ing of the bladder can be influenced by various diseases of the prostate. These organs can also be exposed to bacterial and viral infectious processes that modify the normal milieu and act as confounding variables. All the characteristics just enumerated help to explain why the urinary tract—in both laboratory animals and human beings—is a site of xenobiotic-induced cancer.

BIOLOGIC MARKERS

Humans are often and unavoidably exposed to hazardous environmental chemicals. Biologic markers can be useful in confirming such exposures, estimating their magnitude, and identifying persons adversely affected or most likely to be adversely affected if exposures continue. Biologic markers are indicators of events or conditions in biologic systems or samples. It is hoped that they will clarify the relationship between environmental exposures and human diseases and help to prevent such exposures and diseases. Interest in the use of biologic markers to study the health effects of exposure to environmental toxicants is growing among researchers in clinical medicine, epidemiology, toxicology, and related biomedical fields. Specifically, clinicians can use markers for early detection of disease; epidemiologists can use them as indicators of exposure to determine internal dose or health effects; and toxicologists can use them to estimate dose-response relationships and to facilitate assessment of risk associated with exposures. Biologic markers also can be helpful in clarifying the underlying mechanisms of specific diseases.

New developments in molecular genetic and biochemical approaches to medicine have yielded sensitive markers for assessing toxicant exposures (NRC, 1991). They have also increased our knowledge of disease, improved our ability to predict the outcome of disease, and helped to direct courses of treatment. Many diseases are defined not only by clinical signs and symptoms but also by the assessment of biologic markers at the subcellular and molecular levels. Diseases of the liver and kidney, for example, are often detected by measurement of enzymes in blood, proteins, or cells in urine; diabetes can be suspected if glucose is found in urine; and inborn errors of metabolism, such as phenylketonuria, are found early by biochemical analysis, rather than later as a result of clinical dysfunction. Concentrations of material in the urine are often used to determine the degree of exposure in an occupational setting, in monitoring therapeutic regimens, and in forensic medicine.

Biologic markers are indicators of exposure, effect, or susceptibility. That classification is a useful theoretical scheme by which to characterize biologic markers of any organ system; however, it must be qualified somewhat for practical application. A strict classification with respect to any of the three categories (exposure, effect, or susceptibility) is difficult, for the three categories often are related and can be thought of as elements in a continuum



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