Genetic testing usually means the direct examination of the DNA in a gene associated with a particular disorder. (It can sometimes mean RNA or the protein product of the DNA and RNA.)

The examination is usually made in order to enhance the understanding of symptoms (for example, to confirm a diagnosis of a muscular dystrophy), or to predict the occurrence of a genetic disorder in which symptoms haven't yet appeared.

What kind of sample is needed for genetic testing?

Usually, only a blood sample is needed, but occasionally other tissues are used for the testing.

Why should I get a genetic test?

Just a few years ago, genetic testing was mainly undertaken when people were thinking about starting or adding to a family and wanted more precise information about inheritance risks than they already had; or to confirm a physician's clinical diagnosis and add to information about the probable course and severity of a disease.

However, as of 2007, there are many experimental treatments for neuromuscular diseases in development that require precise knowledge of a person's genetic mutation.

For example, there are compounds in development that are designed to make cells ignore a type of mutation known as a "premature stop codon," which arrests the synthesis of a protein before the genetic instructions are completely processed. And there are others that are targeted to block a cell's ability to "read" specific, error-containing instructions.

When genetic testing was thought to be largely predictive and to have no therapeutic value, many medical centers discouraged testing of children before the appearance of symptoms, for fear of stigmatizing them or jeopardizing insurance coverage.

However, those ideas are being reexamined in light of new knowledge. It's likely that newborns will be screened for an increasing number of genetic disorders in the coming years, as treatments become available, and that the barriers to DNA testing of children will be lowered. Treatments for genetic disorders will almost certainly be more successful if they're started early in life.

Where can I get a genetic test?

You can ask about genetic testing at your MDA clinic. In some cases, the testing can be done at the same institution where the clinic is held. But in most cases, the blood sample will be sent to an outside laboratory.

Athena Diagnostics, for example, is a large commercial laboratory in Worcester, Mass., that can perform DNA testing for a number of MDA's diseases. You also can check GeneTests for a laboratory near you.

How much does genetic testing cost?

If you're part of a study, there's usually no cost for DNA testing. In some cases, an MDA clinic may have set aside funds specifically for genetic testing. And in some instances, insurance will cover the test. If, however, you have to pay for genetic testing, costs range from about $250 to almost $4,000, depending on the specific test and the laboratory.

What's the downside of genetic testing?

In the United States, fears about being discriminated against when applying for health insurance are not without basis. Even fears about discrimination by employers, although less likely, are not out of the question, although privacy regulations generally prohibit this kind of information from being accessed by most potential employers.

Prenatal testing almost always causes anguish for parents if the results show the baby has a disease-causing mutation. Sometimes the test results can't tell the family how seriously the child is likely to be affected, making the choice of whether or not to continue the pregnancy even more difficult. Some parents would rather just take their chances than have to face this type of decision.

Testing that reveals a young child's genetic destiny may affect relationships within the family or may cause parents more sadness than they would have experienced had events simply unfolded, especially if no treatment for the disease in question is available. A positive genetic test that reveals a child is a carrier of a disease may be unsettling, especially if the child is young and the implications for his or her future children are uncertain.

In addition, the results of one person's tests have implications for the genetic status of relatives, whether or not they have volunteered to be tested themselves. For example, if an autosomal recessive disease is diagnosed by genetic testing, it usually means that both the patient's parents are carriers and that the patient's siblings are also at risk.

Last but not least, the results of a DNA test are not always easy to interpret. Some types of tests only reveal the presence or absence of a disease-causing mutation in a certain percentage of cases, leaving the person who gets a negative result (no mutation found) uncertain about the future. In other cases, mutations may be found whose significance is uncertain. Or a mutation may be a known risk factor for a disease, but the degree of risk may be hard to estimate. On occasion, actual errors are made in the testing process.

For more, see the following Quest articles:

• Love Letters and Preimplantation Genetic Diagnosis (2011)
• The Pain and Promise of Prenatal and Newborn Genetic Diagnosis (2007)

Can someone give me guidance?

The best way to make the right decision about genetic testing and make sure you get the right test and understand the results is to work with a certified genetic counselor. Genetic counselors have graduate degrees that reflect extensive knowledge of both biology and counseling principles. They can guide you through the testing process. Almost all major medical centers now have genetic counselors on staff, and your MDA clinic physician can refer you to one.