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What Are Biomarkers?
Biomarkers are useful scientific tools for evaluating an exposure to a foreign substance, and they can be measured in the body of an organism (animal or human) or in the environment. For example, a biomarker of inflammation in the human body is the presence of C-reactive protein. This protein is released in response to stress and it can be measured in laboratory blood tests. Environmental biomarkers can tell us that a foreign substance has entered an ecosystem. For example, the presence of reproductive changes in reptiles may serve as a biomarker to indicate an environmental contaminant. Also, unexplained changes in the eggs and offspring of birds, turtles, frogs, and fish might also suggest that something toxic has been released into the environment. Because animals and ecosystems respond differently to various exposures—just like humans—one must carefully evaluate each perceived change and apply the proper studies to assess these differences and drawn conclusions.
The most reliable biomarkers are very specific and highly sensitive indicators of exposure. For example, beta-human chorionic gonadotropin (β-HCG) is a hormone released from the placenta that surrounds a developing fetus and this hormone is routinely measured in pregnant women. In fact, this hormone is the substance detected by retail pregnancy tests. An indicator in the test changes color when the test is positive. The more advanced the pregnancy is, the more hormone is expressed, and the greater the color change in the test kit. β-HCG is not present in detectable levels in non-pregnant individuals, so β-HCG can be said to be a biomarker of pregnancy. However, β-HCG is secreted by certain tumors in men and women, so a finding of detectable β-HCG may not always indicate pregnancy. Thus, β-HCG is not an absolute biomarker: it is sensitive, but it may not be totally specific for a particular condition. Ideally, a biomarker is only present when a certain exposure occurs, and it is not present—or it should be below a detection limit—prior to or in the absence of the exposure. Thus, the presence of β-HCG should be weighed against supporting data to properly evaluate its meaning. For example, the presence of β-HCG in a male would likely not indicate a pregnancy. Instead, it might indicate a tumor that originated in the reproductive system. The finding of β-HCG in a female might indicate pregnancy if other conditions are met, such as the ability to bear children. Women who have undergone a radical hysterectomy (removal of all pelvic reproductive organs) would not have a childbearing capacity, so the presence of the biomarker would be suspect.
Thus, when evaluating possible exposures to foreign substances, a detectable biomarker by itself may be sufficient to indicate a positive exposure, but this information should also be considered in the context of additional information and given appropriate attention.
Why Are Toxicologists Interested in Biomarkers?
Although biomarkers can indicate events or changes that are not harmful, toxicologists use biomarkers chiefly to determine if an unwanted exposure occurred and how great that exposure might have been. Also, toxicologists are continually evaluating animals, humans, and our environment to look for more and better biomarkers that represent the earliest signs of exposure. That way, toxicologists can rapidly employ the best efforts to reduce or eliminate the exposure and protect vulnerable populations.
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