- to diagnose diseases, and preferably sub-classify it regarding, for example, severity and treatability
- to confirm that a person is free from disease
Companion diagnostics have also been developed to preselect patients for specific treatments based on their own biology. Such targeted therapy holds great promise in the treatment of diseases such as cancer.
Some of the diagnostic tests are parts of a simple physical examination which require only simple tools in the hands of a skilled practitioner, and can be performed in an office environment. Some other tests require elaborate equipment used by medical technologists or the use of a sterile operating theatre environment.
Some tests require samples of tissue or body fluids to be sent off to a pathology lab for further analysis. Some simple chemical tests, such as urine pH, can be measured directly in the doctor's office.
Most diagnostic tests are conducted on the living; however, some of these tests can also be carried out on a dead person as part of an autopsy.
The validity of diagnostic test results produced in each laboratory is entirely dependent on the measures employed before, during, and after each assay. Consistency in the production of good results requires an overall program that includes quality assurance, quality control, and quality assessment.
Medical tests can be classified into three categories:
Psychological effects of diagnostic tests 
Medical tests can have value when results are abnormal by explaining to a patient the cause of their symptoms. In addition, normal test results can have value by reassuring patients that serious illness is not present and even reduce the rates of subsequent symptoms. Understanding the meaning of a normal test in advance of learning the test results may also reduce the rates of subsequent symptoms.
Lack of adequate education about the meaning of test results (especially relevant to tests that may have incidental and unimportant findings) may cause an increase in symptoms. In addition, the possible benefits must be weighed against the costs of unnecessary tests and resulting unnecessary follow-up and possibly even unnecessary treatment of incidental findings.
The aim of a diagnostic test is to have an answer whether a condition is present or not in the test target, or at least contributing in estimating a post-test probability of it.
Interpretation of diagnostic tests should always take sources of inaccuracy and imprecision into account. Sources of inaccuracy and imprecision of diagnostic tests may broadly be categorized as:
- Physical sources within the diagnostic test taking itself
- Interpretational sources of the resultant data in relation to the target condition. Such sources include conversion of continuous values to binary ones (creating artificially binary values), such as designating a blood test for prostate specific antigen as "positive" when having reached a certain cutoff value, which is generally less accurate than considering the value itself.
See also 
- Medical test
- Medical diagnosis
- Medical record
- Nailbed assessment
- Physical examination
- Gold standard (test)
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