PD-L1 Testing in Cancer: IHC Scores and Result Interpretation

A practical guide to what PD-L1 immunohistochemistry measures, how TPS, CPS and immune-cell scores appear on reports, and why the result is only one part of treatment planning.

Key takeaways

• PD-L1 immunohistochemistry measures protein expression in tumour tissue; it is not simply a test for a DNA mutation.
• TPS, CPS and immune-cell scoring use different cells and denominators, so their numbers are not directly interchangeable.
• PD-L1 thresholds are not universal: the score, antibody clone, platform, tumour type, medicine, treatment setting and jurisdiction must match the applicable validated assay and current clinical guidance.^[9][10]
• A high PD-L1 result does not guarantee that immunotherapy will work, and a low or negative result does not always exclude it.
• Your oncologist and pathologist should interpret the score alongside the assay used, tissue sample, cancer type, other biomarkers and your overall health.

Why your doctor may order a PD-L1 test

A pathology report that says “PD-L1 TPS 40%” or “PD-L1 CPS 10” can look as though it contains a direct answer about treatment. It does not. The result is a biomarker—one piece of biological information that your oncology team may use when considering whether an immune checkpoint inhibitor is relevant in your situation.

PD-L1 testing is commonly considered in several solid tumours, including certain lung, head and neck, gastric or gastro-oesophageal, oesophageal, urothelial, cervical and triple-negative breast cancers. The reason for ordering it varies. In one cancer, the score may help determine whether a particular treatment approach meets an established eligibility threshold; in another, it may provide supporting information without being essential for every regimen.^[1]

The decision to test also depends on the cancer subtype, stage, available tissue, previous treatment and biomarkers already known. A PD-L1 score does not describe how advanced a cancer is, and it does not by itself determine whether immunotherapy is appropriate or safe.

How PD-L1 helps cancer hide from the immune system

PD-1 is a checkpoint receptor found on immune cells, particularly activated T cells. PD-L1 is a protein that can be present on tumour cells and immune cells. When PD-L1 binds to PD-1, it sends a braking signal that reduces the T cell’s activity. This normal pathway helps limit excessive immune reactions, but some cancers exploit it to weaken an immune attack against the tumour.^[3]

Checkpoint inhibitor immunotherapy can block PD-1 or PD-L1 and release part of this brake. The immune response that follows is complex, however, and depends on much more than the amount of PD-L1 in one tissue sample. The tumour’s genetic features, surrounding immune cells, prior treatments and the patient’s health can all affect the balance between possible benefit and harm.

What PD-L1 immunohistochemistry actually measures

PD-L1 immunohistochemistry, usually shortened to PD-L1 IHC, detects PD-L1 protein in a thin section of tumour tissue. A laboratory applies an antibody that binds to the protein. A visible stain then allows a pathologist to assess which tumour cells or tumour-associated immune cells show PD-L1 expression.^[4]

This makes PD-L1 an expression biomarker. It records protein staining in a particular sample at a particular time; it is not simply a search for a mutation in the PD-L1 gene. Reports may provide a numerical TPS, CPS or immune-cell score, followed by an interpretation such as positive or negative according to a specified cut-off. They may also name the antibody clone, assay platform, specimen and scoring method.

Under a TPS algorithm, ‘TPS 40%’ means that 40% of viable tumour cells meet that assay’s membrane-staining criteria. It does not mean immunotherapy has a 40% chance of working, that 40% of the cancer is active, or that treatment has been selected.^[12]

From biopsy to lab report: how PD-L1 IHC testing is performed

PD-L1 IHC testing usually follows these broad stages, from collecting tissue to issuing the final pathology report.

1. Collecting the tissue sample
   Tissue is obtained during a biopsy or an operation and preserved for laboratory analysis. If a suitable stored pathology block is available, PD-L1 testing may be possible without another procedure. A new biopsy may be discussed if the existing sample is too small, contains too few viable tumour cells, was processed in a way that limits testing, or no longer reflects the current clinical question. The specific risks of obtaining new tissue depend on the biopsy site and procedure and should be discussed with your treating team.
2. Preparing and staining the tissue in the laboratory
   In the laboratory, tissue is usually preserved in formalin, embedded in paraffin and cut into very thin sections. One section is stained using a validated PD-L1 antibody assay, and laboratory controls help confirm that the staining has worked as expected.
3. Pathologist review and scoring
   The pathologist verifies specimen adequacy and applies the scoring algorithm validated for that tumour and intended treatment decision. The report should identify the antibody clone or assay, platform, specimen, scoring system, numerical score and interpretive cutoff. Where a companion diagnostic is required, the assay and indication must match; another clone, platform or laboratory-developed test should not be assumed interchangeable without appropriate validation.^[9][10]

