PD-L1 combined positive score (CPS)

What it is

PD-L1 combined positive score (CPS) is an immunohistochemistry score from 0 to 100 that estimates PD-L1 expression in tumour and immune cells. The key fact is that CPS is calculated as the number of PD-L1-staining tumour cells, lymphocytes, and macrophages divided by the total number of viable tumour cells, multiplied by 100.

CPS is used on a tumour tissue sample to help predict whether some cancers may respond to immune checkpoint inhibitors, especially drugs that target the PD-1/PD-L1 pathway such as pembrolizumab and nivolumab in selected settings. Unlike the tumour proportion score, which counts only PD-L1 staining in tumour cells, CPS includes certain immune cells in the tumour area. That makes CPS particularly relevant in cancers where the immune microenvironment matters, such as head and neck, gastric, gastroesophageal, cervical, and urothelial cancers.

CPS is a biomarker result, not a diagnosis by itself. A higher CPS can increase the chance that immunotherapy will help in some diseases, but it does not guarantee benefit, and a low CPS does not always rule benefit out. The exact cutoff that matters depends on the cancer type, the drug, the treatment line, and the assay used.

Feature	CPS
Sample type	Tumour tissue biopsy or surgical specimen
Method	PD-L1 immunohistochemistry
Formula	PD-L1-staining tumour cells + lymphocytes + macrophages, divided by viable tumour cells, ×100
Score range	0 to 100
Common cutoffs in practice/trials	1, 5, 10, sometimes higher depending on cancer

How it works

PD-L1 is a protein that can help tumours evade immune attack by engaging the PD-1 receptor on T cells. When this pathway is active, immune cells may become less effective at killing cancer cells. Drugs that block PD-1 or PD-L1 can restore some anti-tumour immune activity.

CPS tries to capture PD-L1 expression in both malignant cells and nearby immune cells. A pathologist examines stained tissue under the microscope and counts eligible PD-L1-positive cells within the tumour area. For tumour cells, membranous staining is counted. For lymphocytes and macrophages, membranous and or cytoplasmic staining may be considered depending on assay guidance and scoring rules.

Because the denominator is the number of viable tumour cells, CPS can be higher than the percentage of tumour cells staining positive. However, the reported CPS is generally capped at 100. The score depends on pre-analytic and analytic factors such as tissue fixation, biopsy size, necrosis, inflammation, prior treatment, and the antibody platform used.

Evidence and uses

CPS has become an important predictive biomarker in several solid tumours, but its value is disease-specific rather than universal. In gastroesophageal cancers, CPS has been widely used in major immunotherapy trials and treatment decisions. In head and neck squamous cell carcinoma, CPS is also commonly used to guide first-line immunotherapy selection. In urothelial carcinoma and cervical cancer, CPS may also be reported depending on the clinical context.

A practical way to think about CPS is:

1. Higher CPS may enrich for response in some cancers.
2. The useful threshold varies by tumour type and drug.
3. CPS is only one part of decision-making, alongside stage, symptoms, prior treatment, performance status, molecular findings, and drug access.

Examples of commonly discussed thresholds include CPS at least 1 or at least 10, but these are not interchangeable across cancers. For instance, some pembrolizumab indications and trial analyses have used CPS 1 or CPS 10 as key cut points. A pathology review in gastroesophageal cancer notes that CPS has been a primary biomarker in several pivotal trials, but also highlights technical and interpretive challenges.

Evidence also shows that scoring reproducibility is imperfect. A multi-institutional study of pathologist reading found variability in CPS interpretation, especially around clinically important cutoffs. This matters because a score near a threshold can affect treatment eligibility. Small biopsies may under- or overestimate PD-L1 expression because tumours are heterogeneous, meaning one area of the tumour may stain differently from another.

In India, the clinical relevance is similar to other countries, but access can vary by cancer centre, pathology platform, and the specific immunotherapy being considered. Patients may see CPS reported from large private laboratories or tertiary cancer centres. It is reasonable to ask which assay was used and whether the result was reviewed by a pathologist experienced in PD-L1 scoring.

Diagnosis / how it's measured

CPS is measured on formalin-fixed, paraffin-embedded tumour tissue using a validated PD-L1 immunohistochemistry assay. The most widely discussed companion diagnostic context is the 22C3 pharmDx assay, although other assays exist and are not always analytically interchangeable.

Important points in measurement include:

Issue	Why it matters
Assay used	Different antibodies and platforms can give different results
Tissue adequacy	Too few viable tumour cells can make scoring unreliable
Tumour heterogeneity	One biopsy may not represent the whole tumour
Reader experience	CPS interpretation can vary between pathologists
Cutoff selection	Eligibility may depend on whether CPS is 1, 5, or 10

A pathology report may simply state a number such as CPS 0, CPS 1, CPS 5, CPS 10, or CPS 20. In some reports, the pathologist may also note the clone or assay used. If the score is close to a treatment cutoff, oncologists sometimes review the pathology in the context of the whole case rather than relying on the number alone.

CPS should not be confused with:

• TPS, or tumour proportion score, which counts only PD-L1-positive tumour cells.
• TAP, or tumour area positivity, another scoring approach discussed mainly in some gastroesophageal pathology literature.
• Other biomarkers such as MSI-H, dMMR, TMB, HER2, EGFR, or ALK, which answer different treatment questions.

When to see a clinician

See your oncologist if your pathology report mentions PD-L1 CPS and you want to know whether it changes treatment options. This is especially important if you have advanced or metastatic cancer and immunotherapy is being considered.

Ask specific questions such as:

1. What is my exact CPS number?
2. Which assay was used?
3. Does this cutoff matter for my cancer type?
4. Is the sample recent enough, or could repeat biopsy be useful?
5. Are there other biomarkers I also need, such as MSI, HER2, or driver mutations?

If you are receiving care in India, it can also help to ask whether the result is being used for an approved indication, guideline-based off-label use, or trial enrollment, since access and reimbursement may differ.

Limitations and open questions

CPS is useful, but it is an imperfect biomarker. Not all patients with high CPS respond to immunotherapy, and some patients with low or negative CPS still benefit. Evidence in humans supports its clinical use in several cancers, but performance differs by tumour type and treatment setting.

There are also technical limits. Inter-observer variability, assay differences, specimen quality, and intratumoral heterogeneity can all change the score. Borderline values near a cutoff are the most difficult to interpret.

Another open question is whether newer scoring systems or multimarker approaches will outperform CPS. Research is ongoing on digital pathology, combined immune signatures, and integration with genomic biomarkers. For now, CPS is best viewed as one decision tool within a broader oncology assessment, not as a stand-alone answer about whether immunotherapy will or will not work.