You have probably heard the term, “biomarker”, in stories about different diseases and their treatment. There are many different definitions of a biomarker, but they are generally similar. The United States National Institutes of Health Biomarkers Definitions Working Group defined a biomarker as a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention. The International Programme on Chemical Safety that included the World Health Organization and the United Nations and the International Labor Organization, defined a biomarker as any substance, structure, or process that can be measured in the body or its products and influence or predict the incidence of outcome or disease. It should be apparent that biomarkers can include a very wide range of parameters that can be measure in patients and/or individuals suspected of having a disease. Pulse rate, blood pressure, serum cholesterol, HIV RNA, brain lesions seen with magnetic resonance imaging (e.g., in patients with multiple sclerosis) are all biomarkers.
So why are biomarkers useful in medicine? Let‘s take the case of blood pressure. It can be easily and reliably measured and if it is chronically elevated, it indicates that the individual is at increased risk for a number of important adverse outcomes, such as stroke, myocardial infarction, and kidney disease. The higher the blood pressure, the greater the risk for these adverse outcomes. It is also known that lowering blood pressure with medication decreases the risk for these outcomes. Thus, if the physician is treating a patient, he or she can measure blood pressure and if it is reduced by treatment, the physician will know that the risks for heart attack, stroke, and kidney disease are also being decreased. Thus, measuring blood pressure is useful for establishing a diagnosis, establishing a prognosis for the patient, and assessing the response to treatment.
Some biomarkers, like blood pressure, are clearly involved in the disease process and treatment aimed at them changes clinical outcomes. Biomarkers that become treatment targets are referred to as surrogate endpoints (endpoints that substitute for the clinical outcome). Not all biomarkers achieve this status. For example, both low numbers of a specific type of white blood cell (CD4+ cells) and HIV RNA are both biomarkers for HIV disease. However, therapy aimed strictly at lowering HIV RNA prolongs survival in patients with this disease while that aimed strictly at raising the CD4+ cell count without lowering HIV RNA does not.
In summary, biomarkers and surrogate endpoints are very important in medicine for several reasons:
- – They may permit early diagnosis and intervention prior to the occurrence of potentially serious clinical events.
- – They can help establish the patient‘s prognosis and guide the intensity of treatment.
- – They can provide early information about the efficacy of treatment and tell the physician whether to continue with it or augment/switch therapy.
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