By definition, genetic testing is carried out by healthcare professionals to look for a known cause of a specific disease in someone who already has the symptoms or signs of the disease. Laboratory studies can determine whether someone has the condition.
Genetic testing is performed when a person has a pattern of physical features that suggests a specific diagnosis. Down syndrome and cleft palate, for examples, are genetic disorders. 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 genetic 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 disorder 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.
Biochemical genetic testing usually is done on blood, urine or other tissue samples. The sample may not have to be sent to a genetics laboratory because many labs do these routine tests.
DNA tests, on the other hand, 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, though other tissues may sometimes be used. The test involves looking at the number and structure of chromosomes under a microscope. Abnormal numbers or shapes of chromosomes are responsible for the disorder.
Biochemical tests may require fasting or special diets prior to collecting the sample needed. DNA tests and chromosome analysis require no special preparation. People being tested, however, must not have had a blood transfusion for several weeks prior to collection of the specimen.
Because biochemical tests are sometimes imprecise, they may need to be repeated. If the first test is definite and the repeat test is positive, the disease is present. DNA and chromosome tests are usually definitive. If an abnormal sequence of coding elements is found, or if the chromosome abnormality can be seen, the disease is present and repeat testing is not necessary.
Kelly TE: Clinical Genetics and Genetic Counseling. Year Book Medical Publishers, 1980.