A diagnostic test is any kind of medical test performed to aid in the diagnosis or detection of disease. For example, such a test may be used to confirm that a person is free from disease, or to fully diagnose a disease, including to sub-classify it regarding severity and susceptibility to treatment. Diagnostic tests help physicians make clinical decisions for patient care.

Some diagnostic tests are parts of a physical examination that 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 in clinical laboratories, or the use of a sterile operating theater 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.

The validity of such 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.

Diagnostic tests can be classified into three categories: invasive, minimally invasive and non-invasive.

Every test that shows an association between test results and the target disease is potentially useful. If it is not on its own thought to be useful, then a combination of it with other test results and/or data can potentially lead to a post-test probability that is thought to be high enough to rule the diagnosis in or low enough to rule the diagnosis out.

Companion diagnostics have also been developed to preselect patients for specific treatments based on their own biology, where such targeted therapy may hold promise in personalized treatment of diseases such as cancer.

Growing acceptance of companion diagnostics is a trend with the potential to greatly increase the value that clinical pathology laboratory testing delivers to physicians, patients, and payers. It has become increasingly common for pharmaceutical companies to make agreements with in vitro diagnostics (IVD) manufacturers to develop a companion diagnostic test specifically for a therapeutic drug under development by that pharmaceutical company.

As most pathologists and clinical managers know, use of a companion diagnostic test is expected to add precision to the physician’s decision to prescribe therapeutic drugs.