Understanding TPS, CPS and immune-cell PD-L1 scores

The tumour proportion score, or TPS, considers viable tumour cells. It is the number of tumour cells with qualifying partial or complete membrane staining divided by the total number of viable tumour cells assessed, multiplied by 100. If 40 of 100 viable tumour cells stain, the TPS is 40%. A TPS of 50% therefore reflects more stained tumour cells than a TPS of 1%, but the treatment meaning of either value still depends on the specific cancer context.

For the 22C3 CPS algorithm, the numerator includes qualifying PD-L1-staining tumour cells, lymphocytes and macrophages, while the denominator is the total number of viable tumour cells; the result is multiplied by 100 and capped at 100. For example, 5 stained tumour cells, 8 lymphocytes and 2 macrophages among 100 viable tumour cells gives CPS 15. Other assays should not be assumed to use identical inclusion rules.^[4][12]

‘IC’ is not one universal formula. For example, the VENTANA SP142 algorithm estimates the proportion of tumour area occupied by PD-L1-staining tumour-infiltrating immune cells; other assays and indications use different tumour-cell or immune-cell algorithms. An IC value must therefore be read with the exact assay, tumour type and medicine.^[11]

Score type	What it measures	Cells counted in numerator	Denominator	How it is reported	Simple example
TPS (tumour proportion score)	Proportion of viable tumour cells with PD-L1 membrane staining.	Only PD-L1-positive viable tumour cells.	Total number of viable tumour cells assessed.	Percentage from 0% to 100% (for example, TPS 40%).	If 40 of 100 viable tumour cells stain, TPS = 40%.
CPS (combined positive score)	PD-L1-positive tumour and immune cells relative to tumour cells.	PD-L1-positive tumour cells, lymphocytes and macrophages.	Total number of viable tumour cells assessed.	Whole number from 0 to 100 (for example, CPS 15).	If 5 tumour cells, 8 lymphocytes and 2 macrophages stain among 100 tumour cells, CPS = (5+8+2)/100×100 = 15.
IC (example: VENTANA SP142 tumour-area algorithm)[11]	Area of tumour occupied by PD-L1-positive immune cells.[11]	PD-L1-positive tumour-infiltrating immune cells within the tumour area.[11]	Total tumour area on the slide.[11]	Percentage of tumour area (for example, IC 10%).[11]	If stained immune cells occupy about one tenth of the tumour area, the IC score is around 10%.[11]

How cut-offs and cancer type change the meaning of a PD-L1 score

Values such as TPS 1% or 50% and CPS 1, 10 or 20 appear in specific trials, regulatory labels and guidelines. They are not rungs on one universal scale: every threshold belongs to a defined tumour type, medicine, line of therapy, assay and jurisdiction, and must not be transferred to another context.^[5][10]

TPS is common in non-small-cell lung cancer with certain assays, while CPS is used in several head-and-neck, gastrointestinal, cervical, breast and other indication-specific contexts. SP142 may use a tumour-cell percentage and/or an immune-cell tumour-area percentage in defined indications. Because indications are revised or withdrawn, interpretation must follow the current local medicine label or guideline—not an old cutoff table.^[10]

Terms such as “negative,” “low,” “intermediate” or “high” only make sense when the report identifies the scoring method and cut-off. “Negative” can mean below a decision threshold rather than a complete absence of staining. Likewise, a score described as high may be associated with a greater likelihood of benefit in some settings, but it cannot predict an individual response with certainty.^[6]

Some immunotherapy regimens may be considered regardless of PD-L1 status, while others require or place greater weight on a defined score. A person with little or no detected expression may still benefit in some circumstances, and someone with high expression may not respond. PD-L1 is therefore an imperfect predictive biomarker, not a treatment instruction.

