A biomarker is a measurable characteristic of your body that indicates what’s happening inside it, whether that’s a normal process, a disease, or a response to treatment. It can be a molecule in your blood, a gene variant in your DNA, a reading on an imaging scan, or even a physiological measurement like blood pressure. The key distinction: a biomarker is something that can be objectively measured, not how you feel or describe your symptoms.
What Counts as a Biomarker
The term is broader than most people expect. The formal definition, established jointly by the FDA and the National Institutes of Health, describes a biomarker as “a defined characteristic that is measured as an indicator of normal biological processes, pathogenic processes, or responses to an exposure or intervention.” That covers an enormous range of things your doctor might test or track.
Some biomarkers are molecular: proteins, hormones, or fragments of DNA circulating in your blood. Others are structural, like the size of a tumor on a CT scan or the thickness of artery walls on an ultrasound. Still others are physiological, like your resting heart rate or blood pressure. What ties them all together is that they can be measured consistently and that the measurement tells a clinician something useful about your health.
The Seven Categories
The FDA recognizes seven distinct types of biomarkers, each serving a different purpose in medicine:
- Susceptibility/risk biomarkers indicate your likelihood of developing a disease you don’t yet have. Certain gene variants that raise breast cancer risk are a classic example.
- Diagnostic biomarkers confirm whether a disease is present. A hemoglobin A1c level of 6.5% or above, for instance, is used to diagnose diabetes, while 5.7% to 6.4% signals prediabetes.
- Monitoring biomarkers are measured repeatedly over time to track how a condition is progressing or responding to treatment.
- Prognostic biomarkers help predict how a disease will likely unfold, regardless of treatment. A tumor’s growth rate on imaging, for example, can signal how aggressive a cancer is.
- Predictive biomarkers tell clinicians which specific treatment is most likely to work for you. These are the backbone of precision medicine.
- Pharmacodynamic/response biomarkers show that a treatment is having its intended biological effect, even before you notice a change in symptoms.
- Safety biomarkers flag potential harm from a treatment, like liver enzyme levels that rise when a medication is causing organ stress.
Everyday Biomarkers You Already Know
You’ve likely had biomarkers measured at a routine checkup without thinking of them that way. LDL cholesterol is one of the most familiar. For adults 20 and older, a level below 100 mg/dL is considered healthy. As LDL rises above that threshold, so does your risk of heart disease and stroke. Blood pressure works the same way: it’s a physiological biomarker that predicts cardiovascular events, and reducing it with medication or lifestyle changes has been shown in numerous trials to lower the risk of stroke.
Hemoglobin A1c is another routine biomarker. It reflects your average blood sugar over the past two to three months. Below 5.7% is normal. Between 5.7% and 6.4% means prediabetes. At 6.5% or higher, you meet the diagnostic threshold for diabetes. Because it captures a longer window than a single blood sugar reading, A1c gives a more reliable picture of how well your body is managing glucose.
How Biomarkers Guide Cancer Treatment
Cancer care is where biomarkers have had their most dramatic impact. Rather than treating every patient with the same chemotherapy, oncologists now test tumors for specific molecular features and choose drugs that target those features directly. This approach has transformed outcomes for several cancer types.
Breast cancer was one of the first success stories. About one in five breast cancers overproduces a protein called HER2 on the surface of tumor cells. Patients whose tumors test positive for HER2 receive targeted drugs that block that protein, significantly improving survival. Similarly, breast cancers that express estrogen receptors respond to hormone-blocking therapy, which reduced breast cancer mortality by roughly one-third in early-stage patients. Patients whose tumors lack these receptors see no benefit from those same drugs, so the biomarker test determines the treatment plan.
The same principle applies across cancers. Certain lung cancers carry specific gene mutations that make them vulnerable to targeted therapies. Melanomas with a particular mutation in the BRAF gene respond to drugs that inhibit that pathway. Patients with inherited BRCA1 or BRCA2 gene mutations, which affect the body’s ability to repair DNA damage, benefit from a class of drugs that exploits that vulnerability in breast, ovarian, prostate, and pancreatic cancers.
Liquid Biopsies: Biomarkers in a Blood Draw
One of the most significant advances in biomarker testing is the liquid biopsy. When tumor cells break down, they release tiny fragments of their DNA into the bloodstream. Detecting and analyzing this circulating tumor DNA from a simple blood draw gives clinicians a window into the tumor’s genetics without a surgical biopsy.
This is particularly valuable when a tumor is difficult to reach with a needle, when tissue from a previous biopsy has run out, or when clinicians need to monitor how a cancer is responding to treatment over time. Rising levels of circulating tumor DNA correlate with tumor growth, while falling levels track with successful treatment. After surgery to remove a tumor, a sharp drop in circulating tumor DNA is expected. If fragments persist or reappear weeks later, it can signal that microscopic disease remains, alerting the care team before a new tumor becomes visible on a scan.
Biomarkers in Drug Development
Biomarkers also play a critical role in getting new medications approved. In clinical trials, the ultimate goal is to show that a drug improves meaningful outcomes like survival or reduced disability. But those outcomes can take years to measure. Biomarkers can serve as stand-ins, called surrogate endpoints, that speed up the process.
Blood pressure reduction is a validated surrogate endpoint. Enough clinical trials have proven that lowering systolic blood pressure reduces stroke risk, so a new blood pressure drug can demonstrate its effect on that biomarker rather than waiting years to count strokes. This allows trials to enroll fewer people and finish faster. For diseases where no validated surrogate exists yet, the FDA’s Accelerated Approval program allows drugs to be approved based on “reasonably likely” surrogate endpoints, giving patients with serious diseases faster access to promising therapies while longer-term studies continue.
Before a new biomarker can be used this way, it goes through a formal FDA qualification process with three stages: a Letter of Intent describing the biomarker and the need it addresses, a Qualification Plan outlining the studies needed to prove its usefulness, and a Full Qualification Package containing all the accumulated evidence. The FDA reviews and accepts or declines each stage before the next can proceed.
Pharmacogenomic Biomarkers and Medication Response
Your genes influence how your body breaks down medications, and certain genetic biomarkers now appear directly on drug labels. The blood thinner warfarin is a well-known example. Variations in two genes affect how quickly your body processes the drug and how sensitive you are to its effects. People with certain variants need lower doses to avoid dangerous bleeding, while others need higher doses to get adequate protection against clots. The FDA label for warfarin includes genetic information to help guide dosing.
The antiplatelet drug clopidogrel, commonly prescribed after heart attacks and stent placements, carries a boxed warning related to a gene that controls how the drug is activated in your body. Patients who carry certain variants of this gene convert the drug poorly, meaning it doesn’t work as intended. For these patients, alternative medications are a better choice. The FDA currently lists pharmacogenomic biomarkers on the labels of hundreds of drugs across cardiology, oncology, psychiatry, and other specialties.
Digital Biomarkers From Wearable Devices
The definition of biomarkers is expanding beyond the lab. Wearable devices like fitness trackers and smartwatches continuously collect health data, including heart rate, step count, sleep duration and quality, and time spent in different levels of physical activity. These streams of real-time data are increasingly being studied as digital biomarkers that could flag health changes earlier than traditional clinical visits.
Daily physical activity levels, for instance, can serve as a proxy for overall health status and recovery from illness. Changes in resting heart rate patterns over weeks might signal an emerging infection or cardiovascular issue. The advantage of digital biomarkers is that they capture data continuously in your normal environment rather than during a single snapshot at a clinic, offering a more complete picture of how your body functions day to day.