Genetic tests vary in their ability to predict disease. The test is said to be positive if evidence of genes that can cause the disease is found. The test is negative if there is no DNA present that will cause the disease.
Both types of errors can occur in tests. If a genetic test fails to detect the specific form of gene when it is actually present, the test result is called a "false negative". On the other hand, test results that indicate a person has or will become affected by the disease when they do not have it are called "false positives".
False negative and false positive results occur with difference frequencies for different tests. Even a very low rate of error can cause major problems. Consider the example below:
A genetic condition is found which affects 5% of a population. In a city with 1,000,000 people that means 50,000 people will be affected by the disease. A genetic test is developed which has an error rate of 1%.
If we test all one million people, there will be 500 false negatives. These people do not know that they have the disease. False positives will also occur - almost 10,000 people will test positive who do not carry the altered gene. But, if we retest the entire population, new false positives might be found and this will only make matters worse -therefore, usually only individuals with false positives are retested.
Since genetic tests are expensive and not 100% accurate, it is important to have a good reason for doing them..
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