The test involves looking at the number and structure of chromosomes under a microscope. Abnormal numbers or shapes of chromosomes cause disease. 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.
Biochemical tests are sometimes imprecise.
Often, such tests are repeated. If the first test is definite and the repeat is positive, the disease is present. DNA tests are definite. If an abnormal sequence of coding elements is found, the disease is present. Repeat testing is not necessary.