PD-L1 expression: Definition, Uses, and Clinical Overview

PD-L1 expression Introduction (What it is)

PD-L1 expression describes how much of the PD-L1 protein is present on tumor cells and/or nearby immune cells.
It is usually measured in a pathology lab using a sample from a biopsy or surgery.
Clinicians most often use PD-L1 expression to help guide immunotherapy decisions in certain cancers.
It is a biomarker test, meaning it provides information about the tumor’s biology rather than treating the cancer by itself.

Why PD-L1 expression used (Purpose / benefits)

PD-L1 expression is used to support cancer treatment planning by estimating how likely a tumor is to respond to immune checkpoint inhibitors (a type of immunotherapy). Immune checkpoint inhibitors—such as drugs that target PD-1 or PD-L1—work by reducing “off signals” that can prevent immune cells from attacking cancer. PD-L1 is one of the key molecules involved in this immune “braking” system.

In general terms, PD-L1 expression helps solve a common problem in oncology: many effective treatments have meaningful risks and costs, and not every patient benefits. By providing an additional piece of tumor-specific information, PD-L1 expression can help clinicians:

• Identify patients who may be more likely to benefit from certain immunotherapies (varies by cancer type and stage).
• Determine whether immunotherapy may be considered alone or in combination with other treatments (such as chemotherapy or radiation), depending on the clinical setting.
• Standardize communication across the care team (oncology, pathology, surgery, radiation oncology) using a shared biomarker result.
• Support discussions about available options when more than one guideline-supported treatment path exists.

Importantly, PD-L1 expression is not a standalone “yes/no” answer. A higher or lower result does not guarantee response or non-response. Many other factors—tumor type, stage, prior treatments, overall health, and additional biomarkers—often influence decisions.

Indications (When oncology clinicians use it)

Oncology clinicians commonly consider PD-L1 expression testing in situations such as:

• Newly diagnosed or suspected advanced/metastatic solid tumors where immunotherapy may be an option (varies by cancer type).
• Non-small cell lung cancer (NSCLC) workups when systemic therapy planning is underway.
• Cases where a clinician is deciding between immunotherapy alone versus immunotherapy combined with chemotherapy (varies by regimen and indication).
• Tumors where PD-L1 is used as an eligibility criterion for a specific drug, line of therapy, or regulatory indication (varies by country/region).
• Recurrence or progression after prior therapy, when the team is reassessing systemic treatment options.
• When tissue is already available from biopsy/surgery and additional biomarker information could affect treatment selection.
• Multidisciplinary tumor board review, to integrate pathology biomarkers with imaging and clinical findings.
• Clinical trial screening, where PD-L1 expression may be required or stratify patients.

Contraindications / when it’s NOT ideal

PD-L1 expression testing may be less suitable, less informative, or harder to interpret in situations such as:

• Insufficient or poor-quality tissue, including very small biopsies with limited tumor cells.
• Suboptimal specimen handling, such as delayed fixation or inadequate preservation, which can affect staining reliability.
• Cancers or clinical settings where PD-L1 is not validated or not routinely used to guide decisions (varies by cancer type and stage).
• When faster decisions are needed and waiting for biomarker results could delay urgent care (the care team may proceed based on clinical urgency).
• When another biomarker is more clinically decisive, such as a targetable driver mutation in certain cancers (e.g., some lung cancers).
• Marked tumor heterogeneity, where PD-L1 may differ between tumor areas or between primary and metastatic sites, potentially limiting representativeness.
• Recent treatments that can influence the tumor microenvironment, such as radiation or systemic therapy, which may change immune-cell presence and PD-L1 staining patterns (interpretation varies by clinician and case).
• When the goal is early detection or screening in healthy individuals; PD-L1 expression is not a screening test.

These are not “hard bans” in most cases; they are reasons a team may prioritize other tests, obtain a better sample, or interpret results more cautiously.

