Your oncologist has mentioned immunotherapy but first wants to run some tests on your tumour. You may see terms like PD-L1, MSI or TMB on your reports and wonder what they mean for your treatment. Cancer biomarker testing helps doctors understand the biology of a tumour so they can judge whether immunotherapy is likely to be useful. These results guide decisions, but they are never the only factor. This guide explains each test in plain language and how the team at IOCI India helps patients interpret them. 

What Is Cancer Biomarker Testing? 

A biomarker is a measurable feature of a tumour, such as a protein, a gene change or a pattern in its DNA. Cancer biomarker testing looks for these features, usually in a sample of tumour tissue taken during a biopsy or surgery. Some tests can also be done on blood. 

Biomarkers help doctors predict how a cancer might behave and which treatments may work better. For immunotherapy, three biomarkers are commonly discussed: PD-L1, MSI (or MMR) and TMB. 

PD-L1: What Does It Tell Doctors? 

PD-L1 is a protein that some cancer cells use to switch off the immune system’s attack. Certain immunotherapy drugs, called checkpoint inhibitors, block this signal so immune cells can recognise and attack the cancer. 

The PD-L1 test measures how much of this protein is present on tumour cells, and sometimes on nearby immune cells. Results are usually reported as a percentage or score. The cut-off that matters differs between cancer types and drugs, so a result that is meaningful in lung cancer may be read differently in another cancer. 

MSI and MMR: What Do They Mean? 

Cells have a repair system, called mismatch repair (MMR), that fixes small mistakes when DNA is copied. When this system does not work properly, errors build up. This is described as MMR-deficient (dMMR) or microsatellite instability-high (MSI-H). 

Tumours with MSI-H or dMMR often carry many abnormal proteins, making them more visible to the immune system. This status is especially relevant in cancers such as colorectal and endometrial cancer, and in some other solid tumours. 

TMB: What Is Tumour Mutational Burden? 

Tumour mutational burden (TMB) counts the number of genetic changes in a tumour’s DNA. A higher TMB may mean the tumour produces more abnormal signals the immune system can detect. Its value depends on the cancer type, the testing method and the threshold used, so doctors interpret it within the full clinical picture. 

Are PD-L1, MSI and TMB the Same Test?

No. Each test answers a different question. PD-L1 looks at a specific protein on cells. MSI or MMR testing checks whether the DNA repair system is working. TMB measures how many mutations are present. The PD-L1, MSI and TMB tests for immunotherapy may be ordered together or separately, depending on the cancer. 

Which Biomarker Tests Determine Eligibility for Immunotherapy? 

Which biomarker tests determine eligibility for immunotherapy? It depends on the cancer type, stage, previous treatment and the specific drug being considered. In some cancers, PD-L1 level guides the choice. In others, MSI-H or dMMR status is key. TMB may help in selected situations. Some immunotherapy approvals do not require any biomarker at all. 

What If a Biomarker Result Is Negative? 

A negative or low result does not automatically mean all treatment options are closed. Some patients with low PD-L1 still benefit from immunotherapy, often combined with chemotherapy. Other options, such as targeted therapy based on different gene changes, may also be available. Your oncologist will explain what the result means in your situation. 

How Can IOCI India Support Testing and Treatment Planning? 

Many patients search for the best hospital for cancer biomarker testing in India when they are told immunotherapy is an option. Interpreting results correctly matters as much as running the tests. IOCI India has a 15-year legacy in oncology care and a network of cancer centres across India. Its oncologists create personalised treatment plans, and complex cases can be reviewed through the International Tumor Board, where specialists discuss results together. Patients who already have reports can also request a second opinion. 

Conclusion 

Cancer biomarker testing gives doctors valuable insight into how a tumour may respond to immunotherapy, but it works best as part of a complete assessment. Understanding what PD-L1, MSI and TMB each measure can help you ask better questions. If you are considering immunotherapy or have received biomarker results you want explained, consulting a qualified oncologist can help you understand the right next step. For consultation and personalised guidance, you can connect with IOCI India. Bring your biopsy report and any molecular test results.