How Accurate is Genetic Testing?

Posted by Caitlin O’Brien

By Caitlin O’Brien, Genome Medical Genetic Counseling Intern

Did you know that testing for genetic diseases has been around since as early as the 1950’s? (1) It has been quite an evolution since then. In the early years, testing rarely actually looked at your genes (more often tests were done to look for substances that accumulated in the body due to a genetic disease). Fast-forward 69 years and we can now look at the entire genome (all 20,000 genes) in just a few days or few weeks, for a few hundred to a few thousand dollars. You might think this is not such a big accomplishment—after all, 69 years is a long time—but it was only 16 years ago that scientists completed sequencing the first human genome, after spending 13 years and 2.7 billion dollars. Since then, genetic testing has exploded, with over 12 million tests being sold as of 2018.

The decreased cost has led to increased access. Genetic testing is now used more often in traditional medical settings as well as in more recreational ones. Testing kits are now being marketed directly to the consumer, available online and even in drugstores. Odds are, you or someone you know has bought a genetic testing kit from a company like 23andMe or Ancestry.com.

This boom in genetic testing has been attributed to Americans’ interest in genealogy. Several tests advertise that they can tell you exactly where in the world your family came from and even connect you with relatives you never knew you had. But genetic heritage was really just the beginning of this market. From there, companies began offering tests covering everything from superficial traits (are you more likely to sneeze after smelling dark chocolate? Which wine is best for your palate?) to health issues like how likely you are to develop diabetes or which type of diet might work best for you. There are even dating sites offering to find your perfect genetic match. Testing for hereditary cancer risk (which has been available through healthcare providers for the past 20 years) has also recently entered the consumer market.

So genetic testing has the potential to reveal information that could have a major impact on you, your health, lifestyle or family relationships. With so many tests out there, the question arises: Just how accurate are genetic tests?

Give it to Me Straight. Are Genetic Tests Accurate?

Unfortunately, there is no simple answer to this question. All genetic tests are not created equal. There are two key parts to any genetic test and a lab can be accurate in both, one or neither. The first part is detection:, Can the lab actually detect a change in a specific gene? The second is interpretation: If a lab finds a change, will it be interpreted correctly?

Let’s talk a little more about detection. This is complicated because not all tests are designed to find every genetic change. Your genes are something like a book. If you were asked to find typos in a book but only told to look on pages 1, 10, and 27, you would miss some that appear elsewhere. That doesn’t mean you didn’t do a good job looking. You may have found every change on each of those pages and had 100% accuracy—but if the other pages have typos, you would not have detected them.

Another issue is the problem of false positives. One study, published in the journal Genetics in Medicine, found that 40% of variants included in the raw data of one direct-to-consumer genetic testing company were false positives. You may be wondering how this can be possible. There are several explanations.

The first is that no genetic test is perfect. There is always the potential for lab error. Samples get mixed up or contaminated and mistakes happen. This may occur more often when testing companies use labs that aren’t held to high standards. For example, when you get genetic testing through a healthcare provider, your provider will use labs that go through a rigorous and thorough certification process to ensure the accuracy and reliability of test results. While there are some direct-to-consumer tests that use labs like this, there are many which do not. This leaves a lot of room for error and a lack of oversight to prevent it.

What’s more, a lot of genetic results can be interpreted in different ways . Different labs use different protocols for interpreting such test results. So while a test itself may have correctly identified a variant in a particular gene, the way that a particular lab interprets it may be incorrect. For example, one lab may say a certain variant likely causes an increased risk for prostate cancer, while another may lab say the same variant likely doesn’t raise this risk. It all depends on what data the lab is using to support its interpretation.

Many of the common variants we know of today for diseases like Duchenne muscular dystrophy were discovered by doing genome-wide association studies (GWAS). Researchers gathered patients who had the same characteristics and sequenced all their genes to look for sequences that are more common among the selected group than in the general population. In the case of Duchenne muscular dystrophy, they saw “hot spots” in one gene on the X chromosome and were able to associate the disease with this gene, which is now called DMD.

It becomes more complicated when we think about complex traits like which diet will work best for you or what flavors you will and won’t like. These traits are not determined by single genes, but rather the interaction of many genes. Different tests may be looking at different combinations of these hot spots and therefore give entirely different results. What’s more, many of these complex traits are also influenced by environmental factors and lifestyle choices.

Some tests may be accurate in that they can correctly identify a variant if it is present, but they may still be incomplete. Many direct-to-consumer genetic tests do not analyze every gene, or even assess every gene associated with a certain health condition. For example, you may have seen the option to receive results for BRCA1 and BRCA2 from 23andMe. These two genes are associated with an increased risk for breast and ovarian cancer. However, 23andMe looks at only three “hot spots” in these genes commonly found in people of Ashkenazi Jewish ancestry, although there are many more places in the gene that can be affected. While this is clearly explained in the report, it may easily be overlooked. This test can accurately tell you if you do or do not have one of the three variants tested for, but may be missing a variant somewhere else in the gene.

All this means that the accuracy of genetic testing is hard to pin down. So how can you be sure you’re getting the information you want, and that it’s even right? The best thing you can do is work with a certified genetic counselor. These healthcare providers are trained to gather your personal health history as well as your family history and determine what testing is appropriate for you based on your goals. They can then work with other qualified healthcare providers to interpret your genetic results and incorporate them into your medical management, if necessary.

If you’ve already had genetic testing through a direct-to-consumer test and want a clearer understanding of what the results, a genetic counselor can help. This type of information can be complicated and confusing. Luckily, many genetic counselors are willing and able to look through these reports with you, help you interpret them and recommend any other testing that may be necessary to meet your needs.

What is the “Most Accurate” Genetic Test?

Unfortunately, there is no one-size-fits-all genetic test—at least not yet. The best test for you depends on the type of information you’re looking for. If you want to learn about your ancestry or how likely you are to have perfect pitch, then a direct-to-consumer test like one from 23andMe or AncestryDNA may be right for you. However, if you are worried you have an increased for cancer based on your family history, these tests may not be as comprehensive as you’re looking for. Picking the right test can be tricky, especially if you’re looking for health information. That is where a genetic counselor comes in. Combining your wants and needs and the counselor’s expertise, you can determine the best test for you and the best way to go about getting it.

If you’d like to speak to a genetic counselor, click here. If you have questions about the services Genome Medical provides click here.

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Resources

1. https://report.nih.gov/nihfactsheets/ViewFactSheet.aspx?csid=43&key=G

2. https://www.technologyreview.com/s/610233/2017-was-the-year-consumer-dna-testing-blew-up/

3. Tandy-Connor, S, Guiltinan, J., Krempely, K., LaDuca, H., Reineke, P., Gutierrez, S., Gray, P., & Tippin Davis, B. (2018). False-positive results released by direct-to-consumer genetic tests highlight the importance of clinical confirmation testing for appropriate patient care. Genetics in Medicine, 20, 1515-1521.

Topics: Blog

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