What Are Biomarkers?

Biomarkers are measurable biological indicators used to assess normal physiological processes, pathological conditions, or pharmacological responses to a therapeutic intervention. The term encompasses a wide range of molecular, cellular, imaging, and physiological signals, from the concentration of a protein in blood plasma to a pattern detected on a medical scan. According to the definition established by the NIH Biomarkers Definitions Working Group, a biomarker is "a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacological responses to a therapeutic intervention". Biomarkers are central to drug development, disease diagnosis, patient stratification, and monitoring of treatment response in clinical medicine.

The disciplinary roots of biomarker science lie in clinical chemistry, molecular biology, and epidemiology, but the field has expanded significantly with the growth of high-throughput omics technologies, digital imaging, and machine learning, which make it possible to detect subtle biological signals in large patient populations.

Types and Classification

Biomarkers are classified by their intended use in clinical or research settings. Diagnostic biomarkers indicate the presence or absence of a disease; the concentration of cardiac troponin in blood, for example, is a diagnostic biomarker for myocardial infarction. Prognostic biomarkers predict the likely course of a disease independent of treatment, such as tumor stage or certain genetic mutations in cancer. Predictive biomarkers identify patients who are likely to respond to a specific therapy; HER2 expression in breast cancer tissue, which determines eligibility for trastuzumab treatment, is a well-established predictive biomarker. Pharmacodynamic biomarkers track biological changes that result from drug exposure, providing evidence that a compound is engaging its intended molecular target. The FDA-NIH Biomarker Working Group's BEST resource provides authoritative definitions for each biomarker type and describes the statistical and regulatory standards applied during clinical development.

Discovery and Validation

Biomarker discovery typically begins with an omics-scale screen: genomic, proteomic, metabolomic, or transcriptomic data from patient cohorts are analyzed to identify candidate molecules or signals associated with a disease state or treatment outcome. The analytical challenge is substantial, since high-dimensional biological datasets contain thousands of candidate variables, and statistical associations can arise by chance when the number of features greatly exceeds the number of samples. Candidate biomarkers identified in discovery studies must therefore pass through a structured validation process that includes technical validation of the assay, confirmation of the association in independent cohorts, and demonstration of clinical utility, meaning that using the biomarker actually improves patient outcomes. Research published through PMC on biomarker definitions and applications describes this staged development process and the standards that distinguish exploratory candidates from analytically validated clinical biomarkers. Regulatory agencies including the FDA and EMA have published guidance documents that specify the validation evidence required before a biomarker can be used in a clinical trial or a diagnostic test.

Measurement Technologies

Enzyme-linked immunosorbent assays (ELISA) remain widely used for quantifying protein biomarkers in blood and urine, offering high sensitivity and compatibility with clinical laboratory workflows. Mass spectrometry provides higher specificity by measuring the mass-to-charge ratios of ionized molecules, enabling the simultaneous quantification of many proteins or metabolites in a single sample. Liquid biopsy techniques extract circulating tumor DNA, cell-free RNA, or extracellular vesicles from blood and analyze them for cancer-associated alterations without invasive tissue biopsy. Imaging biomarkers, including measures derived from MRI, PET, and ultrasound, characterize tissue structure or function in ways not accessible through blood-based assays. Wearable sensors extend biomarker monitoring into continuous, ambulatory settings, capturing physiological signals over days or weeks.

Applications

Biomarkers have applications in a range of fields, including:

• Oncology for early cancer detection and treatment response monitoring
• Neurology for diagnosis and staging of Alzheimer's disease and other dementias
• Cardiovascular medicine for risk stratification and acute event detection
• Drug development for target engagement verification in clinical trials
• Infectious disease for pathogen load quantification and treatment monitoring