How it works (Mechanism / physiology)

PD-L1 (programmed death-ligand 1) is a protein that can be expressed on tumor cells and on some immune cells within the tumor microenvironment. PD-L1 can bind to PD-1, a receptor on activated T cells (a type of immune cell). When PD-L1 binds PD-1, it sends an inhibitory signal that reduces T-cell activity. In everyday terms, this interaction can function like a “brake” on immune attack.

Relevant tumor biology:

• Tumors can exploit the PD-1/PD-L1 pathway to dampen immune responses, helping cancer cells avoid being destroyed.
• PD-L1 expression may reflect an “inflamed” tumor environment where immune cells are present, or it may reflect tumor-intrinsic signaling pathways that increase PD-L1.

What the test measures:
PD-L1 expression is typically measured by immunohistochemistry (IHC), a staining method performed on tumor tissue. A pathologist evaluates staining patterns and reports a score using a defined scoring system. Different cancers and drugs may use different scoring methods.

Onset, duration, reversibility:
PD-L1 expression is not a treatment, so “onset” and “duration” do not apply in the usual way. The result reflects the sampled tissue at a particular time and site. PD-L1 can change over time or after treatments, so the “longevity” of a result is context-dependent and may vary by cancer type and stage.

PD-L1 expression Procedure overview (How it’s applied)

PD-L1 expression is not a procedure performed on the body like surgery; it is a laboratory test performed on tumor material. A typical high-level workflow looks like this:

1. Evaluation/exam
   A clinician evaluates symptoms, medical history, and physical exam findings and considers whether cancer is present and what type is suspected.

2. Imaging/biopsy/labs
   Imaging (such as CT, MRI, or PET/CT) may help identify a tumor and guide a biopsy. A biopsy or surgical specimen provides tissue for diagnosis and biomarker testing. Blood tests may support overall assessment but do not measure PD-L1 expression directly in the same way as tissue IHC.

3. Pathology diagnosis
   A pathologist confirms the cancer type (for example, carcinoma vs lymphoma; specific subtype) and assesses features such as tumor grade when relevant.

4. PD-L1 expression testing (IHC staining and scoring)
   The lab performs IHC using a validated PD-L1 assay. The pathologist examines stained slides and assigns a score according to the method used for that cancer/therapy context (for example, tumor-cell staining percentage or a combined score that includes immune cells).

5. Staging
   Staging integrates imaging, pathology, and clinical information to describe the extent of disease (localized vs regional vs metastatic, depending on the staging system).

6. Treatment planning
   The oncology team combines PD-L1 expression results with stage, performance status, comorbidities, prior treatments, and other biomarkers to discuss systemic therapy options, local therapies (surgery/radiation), or clinical trials.

7. Intervention/therapy
   If immunotherapy is chosen, it may be given alone or with other treatments depending on the cancer type and clinical scenario.

8. Response assessment
   Follow-up imaging, symptom monitoring, and labs are used to evaluate response and manage side effects. PD-L1 expression itself is not typically repeated routinely, but re-biopsy may be considered in selected situations (varies by clinician and case).

9. Follow-up/survivorship
   Ongoing surveillance, rehabilitation, supportive care, and survivorship planning depend on cancer type, stage, and treatments received.

Types / variations

PD-L1 expression testing varies mainly by assay, scoring method, sample type, and clinical context.

• Assays (test “brands” and platforms):
  Multiple PD-L1 IHC assays exist (often referenced by clone names). Labs generally validate specific assays and platforms. Because assays are not always interchangeable, clinicians and pathologists pay attention to which assay was used, especially when a specific drug label or guideline references a particular approach.

• Scoring systems:
  The report may use different scoring frameworks depending on the cancer type and treatment setting, such as:

• Tumor Proportion Score (TPS): focuses on the percentage of tumor cells with PD-L1 staining.

• Combined Positive Score (CPS): includes PD-L1 staining in tumor cells plus certain immune cells relative to tumor cell count.

