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Why does genetic testing take so long?

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Genetic testing is becoming more routine, but it will never be as quick as a blood sugar test, or a white blood cell count. That is because those tests can be run in the same way, with the same equipment, for each patient, no matter what their underlying condition. A genetic test has to be aimed at a specific gene for each individual. We each have many, many genes in our DNA, and each gene has many parts, called exons and introns. It is impossible at this time to test every one of a person’s genes. Your doctor must therefore make a clinical diagnosis based on your symptoms and the doctor’s findings, and then ask us to test the genes most likely to cause those findings. We then try to test the parts of the gene(s) in question that have most commonly been shown to cause disease. If there are no findings in the first gene tested, we may go on to another gene, and another, which may take several months. On the other hand, if you have a common mutation, or change, in a common gene, we may have a result for you in just a few weeks.

Another important part of genetic testing is telling the difference between polymorphisms, differences in DNA between people that don’t cause disease, and disease-causing mutations, which do. We are all very different, we look and grow differently, and so we have many differences in DNA which reflect that. Only a few types of changes cause disease. Before we can report a DNA change to you or your doctor, we have to do a complex analysis of the change, and assign it an EPP. This stands for Estimated Pathologic Probability. We assign the score based on a number of things, including whether this change has been found in other people with the disorder in question, or in normal people without any eye disorder, and whether it changes an amino acid, the building blocks of the proteins whose construction genes direct. Without this information, just knowing that a change was found in your DNA would not be very useful. An EPP of 2 or 3 means the change that was found causes eye disease.

All of these steps take time. Although some steps are automated, checks and double-checks occur at each stage to be sure you receive the most accurate and useful genetic test result. We hope you’ll agree when we say that from our point of view, considering the potential impact genetic testing can have on an individual and his or her family, we believe the accuracy of the information we provide is more important than the speed at which we deliver it. We’re doing our best to meet both expectations as best we can.

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