PD-L1 IHC versus genetic tests and comprehensive genomic profiling

PD-L1 IHC and genomic testing answer different questions. IHC asks whether selected cells in the tissue are expressing a particular protein. Next-generation sequencing examines DNA and sometimes RNA for molecular alterations. Comprehensive genomic profiling evaluates many genes in one analysis and, depending on the panel and sample, may also report broader measures such as tumour mutational burden or microsatellite instability.^[2]

Markers such as EGFR and ALK can identify cancer-driving alterations relevant to targeted treatment in particular cancers. BRCA alterations involve DNA-repair pathways and may be inherited or confined to the tumour, depending on the finding and test. Microsatellite instability, or MSI, reflects defective mismatch repair and can be assessed through more than one laboratory method. Tumour mutational burden, or TMB, estimates the number of acquired mutations across a defined amount of tumour DNA.

These results may complement PD-L1 rather than replace it. Even when a laboratory offers PD-L1 alongside comprehensive genomic profiling, the PD-L1 component is generally assessed by IHC on tissue, while genomic findings come from DNA- or RNA-based methods. Your oncologist may need to weigh potentially competing findings, the strength of evidence in your cancer type and the usual order in which treatments are considered.

Combined PD-L1 and genomic profiling options in India

Verified Oncquest offering facts

1

Current combined-panel details

Oncquest's current OncqSOL CGP 550 with PD-L1 listing (SP10204) describes an FFPE-tissue test covering more than 550 genes plus SNVs, indels, CNVs, TMB, MSI and PD-L1, with an HPE report and clinical history requested, 40% tumour content listed for the combined specimen and an 11-day turnaround time. Its linked sample report identifies the PD-L1 component as Dako 22C3 IHC, requires at least 100 adequate tumour cells and reports TPS or CPS according to tumour site.

Why it matters for you

Genomic profiling and the PD-L1 IHC component have different methods and reporting requirements.

2

PD-L1 is a protein-expression test

PD-L1 IHC evaluates PD-L1 protein expression in tumour tissue and is distinct from NGS-based genomic profiling. A combined CGP plus PD-L1 offering may bundle separate laboratory methods; do not describe PD-L1 expression as an NGS-detected gene alteration.

Why it matters for you

Do not describe PD-L1 IHC as an NGS-detected gene alteration.

3

Scores are indication-specific

PD-L1 interpretation is context-specific: the assay clone/platform, scoring algorithm (such as TPS or CPS), threshold, tumour type and treatment indication must be considered together. A PD-L1 result alone is not a treatment recommendation or guarantee of response.

Why it matters for you

Clone, platform, score, cutoff, tumour type, medicine and jurisdiction must be interpreted together.

4

Specimen-validation limits

Oncquest's current sample report says its 22C3 workflow is validated for non-decalcified, paraffin-embedded tissue fixed in 10% neutral-buffered formalin. It says the workflow has not been validated for tissue subjected to decalcification or the use of alternative fixatives, specifically flagging bone or bone-marrow specimens and cell blocks in that limitation.

Why it matters for you

Fixation and decalcification can determine whether the listed 22C3 workflow is valid for a specimen.

Oncquest currently lists OncqSOL CGP 550 with PD-L1 (SP10204) as a combined NGS plus IHC test on FFPE tumour tissue. The listing describes more than 550 genes plus SNVs, indels, CNVs, TMB, MSI and PD-L1, requests an HPE report and clinical history, specifies 40% tumour content for the combined specimen and lists an 11-day turnaround time. Its sample report identifies Dako 22C3 IHC, requires at least 100 adequate tumour cells and reports TPS or CPS according to tumour site. The genomic and IHC components answer different questions and must each be interpreted for the specific tumour and treatment decision.^[8][13]

Before choosing a combined panel, ask whether both components are relevant, whether the tissue is suitable and how the results could affect a real decision. The presence of the 22C3 clone alone does not establish that a result is an approved companion diagnostic for a proposed therapy; confirm the exact kit, platform, validation and intended use rather than focusing on panel size alone.^[10][13]

Talking with your oncology team about your PD-L1 result

Start by asking which score was reported—TPS, CPS or IC—and which assay or antibody clone was used. Confirm the tissue source and collection date, whether the pathologist considered the sample adequate, and whether the number is a raw score or an interpretation based on a particular cut-off.

Next, ask which threshold applies to your exact cancer type and treatment setting, and whether the test is required for the option being discussed or is only supportive information. If two scores appear on the report, ask why both were requested and which one is relevant. If an older sample was tested, ask whether the team considers it representative enough or whether newer tissue would materially change the decision.