• Immune cell–focused scoring: emphasizes PD-L1 staining in immune cells in or near the tumor (used in some tumor types and contexts).

• Specimen types:

• Core needle biopsy or endoscopic biopsy (common in initial diagnosis).
• Surgical resection specimens (often provide more tissue).

• Cytology samples (like cell blocks) may be used in some settings but can be limited; suitability varies by lab and case.

• Clinical contexts:

• Early-stage vs advanced disease: PD-L1 expression may be used differently depending on whether the intent is curative or palliative (supportive).
• First-line vs later-line therapy: its role can change across lines of treatment (varies by cancer type).
• Solid tumors vs hematologic malignancies: PD-L1 expression is most commonly discussed in solid tumors; in blood cancers, immune markers are used differently and depend on the specific disease.

Pros and cons

Pros:

• Helps personalize systemic therapy selection in certain cancers (varies by cancer type and stage).
• Uses tissue that is often already collected for diagnosis, avoiding an additional procedure in some cases.
• Can support eligibility decisions for specific immunotherapy drugs or clinical trials.
• Provides standardized biomarker information that can improve multidisciplinary planning.
• May reduce trial-and-error by guiding whether immunotherapy is more or less likely to be beneficial (not guaranteed).
• Often reported alongside other key pathology findings, streamlining decision-making.

Cons:

• Not a perfect predictor: responses can occur with low PD-L1 expression, and non-response can occur with high expression.
• Results can vary between tumor sites, between primary and metastases, or over time (tumor heterogeneity).
• Different assays and scoring methods can complicate comparisons across institutions or studies.
• Tissue limitations (small biopsies, necrosis, poor fixation) can lead to uninterpretable or less reliable results.
• Interpretation depends on cancer type, treatment context, and the specific therapy being considered.
• Testing turnaround time may delay decisions in time-sensitive situations.
• Insurance coverage and access can vary by region and health system.

Aftercare & longevity

Because PD-L1 expression is a biomarker result rather than a treatment, “aftercare” mainly involves what happens after the result is reported and how it fits into ongoing care.

What affects how useful the result is over time:

• Cancer type and stage: PD-L1 expression is more clinically influential in some cancers and treatment settings than others.
• Tumor biology and additional biomarkers: Other features (such as targetable mutations, mismatch repair status, or overall immune environment) can outweigh or contextualize PD-L1 expression.
• Site and timing of the sample: A result from an older biopsy may not reflect current disease biology if treatments have occurred or if the cancer has evolved.
• Treatment intensity and combinations: When immunotherapy is combined with chemotherapy or radiation, PD-L1 expression may play a different role than when immunotherapy is used alone (varies by clinician and case).
• Follow-up and monitoring: Imaging schedules, symptom tracking, and lab monitoring influence how quickly response or toxicity is recognized once treatment begins.
• Supportive care and comorbidities: Management of side effects, nutrition, rehabilitation, and mental health support can affect treatment tolerance and continuity.

In some circumstances, clinicians may consider repeat biopsy and updated biomarker testing, particularly if the cancer progresses or if a new treatment decision depends on updated tissue information.

Alternatives / comparisons

PD-L1 expression is one tool among many in modern oncology. Alternatives and complements depend on the clinical question—diagnosis, prognosis, or treatment selection.

• Observation/active surveillance vs immediate systemic therapy:
  In selected early-stage or slow-growing cancers, clinicians may consider close monitoring. PD-L1 expression typically does not replace the core decision factors for surveillance (such as stage, growth rate, and symptoms), but it may be part of a broader biomarker profile in some settings.

• Surgery vs radiation vs systemic therapy:
  Local treatments (surgery and radiation) treat disease in a defined area. PD-L1 expression mainly informs systemic therapy choices (treatments that circulate throughout the body), particularly immunotherapy. Treatment sequencing and combination strategies vary by cancer type and stage.

