How are genetic screening tests different from genetic diagnostic tests? From Genetics Home Reference. Examples of genetic diagnostic tests include: Molecular gene tests : These tests determine the order of DNA building blocks nucleotides in an individual's genetic code, a process called DNA sequencing. Topics in the Genetic Testing chapter What is genetic testing?
What are the different types of genetic tests? What are the uses of genetic testing? How is genetic testing done? What is informed consent? How can I be sure a genetic test is valid and useful? What do the results of genetic tests mean? What is the cost of genetic testing, and how long does it take to get the results? Will health insurance cover the costs of genetic testing? What are the benefits of genetic testing?
Sensitivity and specificity values are published when new tests become available, and are used by clinicians to decide what tests to order. For diagnostic testing, recall that we are trying sometimes to rule in and other times to rule out.
To understand why, look back at the formulae. Thus a patient with a positive result on a highly specific test is probably a true positive.
A negative result on a highly sensitive test is thus almost certainly a true negative. Thus clinicians will choose a test with high sensitivity when they want to rule out as in concussion or stroke, in our example above , and a test with high specificity when they want to rule in.
For screening purposes, we are testing an asymptomatic population—we thus want to minimize false negatives. Screening programs therefore utilize tests with high sensitivities. The positive predictive value is the probability that you actually have the disease given that you tested positive look carefully, and be sure you understand how this is different than sensitivity :. The negative predictive value is the probability that you do not have the disease given that you tested negative:.
Unlike sensitivity and specificity, however, PPV and NPV do change as the prevalence of disease in the sample changes—it is thus important to know something about the prevalence of disease in the target population to which an individual belongs before you can interpret their test results.
This makes intuitive sense: as a condition becomes more rare, then guessing that the patient does not have the disease becomes more and more likely to be correct. Say a new test for anemia is developed that does not require a finger stick to obtain blood no one likes needles! However, we know that the patient is from a population with a lower prevalence of anemia than We begin by deciding arbitrarily that we will again have 1, people in the table:.
Since the sensitivity is There is thus only a We would follow up with a blood draw to confirm—had he tested negative, the new NPV would have been Screening and diagnostic testing are similar procedures; the difference depends on context whether the tested person is symptomatic or not. Accuracy of such tests is quantified by sensitivity and specificity used ahead of time to pick the correct test , and positive and negative predictive values used after the test results are known, to interpret them.
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Applying a clinical test to asymptomatic individuals, on the theory that finding and treating the disease earlier will lead to better outcomes. Applying a clinical test to a person who has presented with symptoms, to aid in determining what condition the person has, so that they can be correctly treated.
The likely course of a disease; how well someone with the disease will fare, given current treatment regimens. The other two positive predictive value and negative predictive value do change as disease prevalence changes.
Sensitivity is the probability that one tests positive, given that one has the disease. Does not vary as disease prevalence varies. Specificity is the probability that one tests negative, given that one does not have the disease. PPV is the probability that one has the disease, given that one tested positive.
Varies as disease prevalence varies. Since large numbers of people will be screened to identify a very small number of cases, the financial resources needed must be justified carefully.
The test provides a definitive diagnosis e. The result of the test is an estimate of the level of risk and determines whether a diagnostic test is justified. Mastering Public Health: A guide to examinations and revalidation.
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