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What do the results of genetic tests mean?

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The results of genetic tests are not always straightforward, which often makes them challenging to interpret and explain. Therefore, it is important for patients and their families to ask questions about the potential meaning of genetic test results both before and after the test is performed. When interpreting test results, healthcare professionals consider a person’s medical history, family history, and the type of genetic test that was done.

A positive test result means that the laboratory found a change in a particular gene, chromosome, or protein of interest. Depending on the purpose of the test, this result may confirm a diagnosis, indicate that a person is a carrier of a particular genetic mutation, identify an increased risk of developing a disease (such as cancer) in the future, or suggest a need for further testing. Because family members have some genetic material in common, a positive test result may also have implications for certain blood relatives of the person undergoing testing. It is important to note that a positive result of a predictive or presymptomatic genetic test usually cannot establish the exact risk of developing a disorder. Also, health professionals typically cannot use a positive test result to predict the course or severity of a condition.

A negative test result means that the laboratory did not find a change in the gene, chromosome, or protein under consideration. This result can indicate that a person is not affected by a particular disorder, is not a carrier of a specific genetic mutation, or does not have an increased risk of developing a certain disease. It is possible, however, that the test missed a disease-causing genetic alteration because many tests cannot detect all genetic changes that can cause a particular disorder. Further testing may be required to confirm a negative result.

In some cases, a negative result might not give any useful information. This type of result is called uninformative, indeterminate, inconclusive, or ambiguous. Uninformative test results sometimes occur because everyone has common, natural variations in their DNA, called polymorphisms, that do not affect health. If a genetic test finds a change in DNA that has not been associated with a disorder in other people, it can be difficult to tell whether it is a natural polymorphism or a disease-causing mutation. An uninformative result cannot confirm or rule out a specific diagnosis, and it cannot indicate whether a person has an increased risk of developing a disorder. In some cases, testing other affected and unaffected family members can help clarify this type of result.

For more information about interpreting genetic test results:

The National Cancer Institute fact sheet Genetic Testing for Hereditary Cancer SyndromesThis link leads to a site outside Genetics Home Reference. provides an explanation of positive and negative genetic test results. (Scroll down to question 6, “What do the results of genetic testing mean?”)

The National Coalition for Health Professional Education in Genetics (NCHPEG) offers more information in their fact sheet Interpreting the Results of a Genetic or Genomic TestThis link leads to a site outside Genetics Home Reference..

The National Women’s Health Resource Center offers a list of questions about genetic testingThis link leads to a site outside Genetics Home Reference., including the meaning of test results, that patients and families can ask their healthcare professional.

Additional information about understanding genetic test resultsThis link leads to a site outside Genetics Home Reference. is available from the National Genetics and Genomics Education Centre of the National Health Service (UK).

Next: What is the cost of genetic testing, and how long does it take to get the results?

Published: February 8, 2016