A PD-L1 test uses immunohistochemistry to measure programmed death ligand 1 protein in a tumor sample. The result can help select immune checkpoint inhibitor therapy in lung, head and neck, gastric, esophageal, cervical, triple-negative breast, urothelial, and other cancers. PD-L1 is not one universal test: different drugs and tumor types use different antibody clones, scoring systems, specimen requirements, and cutoffs. Tumor proportion score (TPS) counts positive tumor cells, while combined positive score (CPS) includes positive tumor cells and certain immune cells. Some assays use immune-cell, tumor-cell, or tumor-area scoring. A “positive” result therefore has meaning only when the report states the assay, score, cutoff, cancer type, and intended treatment. High expression can increase the chance of benefit in some settings, but it does not guarantee response. Low or negative expression does not always rule out immunotherapy because some approvals do not require PD-L1 or use other biomarkers. Tissue handling, tumor heterogeneity, recent treatment, and pathologist interpretation can all affect the score.
- PD-L1 is measured by immunohistochemistry, not by a routine blood test.
- TPS and CPS are different formulas and cannot be substituted for one another.
- The positive cutoff is drug- and cancer-specific; there is no universal normal PD-L1 range.
- A high score may enrich for immunotherapy benefit but does not guarantee response.
- A negative score may still permit immunotherapy in indications that do not require PD-L1 positivity.
Table of Contents
- What PD-L1 Means
- When Testing Is Used
- TPS, CPS, and Other Scores
- Assays, Specimens, and Quality
- Interpreting High, Low, and Negative Results
- PD-L1 by Cancer Type
- Limits of PD-L1 as a Biomarker
- Next Steps and Questions to Ask
What PD-L1 Means
PD-L1 is a surface protein that can bind PD-1 on activated T cells. This interaction normally helps limit immune injury after infection or inflammation. Cancers can exploit the same checkpoint to reduce T-cell attack. Drugs that block PD-1 or PD-L1 can restore antitumor immune activity in some patients.
PD-L1 expression can occur on tumor cells and immune cells. It is dynamic rather than fixed. Interferon released during an immune response can induce PD-L1, creating an “adaptive resistance” pattern. Oncogenic pathways and gene amplification can also drive expression. A biopsy captures one site and one time point in this changing biology.
The test detects protein with an antibody stain. It does not measure a mutation in the CD274 gene and does not directly count immune cells in blood. Brown membranous staining in viable tumor and defined immune-cell populations is scored according to a validated algorithm. Cytoplasmic stain alone is generally not counted for standard companion assays.
PD-L1 is a predictive enrichment biomarker in selected indications. It estimates how likely a group of patients is to benefit from a particular immune checkpoint strategy compared with another treatment. It is not a binary biological truth. Some PD-L1-high tumors do not respond, while some PD-L1-negative tumors do.
The result must remain attached to the cancer type and proposed regimen. A CPS of 10 can be meaningful in one disease and irrelevant in another. A TPS of 50% in non-small cell lung cancer is not interchangeable with CPS 50 in gastric cancer.
When Testing Is Used
PD-L1 testing is commonly ordered at diagnosis of advanced non-small cell lung cancer, before first-line systemic treatment. It is also used in recurrent or metastatic head and neck squamous cell carcinoma, triple-negative breast cancer, gastric or gastroesophageal junction cancer, esophageal cancer, cervical cancer, urothelial cancer, and other settings. Requirements change as drug indications evolve.
The ordering team should specify the tumor type and intended drug or regimen. This lets the laboratory select an assay and scoring algorithm that are analytically and clinically validated. A generic order for “PD-L1” without context can produce a score that does not match the treatment question.
Testing may use the original diagnostic specimen or a newer recurrence biopsy. A recent sample can reflect current biology after chemotherapy, radiation, targeted therapy, or immunotherapy, but obtaining it may be risky or unnecessary. Archived tissue may be acceptable if adequate and appropriately processed.
PD-L1 is usually tested alongside other biomarkers. In lung adenocarcinoma, actionable genomic drivers such as EGFR, ALK, ROS1, BRAF, MET, RET, NTRK, HER2, and KRAS may take priority because targeted therapy can be preferred even when PD-L1 is high. In colorectal and endometrial cancer, MSI/MMR status is generally more informative than PD-L1 for immunotherapy selection.
A tumor mutational burden test or MSI and dMMR testing can provide complementary information, but none is universally required or sufficient across all cancers.
TPS, CPS, and Other Scores
Tumor proportion score (TPS) is the percentage of viable tumor cells showing partial or complete membranous PD-L1 staining. If 60 of 100 viable tumor cells stain, TPS is 60%. Immune cells are not included. TPS is widely used in non-small cell lung cancer.
