Medical Oncologist
Biomarker & Molecular Testing
PD-L1, TMB and MSI: — What These Tests Mean for You
Three biomarkers — PD-L1, TMB and MSI — appear on many cancer reports and cause a great deal of confusion. They predict whether immunotherapy is likely to work. They say nothing about targeted therapy eligibility, which depends on a separate set of tests.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- Immunotherapy markers, not targeted therapy ones — PD-L1, TMB and MSI predict response to checkpoint inhibitors. They do not apply to EGFR, ALK, HER2 or other targeted treatments.
- One biopsy can often answer all three — PD-L1, MMR and MSI testing can usually be done from the same tissue block, though TMB by NGS requires more material.
- A positive result is not a guarantee — High PD-L1, high TMB or MSI-H makes immunotherapy more likely to be considered. It does not mean it will definitely work.
- Negative on all three does not mean no options — Being ineligible for immunotherapy means a different treatment fits your cancer better — not that options have run out.
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PD-L1, TMB and MSI are biomarkers that predict whether immunotherapy — specifically checkpoint inhibitors — is likely to work for your tumour. They do not affect targeted therapy eligibility, which depends on separate tests such as EGFR, ALK or BRAF. Your oncologist uses these results to decide whether immunotherapy is an option worth adding to your plan.
What is the difference between PD-L1, TMB and MSI?
| Marker | PD-L1 | TMB (Tumour Mutational Burden) | MSI / MMR |
|---|---|---|---|
| What it measures | A protein on tumour or immune cells that acts as a brake on the immune system | The total number of mutations in the tumour's DNA | Whether the tumour's DNA repair machinery is working correctly |
| How it is tested | Immunohistochemistry (IHC) on biopsy or surgical tissue | Next-generation sequencing (NGS) — requires more tissue than IHC | IHC for four MMR proteins, or PCR or NGS for MSI status |
| What a high or positive result suggests | Checkpoint inhibitors may have a better chance of working for your cancer | Tumours with many mutations may be more recognisable to the immune system | MSI-H or dMMR tumours often respond well to checkpoint inhibitors, regardless of cancer type |
| Typical result turnaround | Usually one to two weeks | Two to four weeks or more when NGS is required | Usually one to two weeks by IHC; longer if PCR or NGS is used |
| Does it guide targeted therapy? | No | No | No — but MSI-H status changes the choice between chemotherapy and immunotherapy in some cancers such as colorectal cancer |
What should you confirm before these tests are ordered?
- Ask your oncologist whether your cancer type is one where PD-L1, TMB or MSI results change the treatment recommendation.
- Confirm that your original biopsy block is available — testing can usually be done on tissue already collected, without a repeat procedure.
- Check whether the block is large enough for NGS, which TMB testing requires and which needs more tissue than IHC.
- If your biopsy was done at another centre, ask your team to arrange for the tissue block to be sent rather than repeating the procedure.
- Tell your team if a treatment decision is time-sensitive, so the laboratory can prioritise the request.
- Ask for an indicative cost estimate before testing, as charges vary by centre and by test type.
What does your result mean for treatment?
If your PD-L1 score is high, your oncologist will consider whether checkpoint inhibitors are appropriate for your cancer type and stage. A high score increases the likelihood of a response but does not guarantee one — the cancer type and your overall fitness also matter.
If your TMB is high, NCCN and FDA guidance supports checkpoint inhibitors for certain cancer types on this basis. Your oncologist will tell you whether your specific cancer is one where TMB-high status adds an option.
If your tumour is MSI-H or dMMR, checkpoint inhibitors have shown activity across many cancer types, not just the organ where the tumour started. In colorectal cancer, this result often means immunotherapy is preferred over chemotherapy as a first-line treatment.
If all three results are low or negative, immunotherapy is unlikely to be the right choice based on these markers. Your oncologist will recommend the treatment with the strongest evidence for your cancer type — and that is the appropriate next step, not a lesser option.
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Why do these markers not apply to targeted therapy?
Targeted therapy works by blocking a specific abnormal protein driving your tumour's growth — such as a mutated EGFR gene in certain lung cancers or HER2 amplification in some breast and stomach cancers. Whether you are eligible depends on a completely different set of tests.
PD-L1, TMB and MSI describe your immune system's relationship with the tumour. They say nothing about the growth signals inside the tumour that targeted drugs are designed to interrupt.
A report that is negative for PD-L1, TMB and MSI does not mean you are ineligible for targeted therapy. Those tests may not have been ordered yet, or may already have been done separately. Ask your oncologist which molecular tests have been run and what each one found.
What happens next, depending on your result?
My PD-L1 result is high — does immunotherapy definitely apply to me?
A high PD-L1 score makes checkpoint inhibitors more likely to be considered, but it is not the only factor. Your cancer type, stage, and general fitness all influence the decision. For some cancer types, a high PD-L1 score supports immunotherapy alone; for others it supports a combination of immunotherapy and chemotherapy; and for some it remains under investigation. Ask your oncologist exactly how your PD-L1 result changes the options being offered and why.
