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Genetic Test

Test Overview

A genetic test examines the DNA (deoxyribonucleic acid) of a person's cells. Genetic testing can identify abnormal genes, or analyze 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 colour, eye colour, 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 obtainedgenetic test 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 doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. 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 counsellor. This type of counsellor 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 counsellor can help you make well-informed decisions. Ask to have genetic counselling 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.
  • Apply pressure to the site and then a bandage.

Cell sample from a fetus

For this type of testing, cells are collected from the fetus using amniocentesis or chorionic villus sampling. For more information about amniocentesis, see the topic Amniocentesis or Chorionic Villus Sampling.

How It Feels

Blood sample from a heel stick

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. But 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. ASA, 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 linked 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 counselling is recommended before you have genetic testing.
  • Medical treatment choices. If you test positive for a disease-specific gene change (mutation), 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 deliver 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. A law in Canada called the Privacy Act protects people who have DNA differences that may affect their health. It prevents employers and health insurance companies from using DNA information about people to affect decisions.

Results

A genetic test examines the DNA (deoxyribonucleic acid) of a person's cells. Genetic testing can identify abnormal genes, or analyze 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.
  • Help choose appropriate treatment for some diseases, such as HIV infection or some types of cancer.

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 counsellors 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 counsellor can help you make well-informed decisions. Ask to have genetic counselling 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.
  • The discovery of a genetic disease that is not causing symptoms now (such as breast cancer or Huntington's disease) should not affect your future ability to gain employment or health insurance coverage. A law in Canada called the Privacy Act protects people who have DNA differences that may affect their health.
  • A fetal genetic test may detect a serious disease or disorder (such as Down syndrome) that will greatly impact your child's life and the lives of caregivers. 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 topic Karyotype Test.
  • 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 topic Breast Cancer (BRCA) Gene Test.
  • 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 topic DNA Fingerprinting.

References

Other Works Consulted

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Skirton H, Patch C (2002). Genetics for Healthcare Professionals. Oxford, UK: BIOS Scientific Publishers Limited.

Credits

By Healthwise Staff
Primary Medical Reviewer Sarah Marshall, MD - Family Medicine
Primary Medical Reviewer Andrew Swan, MD, CCFP, FCFP - Family Medicine
Specialist Medical Reviewer Siobhan M. Dolan, MD, MPH - Reproductive Genetics
Last Revised June 6, 2011

Last Revised: June 6, 2011

Author: Healthwise Staff

Medical Review: Sarah Marshall, MD - Family Medicine & Andrew Swan, MD, CCFP, FCFP - Family Medicine & Siobhan M. Dolan, MD, MPH - Reproductive Genetics

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