Understanding your PD-L1 score — TPS, CPS and what the numbers mean

TPS, or Tumour Proportion Score, is a percentage: the number of tumour cells staining positive for PD-L1 divided by all tumour cells the pathologist counted on the slide. CPS, or Combined Positive Score, is calculated differently — it adds PD-L1-positive immune cells (lymphocytes and macrophages) sitting alongside the tumour to the count of positive tumour cells, then divides by total tumour cells and multiplies by 100. Because CPS folds in immune-cell staining, a CPS number can look higher than a TPS percentage on the exact same tissue sample, even though they are measuring related biology.

Which score appears on your report is not a random lab choice — it follows your cancer type. Non-small cell lung cancer is scored with TPS. Gastric, gastro-oesophageal junction, cervical, head and neck, oesophageal and triple-negative breast cancers are scored with CPS. If you're holding a report right now and the letters TPS or CPS are unfamiliar, this page walks through what each one means and what typically counts as a meaningful result — without trying to tell you what your own number means for your own treatment, which is a conversation for your oncologist.

PD-L1 testing itself is coordinated through CION's accredited partner pathology laboratories — the staining and scoring is a specialist pathology process, not something performed in-house at a consultation.

Thresholds shown are the commonly cited cut-offs from published regulatory companion-diagnostic labelling and NCCN guideline references, indicative as of August 2026. Guidelines are updated periodically, and your oncologist confirms which cut-off applies to your specific treatment plan — this table is for general education, not an eligibility guarantee.

1. 1

   A tissue sample is prepared

   Usually a tumour block from an existing biopsy or surgical specimen — a fresh biopsy is only needed if the existing tissue is insufficient for staining.

2. 2

   Immunohistochemistry (IHC) staining

   Thin tissue sections are stained with a specific antibody clone validated for the drug and cancer type in question, at an accredited partner pathology lab.

3. 3

   A pathologist scores the slide

   The pathologist counts stained tumour cells (for TPS) or tumour plus immune cells (for CPS) under the microscope and calculates the percentage or score by hand.

4. 4

   The report reaches your oncology team

   The score, along with the antibody clone used, is entered on your pathology report and reviewed against the relevant cut-off for your cancer type and the treatment being considered.

• PD-L1 score (TPS/CPS) — one input into the discussion, and its weight varies by cancer type.
• MSI/dMMR status — microsatellite instability or mismatch-repair deficiency can support eligibility independently of the PD-L1 result.
• Tumour mutational burden (TMB) — another biomarker considered separately from PD-L1 in some cancers.
• Cancer subtype and histology — some cancers use PD-L1 routinely; others do not require it at all for immunotherapy to be considered.
• Overall stage and fitness — general health, organ function and prior treatment history all factor into whether immunotherapy is a safe option.

If your PD-L1 score is low or negative, that does not automatically rule immunotherapy out — read our companion page on PD-L1-negative results for how oncologists think through that specific situation.

PD-L1 testing is not one standardised test — it is a family of related assays, each built around a specific antibody clone (such as 22C3, 28-8, SP263 or SP142) that was validated as a companion diagnostic for a specific drug in a specific cancer's clinical trial. Two accredited labs testing the same sample with different clones can occasionally report a modestly different number, and sample handling, storage time, and tissue quality can also affect staining.

This is a recognised, openly discussed limitation in NCCN and ASCO guidance, not a sign that a lab made an error. It is one reason your oncology team looks at your PD-L1 result alongside your full clinical picture rather than treating it as a stand-alone verdict.

• My PD-L1 Is Low or Negative: Does That Mean Immunotherapy Will Not Work? — why a low score isn't the end of the conversation for every cancer type.
• MSI-High and dMMR: What These Results Mean for Immunotherapy — a tumour-agnostic biomarker that works independently of PD-L1.
• Which Biomarker Tests Do You Actually Need — and What Do They Cost? — sequencing PD-L1, MSI and other tests without paying for panels you don't need.
• Immunotherapy at CION Cancer Clinics — the full picture of how CION supports patients through the immunotherapy decision and journey.

This page explains general PD-L1 terminology for education only and does not interpret any individual patient's report. PD-L1 testing is coordinated at accredited partner pathology laboratories. Bring your report to a consultation for a doctor's assessment of what it means for you.

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