A genetic test examines a
DNA (deoxyribonucleic acid) sample of a person's cells
for abnormal
genes, or analyzes the number, arrangement, and
characteristics of the
chromosomes. Testing may be performed on samples of
blood, semen, urine, saliva, stool, body tissues, bone, or hair.
You inherit half of your genetic information from your mother and the
other half from your father. Genes, alone or in combination, determine what
features (genetic traits) a person inherits from his or her parents, such as
blood type, hair color, eye color, and other characteristics, including risks
of developing certain diseases. Certain changes in genes or chromosomes may
cause medical problems.
Why It Is Done
A genetic test is done to:
Determine if people who have a
family history of a specific disease are likely to pass that disease on to
their children (carrier identification). Information
obtained from this type of testing can help guide a couple as they make
decisions regarding pregnancy.
Determine if a
fetus has a disorder, such as
Down syndrome (prenatal testing). Information obtained from this type of testing can help guide
a couple as they make decisions regarding pregnancy.
Check for
various metabolic diseases, such as
phenylketonuria (PKU) (newborn screening). Information obtained from newborn screening can help guide
medical treatment to ensure the best possible outcome for the
baby.
Determine if you carry a genetic change that increases your
risk for developing a disease, such as breast cancer or
Huntington's disease, later in life (late-onset disease testing). This might be important to you if
you have a parent or other relative diagnosed with the disease. Information
obtained from this type of testing can help you make decisions about disease
prevention or future childbearing.
Check for genetic changes that may affect your treatment for
diseases such as HIV or some cancers.
How To Prepare
No special preparation is needed before
having this test.
Talk to your health professional about any
concerns you have regarding the need for the test, its risks, how it will be
done, or what the results will indicate. To help you understand the importance
of this test, fill out the
medical test information form(What is a PDF document?)
.
Since the information obtained from a
genetic test can have a profound impact on your life, you may want to see a
doctor who specializes in genetics (geneticist) or a
genetic counselor. This type of counselor is trained
to help you understand your risk for developing a genetic disease or having a
child with an inherited (genetic) disease, such as
sickle cell disease,
cystic fibrosis, or
hemophilia. A genetic counselor can help you make
well-informed decisions. Ask to have genetic counseling before making a
decision about genetic testing.
How It Is Done
A genetic test can be done using almost
any cell or tissue from the body.
Blood sample from a heel stick
If genetic testing
is done on a baby, a heel stick will be done instead of a blood draw from a
vein. For a heel stick blood sample, several drops of blood are collected from
the heel of the baby. The skin of the heel is cleaned with alcohol and then
pricked with a small, sterile lancet. Several drops of blood are collected
inside circles on a specially prepared piece of paper. When enough blood has
been collected, a gauze pad or cotton ball is placed over the puncture site.
Pressure is applied to the puncture site briefly, and then a small bandage is
usually placed over it.
Blood sample from a vein
The health professional
drawing your blood will:
Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.
Clean the needle site with
alcohol.
Put the needle into the vein. More than one needle stick
may be needed.
Attach a tube to the needle to fill it with
blood.
Remove the band from your arm when enough blood is
collected.
Apply a gauze pad or cotton ball over the needle site as
the needle is removed.
The baby may feel a
brief sting or a pinch when the lancet pricks the skin. While the blood is
being collected, there is very little or no discomfort.
Blood sample from a vein
You may feel nothing at
all from the needle puncture, or you may feel a brief sting or pinch as the
needle goes through the skin. Some people feel a stinging pain while the needle
is in the vein. However, many people do not feel any pain, or have only minor
discomfort, once the needle is positioned in the vein. The amount of pain you
feel depends on the skill of the health professional drawing the blood, the
condition of your veins, and your sensitivity to pain.
The collection of DNA from saliva, urine, or semen
does not cause discomfort.
Risks
Blood sample from a heel stick
There is very
little risk of complications from having blood drawn from a heel stick. A small
bruise may develop at the puncture site.
Blood sample from a vein
There is very little risk
of complications from having blood drawn from a vein.
You may develop a small bruise at the
puncture site. You can reduce the risk of bruising by keeping pressure on the
site for several minutes after the needle is withdrawn.
