this paper was supported in part by the National Cancer Institute of the National Institutes of Health under award number R44CA213476 (to SAB).
Conflict of interest
N.A.t'H. declares no competing interests. E.E.T. serves as an advisory board member for Merck, Pfizer, BMS, Seagen, Roche, AstraZeneca, and Agilent. S.R.S., K.V., and S.A.B. are principals at Boston Cell Standards, where some of the work was conducted, and are shareholders in the company. Boston Cell Standards holds a patent and other patent applications on the technology used in the study.
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The original online version of this article was revised: Due to an error in figure 3.
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Sompuram, S.R., Torlakovic, E.E., ‘t Hart, N.A. et al. Quantitative comparison of PD-L1 IHC assays against NIST standard reference material 1934. Mod Pathol 35, 326–332 (2022). https://doi.org/10.1038/s41379-021-00884-w