It is also reasonable to ask how PD-L1 fits with the tumour’s histology, stage, genomic findings, MSI or TMB status, earlier treatment and your general health. A PD-L1 report does not evaluate immune-related side-effect risks, other medical conditions or your ability to receive a particular therapy. Those questions require a qualified oncology team familiar with your full clinical record.

Limitations, safety points and when PD-L1 results may change

PD-L1 expression is not evenly distributed through every tumour. A small biopsy may capture an area with different staining from another part of the same tumour, and a primary tumour may differ from a metastatic site. Expression can also change over time or after treatment, so an older specimen may not always answer a new clinical question.^[6]

Validation is specimen- and assay-specific. Oncquest’s current sample report says its 22C3 workflow is validated for non-decalcified paraffin-embedded tissue fixed in 10% neutral-buffered formalin. It says the workflow has not been validated for tissue subjected to decalcification or the use of alternative fixatives, specifically flagging bone or bone-marrow specimens and cell blocks in that limitation. Tissue handling, fixation, low viable-tumour count, sample age and borderline interpretation can all affect the result.^[13]

Repeat testing may be discussed when the first sample is inadequate, when a newly obtained specimen is more clinically relevant, or when a particular validated assay is required for the treatment context. Re-testing is not automatically useful for everyone. The potential value must be balanced against tissue availability, timing and any risk from an additional biopsy.

Common questions about PD-L1 testing and scores

FAQs

Can PD-L1 expression change over time?

Yes. Expression may differ between tumour areas, between a primary and metastatic site, or after treatment. Whether a newer sample or repeat test would be useful is a clinical decision based on tissue availability and the treatment question.^[6]

Does a negative PD-L1 result rule out immunotherapy?

Not universally. Whether a PD-L1-negative result excludes a therapy depends on the medicine, tumour type, disease setting, assay and jurisdiction. Some regimens do not require PD-L1 positivity, while others do. Check the current applicable label or guideline; do not generalize from another cancer.^[10]

Will PD-L1 testing require another biopsy?

Often the laboratory can use stored tissue from an earlier biopsy or operation. Another procedure may be considered if that tissue is unavailable, inadequate or no longer suitable for the clinical question. Your doctor should explain whether a new biopsy could add useful information and what risks it carries.

Why can two PD-L1 reports give different results?

The reports may involve different tissue sites, collection dates, antibody assays, scoring systems or cut-offs. Tumour heterogeneity and borderline interpretation can also contribute. Ask a pathologist or oncologist whether the results are genuinely comparable before drawing a conclusion.

Is PD-L1 testing an inherited cancer test?

No. Standard PD-L1 IHC examines protein staining in tumour tissue and does not determine inherited cancer risk. If a genomic result raises concern about a possible inherited alteration, separate germline testing and genetic counselling may be required.

Sources

1. PDL1 (Immunotherapy) Tests - MedlinePlus / U.S. National Library of Medicine
2. PD-L1, TMB and Lung Cancer | Immunotherapy Biomarker Testing - American Lung Association
3. What Is Immunotherapy? - American Cancer Society
4. PD−L1 immunostaining: what pathologists need to know - National Library of Medicine (PMC)
5. PD-L1 Expression Scoring: Noninterchangeable, Noninterpretable, Neither, or Both - National Library of Medicine (PMC)
6. Quantifying PD-L1 Expression to Monitor Immune Checkpoint Therapy: Opportunities and Challenges - National Library of Medicine (PMC)
7. Immunotherapy and Biomarker Testing in Recurrent and Metastatic Head and Neck Cancers: ASCO Guideline - American Society of Clinical Oncology / Journal of Clinical Oncology
8. Oncqsol CGP-550 with PD-L1 test - Oncquest Laboratories
9. PD-L1 and TMB Testing of Patients With Lung Cancer for Immuno-oncology Therapies - College of American Pathologists / AMP / IASLC
10. List of FDA-Authorized Companion Diagnostic Devices - U.S. Food and Drug Administration
11. VENTANA PD-L1 (SP142) Assay Interpretation Guide - U.S. Food and Drug Administration
12. PD-L1 IHC 22C3 pharmDx Instructions for Use - U.S. Food and Drug Administration
13. OncqSOL CGP 550 with PD-L1 (SP10204) Sample Report - Oncquest Laboratories