The short answer
PD-L1 CPS is a biomarker result. It means a combined positive score for PD-L1 staining. In some cancers, biomarker results can help guide targeted therapy, immunotherapy, or clinical trial options, but the result only matters in context.
PD-L1 CPS is a biomarker or molecular result, not a treatment decision by itself.
It can be relevant in some head and neck, stomach, cervical, and other cancer decisions.
A positive result may or may not change treatment depending on cancer type, stage, prior treatment, and available options.
Ask whether the result is from tumor testing, blood testing, inherited genetic testing, or another method.
Choose how you want to understand this
The full explanation.
The score is a fraction, not a percentage of tumor
CPS stands for Combined Positive Score. It is how pathologists report PD-L1 in several cancers. The arithmetic is odd enough to cause real confusion.
The FDA-approved instructions for the PD-L1 IHC 22C3 pharmDx test define it exactly:
CPS = (number of PD-L1 staining cells, meaning tumor cells plus lymphocytes plus macrophages) divided by (total number of viable tumor cells), multiplied by 100.
Read the bottom of that fraction again. It counts only tumor cells. The top counts tumor cells and immune cells. The two halves do not measure the same set of cells. So the answer can come out above 100. The instructions cap it. The maximum score is defined as CPS 100.
So a CPS is not a percentage. A CPS of 20 does not mean that 20 percent of the tumor is positive. It means the staining cells number a fifth as many as the viable tumor cells. Writing it with a percent sign is wrong, and it is a common error.
CPS and TPS are different scores
The other score you may see is TPS, the Tumor Proportion Score. TPS really is a percentage. It is the share of viable tumor cells with membrane staining at any intensity. Immune cells are not counted at all.
TPS is used for non-small cell lung cancer. Its cutoffs are TPS under 1 percent for no expression, 1 percent or more for expression, and 50 percent or more for high expression.
CPS is used in gastric or gastroesophageal junction adenocarcinoma, cervical cancer, urothelial carcinoma, and head and neck squamous cell carcinoma.
If your report shows a number without saying which score it is, that is worth clarifying. The same tumor can be TPS 5 and CPS 30.
What counts as a staining cell
The instructions are specific about which cells the pathologist may count:
- Tumor cells with clear partial or full linear staining of the cell membrane, at any intensity. It must look distinct from staining in the cell body.
- Lymphocytes and macrophages inside the tumor nests, or in the tissue right around them. They need clear membrane or cell-body staining at any intensity. They must be directly linked to the tumor.
A pathologist reads the slide under a light microscope. A 10x to 20x objective is used to scan it. Scoring requires a 20x objective.
The adequacy rule that can send you back for another biopsy
A minimum of 100 viable tumor cells must be present on the stained slide for the specimen to be considered adequate for PD-L1 evaluation.
That threshold is why some small biopsies come back as insufficient rather than positive or negative. It is not the pathologist being difficult. Below that count, the score is not reliable.
For gastric and urothelial cancers, the trials that set the cutoffs used at least 3 and up to 5 core needle biopsies per patient. The instructions say the reliability of a result from fewer passes is unknown.
The cutoffs that unlock specific drugs
CPS matters because the number crosses a line printed in a drug label. On the pembrolizumab label, different uses have different thresholds. Two examples:
- CPS 1 or more, for several indications, including head and neck squamous cell carcinoma.
- CPS 10 or more, including single-agent use in esophageal squamous cell carcinoma after prior therapy, and for a combination in triple-negative breast cancer.
The label ties each of these to an FDA-authorized test. That wording matters. Each cutoff was validated with one antibody clone on one instrument. Another lab's own PD-L1 test does not automatically substitute.
An honest limitation, printed in the instructions themselves
The instructions carry a caution that rarely reaches patients. Clinicians are told to be careful reading results at the CPS 20 cutoff in head and neck squamous cell carcinoma. At that cutoff the test failed to meet its set targets. It missed positive percent agreement in two separate studies across sites. It also missed overall percent agreement in one of them.
At the CPS 1 cutoff in head and neck cancer, all pre-specified criteria were met.
What that means in practice: a result near CPS 20 repeats less reliably than one near CPS 1. If your treatment turns on a score close to 20, ask whether a second read or a repeat test is worth doing.
Archival tissue may not reflect the tumor now
PD-L1 levels can change over time and with treatment. The instructions say this. If PD-L1 is not found in an archival gastric or gastroesophageal sample, consider whether a new biopsy can be taken.
The trial defined a newly obtained biopsy narrowly. It had to be taken up to 6 weeks, meaning 42 days, before the first dose. No cancer treatment could be given in between. Anything older counted as archival.
So ask how old the block being tested is. A negative result on a three-year-old sample is not the same finding as a negative result on tissue taken last month.
Questions to bring about a PD-L1 result
- Is this a CPS or a TPS, and what is the exact number?
- Which assay was used, and is it the FDA-authorized companion test for the drug we are discussing?
- How old is the tissue that was tested, and was it before or after my last treatment?
- Were there at least 100 viable tumor cells on the slide?
- What cutoff does my proposed treatment require, and how close is my result to it?
- If I am just below the cutoff, what are the alternatives?
Do not wait for the next appointment if
- You develop new breathlessness or a dry cough while on immunotherapy.
- You have four or more loose stools a day above your usual, or blood in stool.
- Your eyes or skin turn yellow.
- You have a fever of 100.4 F (38 C) or higher on treatment.
- You feel dizzy standing up, with deep fatigue. This can point to the adrenal glands.
Related pages
Immunotherapy, Biomarker Testing and Precision Medicine, Liquid Biopsy vs Tissue Biopsy, and Pathology Reports.
Sources
Words to know
Tap any term to see what it means.
Common questions
What is PD-L1 CPS?
PD-L1 CPS means a combined positive score for PD-L1 staining. It is one type of result that may appear on a biomarker, molecular, genomic, or pathology report.
Can it affect treatment?
Sometimes. Certain biomarkers can point toward targeted therapy, immunotherapy, or a clinical trial, but the same result can mean different things in different cancers.
What should I ask my oncologist?
Ask whether the result is actionable for your cancer, whether a matched treatment exists, and whether a trial is relevant.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
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Sources last checked: 2026-07-30 what this meansLast updated: 2026-08-11Next planned review: 2027-07-30
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Editorial review complete — This page completed Cancer Explained's editorial checks (sources, safety, plain language, duplication). It has not been reviewed by a physician or other healthcare professional.
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