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Types of genetic testing

Diagnostic testing

Diagnostic testing is used when someone is already experiencing symptoms, and the test aims to find out or confirm the genetic cause of these symptoms, providing a genetic diagnosis.

NHS diagnostic testing for inherited retinal conditions uses a panel test. A number of specific genes that are linked to a particular genetic condition are examined at the same time. In inherited retinal disease, the NHS test will examine almost 200 genes that are known to be linked to retinal degeneration, looking for changes that are likely or known to be disease-causing. (Although researchers have found associations between retinal damage and more than 300 genes, not all of these are understood well enough yet to be included in diagnostic testing.)

If the disease-causing change is identified, then it may be possible to offer testing to other family members. In family members who are not experiencing sight loss symptoms, this type of test may be a carrier test or a predictive test.

If the diagnostic test result is not certain, this usually cannot be used for testing the wider family as it may not be possible to interpret the results in a helpful way.

Sometimes patients are asked if it is possible to contact family members (usually parents) to ask if they would be willing to give a sample for family testing, which can help confirm the result and the inheritance pattern. This is usually in autosomal recessive conditions.

Carrier testing

For some genetic conditions that need two copies of a faulty gene to cause symptoms (recessive conditions), someone who only has one copy is called a carrier. Carriers of most conditions are themselves unaffected because the working copy of the gene overrides the faulty copy, but they can pass on a copy of the faulty gene to their children. Carrier testing may be helpful to confirm if both parents of an affected child are carriers and their risks of having another affected child. Depending on the family, carrier testing may also be offered to other relatives, such as in families with cousin marriages or women at risk of being carriers of X-linked conditions. If the test confirms that a person is not a carrier, they will definitely not pass on the faulty gene.

Predictive testing

A predictive test can provide information about whether or not someone will develop or is likely to develop a specific condition, usually at a later stage in life. If a retinal condition that happens later in life runs in your family, you might decide you would like to know whether or not you have inherited the faulty gene. There are lots of things to think about when deciding whether or not to have a predictive test as the result has implications for both you and your children, and a genetic test result from an affected family member would be needed in order to look specifically for the disease-causing genetic change.

Predictive testing is generally only offered to adults. However, families are always encouraged to have early conversations together.

Genetic counsellors can discuss all of the issues around predictive testing with you, and there is also information on the Genetic Alliance website: www.geneticalliance.org.uk/information/service-and-testing/predictive-testing.

Research testing is different to all the clinical tests mentioned above. It provides scientists with genetic samples so that they can find unknown disease-associated genes, learn how the protein products of genes work, develop tests for future clinical use, and help to advance our understanding of genetic conditions. You may be offered a research participation opportunity at some point by your ophthalmologist or genetic counsellor. Some specialist centres also hold a database of interested patients as a means to keep in touch. Ask your ophthalmologist about these options.

The results of tests done as part of a research study are not always given to people who take part or their healthcare providers, and may not be as reliable as clinical test results. It is important for people to know whether their test is clinical or for research. In both clinical and research testing patients will be told about what happens in the test, the risks and benefits, and what might happen afterwards, before agreeing to the test. There should be no obligation to take part in research but it may provide an opportunity for you and your family if there are no further clinical tests available. You will never be asked for payment to take part in a bona fide research study.

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