Combined positive score (CPS) is the number of PD-L1-positive tumor cells, lymphocytes, and macrophages divided by the total number of viable tumor cells, multiplied by 100. CPS can exceed the tumor-cell percentage because immune cells are included, but the score is conventionally capped at 100. CPS is used in several head and neck, gastric, esophageal, cervical, and breast cancer indications.
Some assays use tumor-cell (TC) and immune-cell (IC) percentages, measuring positive tumor cells or the tumor area occupied by positive immune cells. Tumor area positivity (TAP) is used in selected newer settings and estimates the proportion of tumor area occupied by PD-L1-positive tumor and immune cells. These systems are not mathematically interchangeable.
| Score | Cells or area included | Common use |
|---|---|---|
| TPS | PD-L1-positive tumor cells / all viable tumor cells | Non-small cell lung cancer |
| CPS | Positive tumor cells, lymphocytes, and macrophages / viable tumor cells × 100 | Head and neck, gastric, cervical, TNBC, and others |
| TC | Percentage of positive tumor cells | Selected drug-specific assays |
| IC | Immune-cell staining within defined tumor area | Selected urothelial, breast, and lung settings |
| TAP | PD-L1-positive tumor and immune-cell area / total tumor area | Selected emerging indications |
Cutoffs may include TPS 1% or 50%, CPS 1, 5, 10, or 20, and assay-specific IC/TC thresholds. These numbers are not severity grades. A CPS of 20 does not mean the tumor is 20% likely to respond.
Assays, Specimens, and Quality
Major antibody clones include 22C3, 28-8, SP263, and SP142, each linked historically to specific platforms and drug development programs. Some assays show substantial analytic similarity in certain tumors, while others—especially SP142 tumor-cell staining—can produce systematically different results. Laboratories must validate any assay used as a companion or complementary diagnostic.
Adequate viable tumor is essential. Many algorithms require at least 100 viable tumor cells, though requirements vary. Small biopsies and cytology cell blocks can be acceptable when validated. Necrosis, crush artifact, cautery, and scant tissue reduce reliability.
Fixation matters. Neutral-buffered formalin is standard. Strong-acid decalcification can destroy antigen and produce false-low staining, which is a concern in bone metastases. A nondecalcified specimen or validated alternative may be preferable. Old unstained slides can lose antigenicity; cutting fresh sections from the block is often better.
Intratumoral heterogeneity can create discordance between primary and metastatic sites or between small biopsy and resection. Immune-cell-rich edges may score differently from tumor centers. A pathologist distinguishes tumor cells from macrophages and other cells and excludes necrotic areas.
The report should include the clone or assay, platform when relevant, score type, numerical result, cutoff interpretation for the intended indication, specimen, adequacy, and limitations. “PD-L1 positive” without a score is inadequate for many decisions.
Interpreting High, Low, and Negative Results
A high PD-L1 result can support immunotherapy alone or in combination in selected cancers. In metastatic non-small cell lung cancer without an actionable driver, TPS at least 50% may permit first-line anti-PD-1 monotherapy for appropriate patients, while lower positive levels can support other regimens. Clinical factors and current labels determine the exact choice.
A low-positive result can still be actionable. CPS 1 or TPS 1% may meet thresholds in some diseases, but the expected benefit may differ from a very high score. Combination therapy may be favored when rapid tumor shrinkage is needed or when disease burden is high.
A negative result means staining is below the specified cutoff in that specimen. It does not prove that checkpoint blockade cannot work. Chemotherapy–immunotherapy combinations in several cancers may be used regardless of PD-L1. MSI-H/dMMR, TMB-high, viral association, or other biological features can also support immunotherapy independently.
An indeterminate result occurs when there are too few viable tumor cells, tissue is damaged, controls fail, or scoring cannot be performed. It should not be treated as negative. Another block, biopsy, or validated cytology specimen may be tested.
A score close to the cutoff has measurement uncertainty. Observer variation, heterogeneous staining, and sampling can move a case across a threshold. This does not make the test useless, but it argues for careful pathology review when the score will determine access to a major treatment.
PD-L1 by Cancer Type
In non-small cell lung cancer, TPS is the dominant score. PD-L1 must be interpreted with driver testing. A tumor with EGFR mutation or ALK fusion may receive targeted therapy first even when TPS is high, because immunotherapy response patterns and toxicity sequencing differ.
In head and neck squamous cell carcinoma, CPS helps select pembrolizumab alone or with chemotherapy in recurrent or metastatic disease. Disease symptoms, pace, and need for rapid response influence whether monotherapy is appropriate.
