Genetic Testing

Genetic Testing

If you have symptoms of a disease that may be caused by genetic changes, sometimes called mutated genes, genetic testing can reveal if you have the suspected disorder. For example, genetic testing may be used to confirm a diagnosis of cystic fibrosis or Huntington's disease. Presymptomatic and predictive testing. Although genetic testing can provide important information for diagnosing, treating and preventing illness, there are limitations. For example, if you're a healthy person, a positive result from genetic testing doesn't always mean you will develop a disease. On the other hand, in some situations, a negative result doesn't guarantee that you won't have a certain disorder.The variety of genetic tests has expanded throughout the years. Early forms of genetic testing which began in the 1950s involved counting the number of chromosomes per cell. Deviations from the expected number of chromosomes (46 in humans) could lead to a diagnosis of certain genetic conditions such as trisomy 21 (Down syndrome) or monosomy X (Turner syndrome). In the 1970s, a method to stain specific regions of chromosomes, called chromosome banding, was developed that allowed more detailed analysis of chromosome structure and diagnosis of genetic disorders that involved large structural rearrangements.In addition to analyzing whole chromosomes (cytogenetics), genetic testing has expanded to include the fields of molecular genetics and genomics which can identify changes at the level of individual genes, parts of genes, or even single nucleotide "letters" of DNA sequence. According to the National Institutes of Health, there are tests available for more than 2,000 genetic conditions,  and one study estimated that as of 2017 there were more than 75,000 genetic tests on the market.

Why it's done

• Diagnostic testing.If you have symptoms of a disease that may be caused by genetic changes, sometimes called mutated genes, genetic testing can reveal if you have the suspected disorder. For example, genetic testing may be used to confirm a diagnosis of cystic fibrosis or Huntington's disease.
• Presymptomatic and predictive testing. If you have a family history of a genetic condition, getting genetic testing before you have symptoms may show if you're at risk of developing that condition. For example, this type of test may be useful for identifying your risk of certain types of colorectal cancer.
• Carrier testing.If you have a family history of a genetic disorder — such as sickle cell anemia or cystic fibrosis — or you're in an ethnic group that has a high risk of a specific genetic disorder, you may choose to have genetic testing before having children. An expanded carrier screening test can detect genes associated with a wide variety of genetic diseases and mutations and can identify if you and your partner are carriers for the same conditions.
• If you have a particular health condition or disease, this type of genetic testing may help determine what medication and dosage will be most effective and beneficial for you.
• Prenatal testing.If you're pregnant, tests can detect some types of abnormalities in your baby's genes. Down syndrome and trisomy 18 syndrome are two genetic disorders that are often screened for as part of prenatal genetic testing. Traditionally this is done looking at markers in blood or by invasive testing such as amniocentesis. Newer testing called cell-free DNA testing looks at a baby's DNA via a blood test done on the mother.
• Newborn screening.This is the most common type of genetic testing. In the United States, all states require that newborns be tested for certain genetic and metabolic abnormalities that cause specific conditions. This type of genetic testing is important because if results show there's a disorder such as congenital hypothyroidism, sickle cell disease or phenylketonuria (PKU), care and treatment can begin right away.
• Preimplantation testing.Also called preimplantation genetic diagnosis, this test may be used when you attempt to conceive a child through in vitro fertilization. The embryos are screened for genetic abnormalities. Embryos without abnormalities are implanted in the uterus in hopes of achieving pregnancy.

What a genetic test can tell me

A genetic test can:

• help to diagnose a rare health condition in a child
• help you understand whether an inherited health condition may affect you, your child or another family member, and help you decide whether to have children
• show if you are at higher risk of getting certain health conditions, including some types of cancer
• guide doctors in deciding what medicine or treatment to give you
• guide doctors on whether you're able to join a clinical trial

Genetic counselling

If you’re offered a genetic test, you may be referred to a genetic counsellor to help you think through what the test means for you and your family.

A genetic counsellor can help you understand:

• the risks and benefits of you having a genetic test
• the potential results of your test and what they mean
• how your family members may be affected if the test shows a serious health condition runs in your family
• the risk of you and your partner passing on a health condition to your children
• your options if you have a child with an inherited health condition and you do not want your next child to inherit it

How you prepare

Before you have genetic testing, gather as much information as you can about your family's medical history. Then, talk with your doctor or a genetic counselor about your personal and family medical history to better understand your risk. Ask questions and discuss any concerns about genetic testing at that meeting. Also, talk about your options, depending on the test results.

If you're being tested for a genetic disorder that runs in families, you may want to consider discussing your decision to have genetic testing with your family. Having these conversations before testing can give you a sense of how your family might respond to your test results and how it may affect them.

Not all health insurance policies pay for genetic testing. So, before you have a genetic test, check with your insurance provider to see what will be covered.

In the United States, the federal Genetic Information Nondiscrimination Act of 2008 (GINA) helps prevent health insurers or employers from discriminating against you based on test results. Under GINA, employment discrimination based on genetic risk also is illegal. However, this act does not cover life, long-term care or disability insurance. Most states offer additional protection.

Medical procedure

Genetic testing is often done as part of a genetic consultation and as of mid-2008 there were more than 1,200 clinically applicable genetic tests available. Once a person decides to proceed with genetic testing, a medical geneticist, genetic counselor, primary care doctor, or specialist can order the test after obtaining informed consent.

Genetic tests are performed on a sample of blood, hair, skin, amniotic fluid (the fluid that surrounds a fetus during pregnancy), or other tissue. For example, a medical procedure called a buccal smear uses a small brush or cotton swab to collect a sample of cells from the inside surface of the cheek. Alternatively, a small amount of saline mouthwash may be swished in the mouth to collect the cells. The sample is sent to a laboratory where technicians look for specific changes in chromosomes, DNA, or proteins, depending on the suspected disorders, often using DNA sequencing. The laboratory reports the test results in writing to a person's doctor or genetic counselor.

Routine newborn screening tests are done on a small blood sample obtained by pricking the baby's heel with a lancet.

What you can expect

Depending on the type of test, a sample of your blood, skin, amniotic fluid or other tissue will be collected and sent to a lab for analysis.

• Blood sample.A member of your health care team takes the sample by inserting a needle into a vein in your arm. For newborn screening tests, a blood sample is taken by pricking your baby's heel.
• Cheek swab.For some tests, a swab sample from the inside of your cheek is collected for genetic testing.
• In this prenatal genetic test, your doctor inserts a thin, hollow needle through your abdominal wall and into your uterus to collect a small amount of amniotic fluid for testing.
• Chorionic villus sampling.For this prenatal genetic test, your doctor takes a tissue sample from the placenta. Depending on your situation, the sample may be taken with a tube (catheter) through your cervix or through your abdominal wall and uterus using a thin needle.