Unraveling the Complexity of PD-L1 Assays: A Descriptive Review of the Methodology, Scoring, and Practical Implications

Gilad W Vainer¹Sunil S Badve²Pathmanathan Rajadurai³Fernando A Soares^4,5Giuseppe Viale⁶Reinhard Büttner⁷Shanthy Nuti⁸Jonathan Juco⁸Radha Krishnan⁸Ming-Sound Tsao^9*

• ¹Hadassah–Hebrew University Medical Center, Jerusalem, Israel
• ²Emory Winship Cancer Institute, Atlanta, Georgia, United States
• ³Monash University Malaysia, Selangor, Malaysia Subang Jaya Medical Centre, Subang Jaya, Selangor Darul Ehsan, Malaysia
• ⁴Instituto de Anatomia Patológica, Rede D’Or São Luiz, SAO PAULO, Brazil
• ⁵Instituto de Pesquisa e Ensino D’Or, Sao Paula, Brazil
• ⁶IEO, European Institute of Oncology IRCCS, Milan, Lombardy, Italy
• ⁷University of Cologne Medical Faculty, Cologne, North Rhine-Westphalia, Germany
• ⁸Merck & Co., Inc, Rahway, New Jersey, United States
• ⁹University of Toronto, Toronto, Canada

Purpose: Over the years, immune checkpoint inhibitors targeting the programmed cell death protein 1 (PD-1) and programmed cell death ligand 1 (PD-L1) axes have substantially improved clinical outcomes for patients with various types of cancer and stages. As a result, PD-L1 is the most recognized biomarker used to guide the selection of patients for treatment with anti–PD-(L)1 therapy. To date, there are 4 regulatory agency-approved and commercially available immunohistochemistry assays used to quantify PD-L1 tumor expression, with each assay approved for use with a specific PD-(L)1 inhibitor. In this descriptive review, we concisely summarize the methodology and scoring methods of each assay, as well as some of the challenges associated with real-world use of these assay systems. Results: Each assay system is optimized for specific therapies, with its own anti-PD-L1 antibody, protocol, scoring, and interpretation guidelines. Although the methodologies of the 4 PD-L1 immunohistochemistry assay systems are similar, differences in their antibody clones, protocol conditions, instrumentation, and scoring methods limit assay interchangeability. The assays are also highly sensitive; slight deviations to the protocol can increase the risk of misclassifying the PD-(L)1 tumor status of patients. As a result, pathologists are faced with choosing which assay to perform with a limited tumor sample as well as the challenges associated with the scoring methods and differences in regional regulatory approvals and infrastructure. Conclusion: While the 4 approved PD-L1 immunohistochemistry assays provide clinical value, we offer pathologists suggestions to reduce the challenges associated with PD-L1 testing based on assay systems.