In rare
cases, the vein may become inflamed after the blood sample is taken. This
condition is called phlebitis and is usually treated with a warm compress
applied several times daily.
Continued bleeding can be a problem
for people with bleeding disorders. Warfarin (Coumadin) and other
blood-thinning medicine can also make bleeding more likely. If you have
bleeding or clotting problems, or if you take blood-thinning medicine, tell
your health professional before your blood is drawn.
There are no risks associated with collecting DNA from
saliva, urine, or semen.
Other factors
The information obtained from a
genetic test can affect your life and the lives of your family in many ways,
including:
Psychological effects. The emotions you
may experience if you learn that you are likely to develop a serious disease or
have an affected child can cause you to feel anxious or depressed. This
information may also affect your relationship with your partner or other family
members.
Genetic counseling is recommended before you have
genetic testing.
Medical treatment choices.
If you test positive for a disease-specific gene, you may decide to use
preventive or treatment options, if they are available, to reduce the impact or
severity of the disease. While many treatment options are proven effective,
others may be potentially dangerous or of unproven
value.
Pregnancy decisions. Finding out that your
unborn child (fetus) is or may be affected by a genetic disease can impact the
decisions you make about the pregnancy. You may want to consider ending the
pregnancy. Or you may need to change your delivery plans. If you had planned on
giving birth at home, you may need to have your baby in a hospital. If your
child is likely to need special care after birth, you may need to delivery in a
hospital other than the one you first chose. You may also need to have special
health professionals present at the birth.
Privacy issues. Because genetic testing is expensive, most people cannot afford
it without help from their insurance companies. Many people worry that genetic
information released to insurance companies may affect future employment
options or the cost or availability of insurance.
Results
A genetic test examines a
DNA (deoxyribonucleic acid) sample of a person's cells
for abnormal genes, or analyzes the number, arrangement, and characteristics of
the
chromosomes.
The results of genetic
testing depend on the type of test done. Genetic testing is used to:
Diagnose an inherited
disease.
Provide information about how likely it is that you will
develop a disease in the future.
Determine whether you are a
carrier of the disease.
Provide information about the severity of
an inherited disease.
What Affects the Test
Having a blood transfusion
within a week before the test can interfere with your test or the accuracy of
the results.
What To Think About
The information obtained from a
genetic test can have a profound impact on your life. Genetic counselors are
trained to help you understand your risk for having a child with an inherited
(genetic) disease, such as sickle cell disease, cystic fibrosis, or hemophilia.
A genetic counselor can help you make well-informed decisions. Ask to have
genetic counseling before making a decision about genetic testing.
A genetic test result is sensitive information.
Your confidentiality should be maintained, and the release of information
should be limited to those who are authorized to receive it.
A
fetal genetic test may detect a serious disease or disorder that will greatly
impact your child's life and the lives of caregivers, such as Down syndrome. A
pregnant woman who is considering genetic testing may want to consider her
ethical, social, and religious beliefs to help her determine the actions she
would take if test results are positive.
A karyotype can be used to
examine the size, shape, and number of chromosomes. Extra, missing, or abnormal
positions of chromosome pieces can cause problems with a person's growth,
development, and body functions. For more information, see the medical test
Karyotype.
A genetic test can sometimes
reveal unintended information, such as the identity of a child's father
(paternity).
A breast cancer (BRCA) gene test is done to evaluate
your risk of developing breast cancer. For more information, see the medical
test
Breast Cancer (BRCA) Gene Test.
The discovery of a genetic disease that is not causing current
symptoms may affect your future ability to gain employment or some types of
private insurance coverage. This may be true for a disease that affects you
late in life, such as breast cancer or Huntington's disease.
A type
of genetic test (DNA fingerprinting) can be used to determine paternity, help
solve crimes, and identify a body. DNA fingerprinting is more accurate than
dental records, blood type, or traditional fingerprints. For more information,
see the medical test
DNA Fingerprinting.
References
Other Works Consulted
Fischbach FT, Dunning MB III, eds. (2004).
Manual of Laboratory and Diagnostic Tests, 7th ed.
Philadelphia: Lippincott Williams and Wilkins.
Skirton H, Patch C (2002). Genetics for Healthcare Professionals. Oxford, UK: BIOS Scientific Publishers
Limited.
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