My result shows MSI-H — which cancers benefit most from this finding?
MSI-H or dMMR status is significant across many cancer types, including colorectal, endometrial, gastric and biliary cancers, among others. ASCO and NCCN guidance supports checkpoint inhibitor treatment for MSI-H or dMMR tumours across multiple cancer types regardless of where the cancer started. In colorectal cancer specifically, MSI-H status is now a major factor in first-line treatment decisions. Ask your oncologist how it applies to your specific diagnosis and what options it opens.
My TMB result is low — does that rule out immunotherapy entirely?
Not necessarily. TMB is one predictive marker among several. A tumour can be low TMB but still be PD-L1-high or MSI-H, and those results carry their own implications for immunotherapy eligibility. Some cancer types are treated with checkpoint inhibitors regardless of TMB because the overall evidence for that cancer type supports it. Low TMB means this particular marker does not support immunotherapy — it does not mean no marker does, and it has no bearing on your targeted therapy options.
All three of my results are negative — what are my treatment options now?
Being negative for PD-L1, TMB and MSI means the available evidence does not support immunotherapy based on these markers. It does not mean your cancer is untreatable or that options have run out. Chemotherapy, targeted therapy, radiation, and combinations remain available and are the recommended path for many cancer types. Ask your oncologist which treatment the evidence supports most strongly for your cancer type and stage, and what that treatment is expected to achieve for you.
Can all three tests be done from the same biopsy sample?
Usually yes, though this depends on the size and quality of the sample. IHC for PD-L1 and MMR proteins requires relatively little tissue. TMB testing by NGS requires a larger sample with enough intact DNA. If your original biopsy block is available, the pathology team will assess whether it is sufficient before proceeding. In some cases one test may need to wait, or a repeat biopsy may be required. Your oncologist or the pathology team can tell you what the current sample allows.
Did you know?
MSI-H or dMMR status was the first biomarker to earn regulatory approval for a cancer treatment that applies regardless of where in the body the cancer started.
ASCO and NCCN guidance now supports checkpoint inhibitor treatment for MSI-H or dMMR tumours across more than a dozen cancer types — making the molecular fingerprint of the tumour more important than the organ it came from.
Source: NCCN Guidelines; ASCO and FDA guidance on MSI-H and dMMR tumour-agnostic immunotherapy
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Common questions
Frequently asked questions
Which of these three markers is the most important?
That depends on your cancer type. PD-L1 is the most commonly ordered because IHC testing is widely available and relevant to many tumour types. MSI or MMR testing is standard for colorectal, endometrial and gastric cancers. TMB is less routinely ordered and matters most when the other two are equivocal or when your oncologist is considering a broader NGS panel. None of the three is universally the most important — their value is determined by your specific diagnosis and what your oncologist is deciding.
Do all cancer types need PD-L1, TMB and MSI testing?
No. The markers that are clinically meaningful depend on your cancer type. Some cancers have well-established biomarker-driven immunotherapy indications; others do not. Your oncologist will order the tests relevant to your diagnosis rather than a blanket panel. Testing for markers that do not apply to your cancer type adds cost and time without changing the treatment decision.
How long does it take to get results for these tests?
PD-L1 and MMR results by IHC are usually available within one to two weeks of the laboratory receiving the sample. TMB testing by NGS typically takes longer because sequencing and analysis require additional processing steps. Your oncologist or pathology team can give you a specific timeline for your laboratory. If a treatment decision is urgent, let your team know so the laboratory can prioritise where possible.
If my PD-L1 is negative, can TMB or MSI still support immunotherapy?
Yes. The three markers are independent of each other, and a positive result for any one of them may support immunotherapy depending on your cancer type. MSI-H in particular carries a tumour-agnostic indication under NCCN and FDA guidance, meaning it can support immunotherapy regardless of cancer type and regardless of PD-L1 or TMB status. Your oncologist will look at all three results together, not in isolation, when deciding whether immunotherapy is appropriate.
What is the difference between MSI and MMR on my report?
They describe the same underlying problem from two angles. MMR refers to the proteins that correct errors in DNA replication. A result of dMMR means those proteins are deficient. MSI measures the DNA errors that accumulate when MMR proteins fail — MSI-H means high instability. Clinically, MSI-H and dMMR are equivalent and carry the same implications for treatment. Some laboratories use IHC to test MMR proteins; others test directly for MSI by PCR or NGS. Either way, the treatment implication is the same.
Do these tests need to be repeated if my cancer comes back?
It is worth asking your oncologist at the time of recurrence. MSI and MMR status are relatively stable between primary and recurrent tumours in most cases, but PD-L1 expression can change over time or after treatment. If new tissue is being biopsied for any reason at recurrence, asking for biomarker testing to be repeated adds little burden and may reveal new information. Your oncologist will advise whether repeat testing is clinically justified in your situation.