In triple-negative breast cancer, CPS with a specified assay can determine eligibility for pembrolizumab plus chemotherapy in advanced disease. Early-stage high-risk TNBC immunotherapy regimens may not use the same PD-L1 requirement, illustrating why stage and regimen matter.
In gastric, gastroesophageal junction, and esophageal cancers, CPS thresholds differ by histology, drug, and combination. HER2, MSI/MMR, and sometimes CLDN18.2 or other markers also influence therapy. A single CPS should be considered within the full biomarker panel.
In cervical cancer, CPS can be required for selected pembrolizumab indications, while other settings use immunotherapy regardless of score. In urothelial cancer, assay-specific IC or CPS criteria have changed over time with indication updates.
In colorectal cancer, PD-L1 staining is not routinely used to select immunotherapy. MSI-H/dMMR status is the established biomarker. Applying a lung-cancer TPS framework to colon cancer would be misleading.
Limits of PD-L1 as a Biomarker
PD-L1 is imperfect because antitumor immunity depends on more than one checkpoint protein. A tumor needs recognizable antigens, T-cell infiltration, antigen presentation, interferon signaling, and an immune environment capable of responding. Defects in any step can produce resistance despite high PD-L1.
Expression changes over time and under treatment pressure. Chemotherapy, radiation, targeted therapy, infection, and inflammation can alter staining. A biopsy from one metastasis may not represent another. Small samples are especially vulnerable to heterogeneity.
Assay diversity adds complexity. Different clones, platforms, scoring algorithms, and cutoffs were developed with different drugs. Although some assays can be harmonized, substitution must be validated rather than assumed. The College of American Pathologists recommends validated PD-L1 IHC for advanced lung cancer and appropriate validation across specimen types and fixatives.
PD-L1 can also correlate with poor prognosis in some untreated cancers and favorable immunotherapy response in others. Predictive and prognostic meanings should not be confused. The test does not estimate how long a person will live.
A complete tumor profile, stage, symptoms, autoimmune history, organ transplant status, steroid use, and treatment goals often matter as much as the score.
Next Steps and Questions to Ask
Ask for the full pathology addendum rather than a portal label. Useful questions include:
- Which PD-L1 antibody clone and platform were used?
- Is the result TPS, CPS, IC, TC, or TAP?
- What is the exact numerical score and drug-specific cutoff?
- Was there enough viable tumor, and was the specimen decalcified?
- Does the proposed regimen require PD-L1 positivity?
- Are there actionable driver mutations that should be treated first?
- Were MSI/MMR and TMB tested where relevant?
- Would a newer biopsy be more representative than archival tissue?
- How do autoimmune disease, transplant history, or lung disease affect immunotherapy risk?
- What immune-related symptoms require urgent contact?
Checkpoint inhibitors can cause immune-related inflammation of the lungs, colon, liver, thyroid, pituitary, skin, kidneys, heart, nerves, and other organs. New shortness of breath, persistent diarrhea, severe abdominal pain, jaundice, confusion, weakness, chest pain, or marked fatigue should be reported promptly. Toxicity can appear during treatment or months after it stops.
Common report-reading mistakes
The first mistake is reading “10” without checking whether it is TPS 10%, CPS 10, IC 10%, or another score. These values use different denominators and cell populations. The second is treating the cutoff as a biological cliff. A CPS of 9 and CPS of 10 may be analytically close, even though a regulatory threshold creates different treatment access. Borderline cases deserve careful review, not creative rescoring.
The third mistake is assuming a newer biopsy is always better. A recent specimen may be tiny, necrotic, or decalcified, while an older resection has abundant well-fixed tumor. The best sample is the most biologically relevant specimen that also meets analytic requirements. The pathologist can compare blocks and choose deliberately.
The fourth is ignoring molecular drivers. In lung cancer, a never-smoker with an EGFR mutation and TPS 90% does not automatically follow the same first-line pathway as a driver-negative smoker with TPS 90%. Targeted therapy evidence, risk of overlapping toxicities, and sequencing matter.
How treatment decisions use the score
A high score may allow checkpoint inhibitor monotherapy, reducing chemotherapy exposure for selected patients. Combination therapy can still be preferred when disease is symptomatic, bulky, or rapidly progressive. Lower or negative expression may support chemotherapy–immunotherapy when the indication permits. Performance status, autoimmune disease, organ transplant, chronic immunosuppression, viral infection, and prior pneumonitis influence safety.
PD-L1 can also be one biomarker among several. In gastric cancer, HER2, MSI/MMR, CLDN18.2, and tumor site influence the regimen. In triple-negative breast cancer, germline BRCA status and disease-free interval matter. In head and neck cancer, symptom burden and need for rapid response can determine whether pembrolizumab is used alone or with chemotherapy.