• Chemotherapy vs targeted therapy vs immunotherapy:

• Chemotherapy attacks rapidly dividing cells and is not usually chosen based on PD-L1 expression alone.
• Targeted therapy is guided by specific genetic alterations (for example, certain mutations or fusions) rather than PD-L1. In some cancers, finding a targetable alteration may strongly shape first-line treatment choices.

• Immunotherapy is where PD-L1 expression most often has direct clinical relevance, although it is rarely the only deciding factor.

• Other biomarkers compared with PD-L1 expression:
  Depending on the cancer, clinicians may also use markers such as mismatch repair deficiency/microsatellite instability, tumor mutational burden, and broad genomic profiling. These tests answer different questions, and they may be used together rather than as substitutes.

• Standard care vs clinical trials:
  Clinical trials may offer access to newer immunotherapy combinations or novel immune targets. PD-L1 expression can be used for eligibility or subgroup analysis, but many trials enroll patients regardless of PD-L1 expression.

PD-L1 expression Common questions (FAQ)

Q: Is PD-L1 expression a genetic test?
No. PD-L1 expression is usually measured by immunohistochemistry, which detects protein levels in tissue. Genetic tests look for DNA or RNA changes (mutations, fusions, or expression profiles). Many patients have both types of testing because they provide different kinds of information.

Q: Does PD-L1 expression testing hurt? Do I need anesthesia?
The test itself is done on tissue in a lab and does not cause pain. Discomfort, numbing medication, or sedation relates to the biopsy or procedure used to obtain the tissue, not the PD-L1 expression test. The type of anesthesia depends on the biopsy location and method and varies by clinician and case.

Q: How long does it take to get PD-L1 expression results?
Turnaround time varies by lab workflow, whether the sample needs to be sent out, and whether additional stains are needed. In many cases, PD-L1 expression is reported along with other pathology results during the diagnostic workup. Your care team can explain what is typical for the specific clinic and specimen type.

Q: If my PD-L1 expression is high, does that mean immunotherapy will work?
Not necessarily. Higher PD-L1 expression can be associated with a higher chance of benefit in some cancers and settings, but it does not guarantee response. Tumor biology is complex, and factors such as tumor subtype, disease burden, other biomarkers, and prior treatments can also influence outcomes.

Q: If my PD-L1 expression is low or negative, does that mean immunotherapy won’t help?
Not always. Some patients with low PD-L1 expression still respond to immunotherapy, and some treatment approaches use immunotherapy regardless of PD-L1 expression (depending on cancer type and stage). Clinicians interpret the result in context rather than using it as the only decision point.

Q: Are there side effects from PD-L1 expression testing?
The laboratory test has no direct side effects. Possible risks come from the biopsy or surgery used to obtain tissue (such as bleeding, infection, or pain), and those risks vary by procedure and patient factors. If immunotherapy is chosen based on the result, immunotherapy itself can cause side effects, which your oncology team monitors closely.

Q: What does PD-L1 expression mean for the length of treatment?
PD-L1 expression does not set a fixed treatment length by itself. Treatment duration depends on the specific cancer, stage, chosen therapy, response, side effects, and care goals. Plans are typically reassessed over time using imaging, symptom review, and lab monitoring.

Q: How much does PD-L1 expression testing cost?
Costs vary widely based on the health system, insurance coverage, whether the lab is in-network, and whether the test is bundled into a larger pathology or biomarker panel. Some patients have no separate out-of-pocket cost, while others may face copays or coverage limits. A clinic financial counselor or billing team can often help clarify coverage in general terms.

Q: Can PD-L1 expression affect fertility or pregnancy?
The PD-L1 expression test itself does not affect fertility or pregnancy because it is performed on tissue outside the body. However, treatment decisions influenced by the result—such as immunotherapy, chemotherapy, or radiation—can have fertility and pregnancy implications. These issues are highly individualized and are typically discussed as part of treatment planning and supportive care.