PD-L1 expression is tested using immunohistochemistry (IHC) on a tissue sample from the tumor, typically using an FDA-approved clone and scoring system like the Tumor Proportion Score (TPS) or Combined Positive Score (CPS). The pathologist performs the test, and the oncologist uses the results to determine if certain immunotherapy drugs are likely to be effective for the patient. Blood tests are also being explored as a less invasive way to assess PD-L1 expression.
How the test is performed
Sample collection: A biopsy is performed to obtain a tissue sample from the tumor.
Lab analysis: The tissue sample is sent to a lab for testing.
IHC: The lab uses an immunohistochemistry (IHC) assay to stain the cells in the tissue for the PD-L1 protein.
Scoring: A pathologist evaluates the stained sample and calculates a score based on a specific algorithm (e.g., TPS, CPS) approved for the intended immunotherapy drug.
Reporting: The oncologist receives the results, which indicate the level of PD-L1 expression.
What the results mean
Positive result: A "positive" result indicates a high enough level of PD-L1 protein for the patient to be a candidate for a specific immunotherapy medicine.
Negative result: A "negative" result means there is not enough PD-L1 protein, and the immunotherapy is not likely to be helpful.
Important considerations
Specific test: The specific IHC clone and scoring system used must be approved for the particular cancer and the specific immunotherapy drug being considered.
Tissue source: PD-L1 expression can vary between tumors, so testing the primary tumor site is crucial.
Blood tests: While less common, blood tests analyzing PD-L1 expression in circulating tumor cells (CTCs) are being studied as a non-invasive alternative, especially in advanced cancers.

Leronlimab lowers CTCs very early during treatment, rendering the blood test implausible. That means the patient's primary tumor would be biopsied every two weeks. That could be very tough to go through.