Cancer biomarkers are biological molecules found in blood, body fluids, or tissue that signal the presence of cancer or help guide its treatment. They can be proteins produced by tumors, genetic mutations in cancer cells, or even inherited gene changes that raise your risk. In practical terms, biomarkers are what allow oncologists to move beyond a one-size-fits-all approach and match you with the treatment most likely to work for your specific cancer.
Types of Cancer Biomarkers
Not all biomarkers serve the same purpose. Some help detect cancer early, others predict how aggressive it will be, and still others determine which drugs you’re eligible for. These three roles overlap but are worth understanding separately.
Diagnostic biomarkers help identify whether cancer is present. PSA (prostate-specific antigen), for example, is a protein measured in blood that rises in prostate cancer. CA-125 is a protein linked to ovarian and other gynecological cancers. CEA (carcinoembryonic antigen) is elevated in colorectal, lung, and pancreatic cancers. These markers are often used alongside imaging and biopsies rather than on their own, because no single blood test is perfectly accurate.
Prognostic biomarkers tell your care team something about how the cancer is likely to behave. HER2 overexpression in breast cancer, for instance, historically signaled a more aggressive tumor. That prognostic picture has shifted dramatically now that targeted therapies exist for HER2-positive disease.
Predictive biomarkers are the ones that directly influence which treatment you receive. They answer the question: will this specific drug work for this specific tumor? PD-L1 expression, BRCA mutations, and BRAF mutations all fall into this category. A drug may only be prescribed if your tumor tests positive for the relevant marker.
Proteins, Genes, and What Gets Tested
Biomarkers come in different molecular forms. The traditional ones are proteins that tumors shed into the bloodstream or express on their surface. PSA, CA-125, and CEA are all protein biomarkers measured through standard blood tests or tissue staining.
Genetic biomarkers are mutations in DNA. These fall into two important categories. Somatic mutations are changes that occur only in the tumor itself, acquired after birth. They’re found by testing tumor tissue or circulating tumor DNA in blood. BRAF V600E, EGFR mutations, and HER2 activating mutations are all somatic markers used to select targeted therapies.
Germline mutations, by contrast, are inherited. They’re present in every cell of your body from conception. BRCA1 and BRCA2 are the most well-known examples. Having a germline BRCA mutation both raises your lifetime cancer risk and may qualify you for specific drugs if cancer develops. Here’s an important nuance: a somatic test might find a BRCA variant in your tumor, but unless germline testing is also done, there’s no way to know if you inherited it. That distinction matters for both treatment decisions and family screening.
How Biomarkers Shape Treatment Decisions
The concept of “companion diagnostics” has become central to cancer care. Before you can receive certain drugs, your tumor must be tested for a specific biomarker. The drug and the test are paired together by regulatory approval.
PD-L1 is a protein some tumors use to hide from the immune system. If your tumor has high PD-L1 expression, immune checkpoint inhibitors can block that hiding mechanism and let your immune system attack. Your test results will include a scoring system, either a Combined Positive Score (CPS) measuring PD-L1 on both tumor cells and immune cells, or a Tumor Proportional Score (TPS) measuring the percentage of tumor cells expressing it. If your score is too low, immunotherapy is unlikely to help, and your oncologist will pursue other options.
BRAF V600E is a mutation that drives tumor growth in melanoma, colorectal cancer, and non-small cell lung cancer. Drugs that specifically inhibit BRAF only work when this mutation is present, so testing is required before prescribing them. Similarly, HER2 mutations in lung cancer now qualify patients for targeted kinase inhibitors, a use that has expanded beyond the protein’s long-established role in breast cancer.
The impact of this biomarker-driven approach is measurable. HER2-positive metastatic breast cancer, once among the most aggressive subtypes, now has a median overall survival approaching five years (roughly 56 months in a large analysis of patients diagnosed between 2010 and 2020). Before targeted HER2 therapies existed, outcomes were far worse. That transformation is entirely the result of identifying a biomarker and building drugs around it.
Limitations of Biomarker Tests
Biomarkers are powerful tools, but none is perfect. PSA illustrates the problem well. At the standard cutoff of 4.0 ng/mL, PSA testing has a sensitivity of about 98%, meaning it catches nearly all prostate cancers. But its specificity is only around 9%, meaning the vast majority of men who test positive don’t actually have cancer. Conditions like an enlarged prostate or infection can raise PSA levels. This is why elevated PSA triggers further testing rather than an immediate diagnosis.
PD-L1 testing has its own challenges. Immunohistochemistry, the staining method used to measure PD-L1 expression, has known reliability issues. Different scoring systems and different antibody tests can produce inconsistent results, which is why some oncologists recommend broader genomic testing alongside it.
No single biomarker is a standalone answer. They work best as part of a larger diagnostic picture that includes imaging, pathology, and clinical assessment.
How Biomarker Testing Works
There are two main ways biomarkers are collected: tissue biopsy and liquid biopsy.
Tissue biopsy is the traditional method. A surgeon or radiologist removes a sample of the tumor, and pathologists examine it under a microscope or run molecular tests on it. This provides a detailed snapshot of the tumor’s biology, but it’s invasive, can only sample one area of the tumor at a time, and can’t be easily repeated to track changes.
Liquid biopsy draws a simple blood sample and analyzes it for circulating tumor DNA, circulating tumor cells, or other tumor-derived fragments. It’s quicker, less invasive, and can be repeated over time to monitor how a tumor evolves or whether it’s developing resistance to treatment. The tradeoff is that tumor-derived material in blood can be scarce, making some results less reliable than tissue testing. Both methods are used in practice, sometimes together.
How Long Results Take
If you’re waiting on biomarker results, the timeline depends on which tests are run. Immunohistochemistry, the staining method used for markers like PD-L1 and ALK, typically comes back in about 12 days (with a range of 7 to 14). This is often issued as a partial report.
Next-generation sequencing, which screens for a panel of genetic mutations like EGFR and BRAF, takes longer. In one large study of lung cancer patients, the additional wait from the partial report to the final genomic report was a median of 15 days, with some patients waiting up to 20. That means total turnaround from biopsy to complete results can stretch to nearly four weeks. For patients eager to start treatment, this wait can feel agonizing, but starting the wrong therapy can be worse than waiting for the right one. Your oncology team will typically use the partial results to begin planning while the full picture comes together.