Through genetic testing, healthcare providerslook for the known cause of a specific disease in someone who already has the symptoms. Laboratory studies can determine whether someone has a genetic disease. Genetic screening, on the other hand, is a more general search for inherited problems, to see if any exist. Genes are inherited building blocks that determine a person's physical traits such as height or eye color. A defect in a gene can cause inherited diseases.

Genetic testing may look at a chemical in blood or urine that is known to be associated with a disease. Genetic testing may involve DNA testing. If a DNA defect is known to cause a disease, healthcare providers can look for it to make a diagnosis. Genetic testing also may involve chromosome analysis. Chromosomes contain many genes.

Who is a candidate for the test?

Genetic testing is performed when a person has a pattern of physical features that suggests a specific diagnosis. Down syndrome or a cleft palate, for examples, are genetic defects. They can be observed and then confirmed through genetic testing. The decision about which genetic test to do is determined by the features observed.

The test itself is based on what is known about the defect that causes the disease. If a disease caused by a biochemical marker is suspected, the doctor will order a biochemical test. If the basic defect is known, the doctor will order a DNA test or chromosome analysis. Relatives of people with genetic disease may be tested to search for inherited patterns.

How is the test performed?

Biochemical genetic testing usually is done on blood or urine. The sample may not have to be sent to a genetics laboratory because many labs do these routine tests. DNA tests usually are done on blood or other tissue in specialized laboratories throughout the country. The test itself determines the sequence of coding elements along the DNA strand. Errors in the code lead to defects that cause disease. Chromosome analysis usually is done on blood or bone marrow. Other tissues may be used. The test involves looking at the number and structure of chromosomes under a microscope. Abnormal numbers or shapes of chromosomes cause disease.