Interpreting the results

Deciding how to use information from genetic carrier screening tests may be challenging. It's important to talk with someone experienced in genetics and disease before and after testing.

If test results indicate you're not a carrier, no special precautions with pregnancy are necessary, unless they're related to a separate condition. However, the test results shouldn't be taken as a guarantee that your child will be healthy. Although genetic carrier screening is accurate most of the time, it doesn't identify all carriers. Some diseases, such as cystic fibrosis, have many mutations. The test may focus only on the most common ones. Even if the test says you're not a carrier, it's possible that you do, in fact, carry a little-known or unknown mutation of the disease.

When discussing the genetic carrier screening test results, your genetic counselor or health care provider may review your probability of being a carrier. For example, whites have a four or five in 100 (4 percent to 5 percent) chance of carrying a cystic fibrosis gene alteration. If the cystic fibrosis carrier test is negative, that chance is reduced to one in 240 (0.4 percent) for those of Northern European descent.

The test may tell you that you're a carrier of a specific altered gene, but it may not reveal how severe the disease may be if it occurs in one of your

Explaining the conditions

Alpha-thalassemias result in a deficiency of red blood cells (anemia). The most severe form results in fetal or newborn death. Most cases are much less severe.

Beta-thalassemias also result in anemia. In its most severe form (thalassemia major), children require regular blood transfusions. With proper treatment, most people with this condition live into adulthood. Less severe forms cause varying degrees of complications related to the need for more red blood cells.

Canavan disease is a severe condition of the nervous system that's usually diagnosed soon after a child's birth. Death usually occurs in early childhood.

Cystic fibrosis affects the respiratory and digestive systems, causing severe chronic respiratory disease, diarrhea, malnutrition and exercise limitation. Recent treatments enable most affected people to live into adulthood.

Duchenne's muscular dystrophy (MD) affects the muscles of the pelvis, upper arms and upper legs. It occurs in young boys and is the most common form of MD that affects children. It can lead to muscle weakness and, in severe cases, death.

Fragile X syndrome is the most common genetically inherited cause of mental retardation, caused by alterations on the X chromosome.

Sickle cell disease prevents blood cells from moving smoothly through the body. Affected infants have an increased susceptibility to infection and slow growth rates. The disease can cause bouts of severe pain and damage to vital organs. With early and consistent medical treatment, complications can be minimized.

Tay-Sachs disease is a condition in which the enzyme needed to break down certain fats (lipids) is absent. These substances build up and gradually destroy brain and nerve cells until the central nervous system stops working. Death usually occurs in early childhood.

For in-depth information on these and other genetic diseases, see the National Center for Biotechnology Information web site at www.ncbi.nih.govdmim.

children. If an increased risk of a specific disease is indicated, you may wish to talk with your health care provider, a geneticist or a genetic counselor about the implications of the disease.

If the test indicates that you're a carrier, your genetic counselor will help you assess the options available to you. These may include:

• Foregoing conception

• Adopting a child

• Using a donor egg or donor sperm

• Using pre-implantation genetic diagnosis (see page 295)

• Using diagnostic tests such as amniocentesis (see page 299) or chorionic villus sampling (see page 304) during pregnancy to determine if your baby has the disease

If you're pregnant, a genetic counselor can help you consider the next steps in your pregnancy. The genetic counselor's goal is to help you make decisions consistent with your own values, so don't be afraid to ask questions or to request that information be repeated.

If you and your partner plan a pregnancy in the future and have reasons to be concerned about genetic disorders, you may want to talk with a genetic counselor now to see if any new tests or treatment options are available now or will be soon. Remember that genetic screening information pertains to a unique couple. If, in the future, you plan a pregnancy with a different partner, he or she may wish to be screened as well.

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