Response is evaluated clinically and radiographically, not by repeating PD-L1 after every cycle. Immune-related pseudoprogression can occur but is uncommon and must be distinguished from true progression. Continuing therapy through apparent progression is appropriate only in selected clinically stable patients under oncology supervision.
Repeat testing and discordance
Repeat PD-L1 testing may be considered after a new recurrence, a long interval, major intervening therapy, an inadequate first specimen, or a change in the proposed drug and scoring system. It is not automatically useful after every progression. The new result can differ because biology changed or because a different site, clone, block, or scoring algorithm was used.
When two results conflict, compare clone, platform, score type, tumor type, specimen date, fixation, tumor-cell count, and treatment exposure. A pathologist can rescore the same specimen with the required algorithm or test another block. Averaging two scores is not valid.
Patients should keep the exact score and assay in their treatment summary. “PD-L1 positive” may be insufficient if they later seek a trial or transfer care. The report should remain linked to the specimen and cancer diagnosis from which it came.
Preanalytic and interpretive checkpoints
Cold ischemia—the time between tissue removal and formalin fixation—should be minimized. Underfixation and overfixation can affect antigen preservation. The laboratory validates a fixation window and specimen types, including cytology cell blocks. A sample prepared with an unvalidated fixative should be reported with a limitation or retested.
Pathologists use external controls and internal cells to verify staining. Macrophages can be strongly positive and mistaken for tumor cells, especially in small biopsies. Melanin, anthracotic pigment, crush artifact, and necrosis can obscure interpretation. Training and participation in quality-assurance programs improve reproducibility.
A score is based on viable tumor. If only a few clusters remain after deeper sections or molecular testing, the PD-L1 slide may not represent the original block. Tissue stewardship matters in lung cancer, where one small biopsy must support diagnosis, driver testing, and PD-L1.
What PD-L1 does not tell you
The test does not show whether a person has an autoimmune disease, whether a transplant will reject, or whether immune toxicity will occur. It does not measure PD-1 on circulating T cells and does not determine a safe dose. These questions require clinical assessment.
PD-L1 also does not replace tumor histology. Small cell lung cancer, mesothelioma, melanoma, and renal cancer have immunotherapy strategies that may not use the same scoring requirements as NSCLC. Applying a familiar cutoff outside its validated disease can misdirect care.
A score cannot distinguish a durable complete response from a brief partial response. It shifts probabilities at a population level. Other features—tumor burden, liver metastases, performance status, genomic drivers, microbiome, steroid exposure, and immune-cell composition—contribute to outcome.
After immunotherapy starts
Baseline thyroid tests, liver enzymes, kidney function, glucose, and symptom review help detect immune toxicity. Follow-up schedules vary by drug and combination. Patients should carry information that they are receiving a checkpoint inhibitor because an emergency clinician may otherwise miss delayed immune-related disease.
High-dose corticosteroids or other immunosuppression may be needed for serious toxicity. Treating toxicity promptly does not necessarily erase anticancer benefit. Rechallenge after a severe event is individualized by organ, grade, recovery, alternatives, and treatment response.
PD-L1 is rarely repeated to decide whether toxicity has resolved or whether therapy can restart. The decision is clinical. This distinction prevents the biomarker from being used for tasks it was never designed to perform.
The report date should be separated from the specimen date. A score issued today from tissue collected years earlier reflects the older tumor state and treatment-naive or pretreated context at collection. This may still be clinically adequate, but the oncology note should acknowledge it when interpreting a borderline or unexpected result.
References
- PD-L1 and TMB Testing of Patients With Lung Cancer for Immunooncology Therapies 2024 (Guideline)
- Programmed Death Ligand-1 and Tumor Mutation Burden Testing of Patients With Lung Cancer for Selection of Immune Checkpoint Inhibitor Therapies: Guideline From the College of American Pathologists, Association for Molecular Pathology, International Association for the Study of Lung Cancer, Pulmonary Pathology Society, and LUNGevity Foundation 2024 (Guideline)
- Refining PD-1/PD-L1 assessment for biomarker-guided immunotherapy: A review 2024 (Review)
- PD-L1 Testing by Immunohistochemistry 2022 (Official Laboratory Review)
- KEYTRUDA (pembrolizumab) Prescribing Information 2025 (Official Label)
Disclaimer
This article provides general information and cannot determine whether immunotherapy is appropriate. PD-L1 must be interpreted using the cancer-specific assay, score, cutoff, pathology, complete biomarker profile, and current drug indication. New or severe symptoms during or after checkpoint therapy should be reported promptly because immune-related reactions can affect any organ.
