Biomarker testing is a laboratory method that analyzes a sample of tissue, blood, or other body fluid to look for specific genes, proteins, or other molecules that signal a disease, guide treatment choices, or predict how a condition will progress. It’s most commonly associated with cancer care, where it helps oncologists match patients with targeted therapies, but it’s also used in cardiology, neurology, and other fields. The core idea is simple: rather than treating everyone with the same disease identically, biomarker testing reveals biological details that make treatment more precise.
What Biomarker Testing Actually Looks For
Every disease leaves molecular fingerprints. Cancer cells, for example, often carry specific genetic mutations or produce unusual proteins on their surface. Biomarker testing identifies these fingerprints so doctors can understand what’s driving the disease at a molecular level. The test might look for a single gene change, a protein that’s overproduced, or dozens of genetic alterations at once using broader panels.
Not all biomarkers serve the same purpose. They generally fall into three categories:
- Diagnostic biomarkers help confirm whether you have a particular disease or subtype of disease. They answer the question “what is this?”
- Prognostic biomarkers indicate how a disease is likely to behave over time, including the chance of recurrence or progression. They answer “how serious is this?”
- Predictive biomarkers identify whether a specific treatment is likely to work for you. They answer “will this drug help?” A predictive biomarker is confirmed by comparing outcomes in patients with and without the biomarker who receive the same treatment.
A single biomarker can sometimes serve more than one role. A genetic mutation might both indicate a worse prognosis and predict a strong response to a particular drug.
Biomarkers Commonly Tested in Cancer
Cancer care is where biomarker testing has had the greatest impact. Several markers are now tested routinely because they directly determine which therapies a patient is eligible for:
- HER2 is a protein that, when overproduced, fuels the growth of certain breast, stomach, bladder, and lung cancers. Patients who test positive for HER2 can receive drugs specifically designed to block it.
- EGFR mutations are common in non-small cell lung cancer and colorectal cancer. Targeted drugs that block the signals from these mutations can slow or stop tumor growth.
- BRAF V600 mutations appear in melanoma, thyroid cancer, colorectal cancer, and several other types. Drugs targeting this mutation have dramatically changed outcomes in melanoma.
- PD-L1 is a protein some tumors use to hide from the immune system. High PD-L1 levels often indicate that immunotherapy drugs will be effective. It’s tested across many cancer types, including lung, breast, liver, bladder, and head and neck cancers.
- NTRK gene fusions are rare but can appear in virtually any solid tumor. Drugs targeting this fusion work regardless of where the cancer originated.
The FDA currently lists over 200 approved companion diagnostic tests, each one tied to a specific therapy. A companion diagnostic is a test that must be performed before a particular drug can be prescribed, because the drug only works in patients with a specific biomarker.
Inherited Mutations vs. Tumor Mutations
Biomarker testing can look at two fundamentally different types of genetic changes, and the distinction matters. Germline mutations are inherited from a parent and exist in every cell of your body from birth. These are the mutations tested when doctors evaluate hereditary cancer risk, such as BRCA1 or BRCA2 in breast and ovarian cancer.
Somatic mutations, by contrast, develop randomly during your lifetime in specific cells. They aren’t inherited and can’t be passed to your children. Most cancer-driving mutations are somatic. They arise when cells copy themselves imperfectly, and the resulting errors give those cells a growth advantage. When oncologists order biomarker testing on a tumor, they’re typically looking for somatic mutations that reveal what’s driving that particular cancer and which drugs might stop it.
Some patients undergo both types of testing. A tumor biopsy might reveal a somatic mutation that’s targetable with a drug, while a separate blood test might check for inherited mutations that affect cancer risk for the patient and their family members.
How Samples Are Collected
There are two main approaches to collecting material for biomarker testing: tissue biopsy and liquid biopsy.
A tissue biopsy involves removing a small piece of the tumor, either through a needle or during surgery. The sample goes to a molecular pathology lab, where technicians extract DNA and run it through sequencing technology. Tissue biopsies provide a detailed genetic portrait of the tumor but require an invasive procedure and enough tumor material to work with.
A liquid biopsy uses a standard blood draw. Tumors shed tiny fragments of their DNA into the bloodstream, and labs can capture and analyze these fragments. The process is far less invasive, making it easier to repeat over time, which is especially useful for tracking whether a cancer is developing resistance to treatment. The trade-off is that liquid biopsies may miss some mutations that a tissue biopsy would catch, particularly when tumors are small or shed little DNA.
How Long Results Take
Turnaround time depends on what’s being tested. Simple tests using a technique called immunohistochemistry, which looks for specific proteins, typically return results in about 12 days. These partial reports can provide information on markers like PD-L1 status relatively quickly.
Broader genetic panels that use next-generation sequencing take longer. In a large study of lung cancer patients, the median total testing time was 21 calendar days across all patients. For those needing the full sequencing panel, results took a median of 23 days, with an additional 15 days required after the initial protein-based results came back. The bottleneck is the sequencing step itself, not the initial sample processing.
This wait can feel long when you’re anxious to start treatment. In some cases, doctors begin a standard therapy while waiting for biomarker results and then adjust the plan once the full picture is available.
Why It Changes Treatment Decisions
Biomarker testing meaningfully shifts which treatments patients receive. In a study published in JAMA Network Open, patients with non-small cell lung cancer and colorectal cancer who underwent comprehensive genomic profiling were significantly more likely to receive targeted therapy compared to those who had more limited testing. The effect was substantial: patients with comprehensive testing had roughly 1.5 to 2.3 times higher odds of receiving a targeted drug.
This matters because targeted therapies, when matched to the right biomarker, tend to be more effective and often cause fewer side effects than broad chemotherapy. A lung cancer patient whose tumor carries an EGFR mutation, for instance, will typically respond far better to an EGFR-targeting drug than to standard chemotherapy. Without biomarker testing, that match would never be identified.
Beyond Cancer
While oncology dominates the conversation around biomarker testing, the approach is expanding into other areas of medicine. In cardiology, blood tests measuring proteins released by damaged heart muscle help diagnose heart attacks, assess heart failure severity, and predict outcomes after a stroke. Elevated levels of these cardiac proteins in stroke patients have been linked to a higher risk of heart rhythm problems and reduced heart function.
In neurology, researchers are studying blood-based biomarkers for dementia. Population studies have found associations between cardiac biomarker levels and cognitive decline, particularly for vascular dementia. The goal is to eventually identify neurodegenerative diseases earlier, before significant brain damage occurs, when interventions might be more effective.
Insurance Coverage and Cost
Coverage for biomarker testing varies by insurer, test type, and diagnosis. Medicare covers many oncology biomarker tests, though some comprehensive panels are limited to once per lifetime per patient. Certain tests require documentation that a standard workup couldn’t answer the clinical question before the molecular test will be approved for payment.
Private insurers have their own criteria, and coverage has been expanding as more companion diagnostics receive FDA approval. If a test is tied to an FDA-approved companion diagnostic and your cancer type matches the indication, coverage is more likely. For tests ordered outside those clear-cut scenarios, prior authorization or appeals may be necessary. Many testing companies offer financial assistance programs for patients facing high out-of-pocket costs, and your oncology team can often help navigate the process.