Genetic Testing - The Quality And Utility Of Genetic Tests, Pregnancy, Childbirth, Andgenetic Testing, Genetic Diagnosis In Children Andadults

Diagnosis means finding the cause of a disorder, not just giving it a name.

—Sydney Walker III

Over the course of the last decade, the definitions of health and disease have been transformed by advances in genetics. Genetic testing has enabled researchers and clinicians to detect inherited traits, diagnose heritable conditions, determine and quantify the likelihood that a heritable disease will develop, and identify genetic susceptibility to familial disorders. Many of the strides made in genetic diagnostics are direct results of the Human Genome Project, an international thirteen-year effort begun in 1990 by the Department of Energy and the National Institutes of Health, which mapped and sequenced the human genome in its entirety. The increasing availability of genetic testing has been one of the most immediate applications of this groundbreaking research.

A genetic test is the analysis of human deoxyribonucleic acid (DNA), ribonucleic acid (RNA), chromosomes, and proteins in order to detect heritable disease-related genotypes, mutations, phenotypes, or karyotypes (standard pictures of the chromosomes in a cell) for the purposes of diagnosis, treatment, and other clinical decision making. Most genetic testing is performed by drawing a blood sample and extracting DNA from white blood cells. Genetic tests may detect mutations at the chromosomal level, such as additional, absent, or rearranged chromosomal material, or even subtler abnormalities such as a substitution in one of the bases that make up the DNA. There is a broad range of techniques that can be used for genetic testing. Genetic tests have diverse purposes, including screening for and diagnosis of genetic disease in newborns, children, and adults; the identification of future health risks; the prediction of drug responses; and the assessment of risks to future children.

There is a difference between genetic tests performed to screen for disease and testing conducted to establish a diagnosis. Diagnostic tests are intended to definitively determine whether a patient has a particular problem. They are generally complex tests and commonly require sophisticated analysis and interpretation. They may be expensive and are generally performed only on people believed to be "at risk," such as patients who already have symptoms of a specific disease.

In contrast, screening is performed on healthy, asymptomatic (showing no symptoms of disease) people and often to the entire relevant population. A good screening test is relatively inexpensive, easy to use and interpret, and helps identify which individuals in the population are at higher risk of developing a specific disease. By definition, screening tests identify people who need further testing or those who should take special preventive measures or precautions. For example, people who are found to be especially susceptible to genetic conditions with specific environmental triggers are advised to avoid the environmental factors linked to developing the disease. Examples of genetic tests used to screen relevant populations include those that screen people of Ashkenazi Jewish heritage (the east European Jewish population primarily from Germany, Poland, and Russia, as opposed to the Sephardic Jewish population primarily from Spain, parts of France, Italy, and North Africa) for Tay-Sachs disease, African-Americans for sickle-cell disease, and the fetuses of expectant mothers over age thirty-five for Down's syndrome. Table 6.1 lists a few of the more than 900 DNA-based genetic tests that were available as of 2004.