A diagnostic test is any kind of medical test performed to aid the diagnosis or detection of a suspected disease or condition. This is in contrary to a screening test which is used to detect disease markers of a specific disease and is offered to those have no symptoms (that is, are "asymptomatic") but who are thought to have a high risk of developing the disease tested for. A positive screening test does not equal a diagnosis. People with a positive screening test are offered a diagnostic test to establish a diagnosis. Sometimes two or more diagnostic tests are necessary before a diagnosis can be made. These may be offered at the same time (parallel testing) or sequentially (serial testing).
Diagnostic tests frequently test for a variable which is continuous. In such situations a cutoff value has to be set. The positioning of a cutoff value will always constitute a compromise between the sensitivityand the specificity of a test. A receiver operating curve (ROC) is a plot of the sensitivity of a test versus 1-specificity of the same test. This is used to identify potential cut-off values below which a test is considered negative or above which a test is considered positive.
A good diagnostic test has a high validity and accuracy in order to be able to discriminate between those who have and those who do not have the disease or condition tested for. For a diagnostic test to be of use, the test needs to give consistent results indepent of whom performs and/or interpretates the results of the test. This is called the test reliability