| Medical test | |
|---|---|
 X-ray of a hand. X-rays are a common medical test. | |
| MeSH | D019937 |
Amedical test is amedical procedure performed todetect,diagnose, ormonitor diseases, disease processes, susceptibility, or to determine a course of treatment. Medical tests such as, physical and visual exams,diagnostic imaging, genetic testing, chemical and cellular analysis, relating toclinical chemistry andmolecular diagnostics, are typically performed in amedical setting.
Medical tests can be classified by their purposes, including diagnosis, screening or monitoring.
A diagnostic test is a procedure performed to confirm or determine the presence of disease in an individual suspected of having a disease, usually following the report of symptoms, or based on other medical test results.[1][2] This includesposthumous diagnosis. Examples of such tests are:
Screening refers to a medical test or series of tests used to detect or predict the presence of disease in at-risk individuals within a defined group such as a population, family, or workforce.[4][5] Screenings may be performed to monitor disease prevalence, manage epidemiology, aid in prevention, or strictly for statistical purposes.[6]
Examples of screenings include measuring the level ofTSH in theblood of a newborninfant as part ofnewborn screening forcongenital hypothyroidism,[7] checking forLung cancer in non-smoking individuals who are exposed tosecond-hand smoke in an unregulated working environment, andPap smear screening for prevention or early detection ofcervical cancer.[citation needed]
Some medical tests are used tomonitor the progress of, or response tomedical treatment.
Most test methods can be classified into one of the following broad groups:
In vitro tests can be classified according to the location of the sample being tested, including:
Tests performed in aphysical examination are usually aimed at detecting asymptom orsign, and in these cases, a test that detects a symptom or sign is designated apositive test, and a test that indicated absence of a symptom or sign is designated a negative test, as further detailed in a separate section below.A quantification of a target substance, a cell type or another specific entity is a common output of, for example, mostblood tests. This is not only answeringif a target entity is present or absent, but alsohow much is present. In blood tests, the quantification is relatively well specified, such as given inmass concentration, while most other tests may be quantifications as well although less specified, such as a sign of being "verypale" rather than "slightly pale". Similarly, radiologic images are technically quantifications of radiologic opacity of tissues.[citation needed]
Especially in the taking of amedical history, there is no clear limit between a detecting or quantifying test versus ratherdescriptive information of an individual. For example, questions regarding the occupation or social life of an individual may be regarded as tests that can be regarded as positive or negative for the presence of various risk factors, or they may be regarded as "merely" descriptive, although the latter may be at least as clinically important.[citation needed]
The result of a test aimed at detection of an entity may bepositive ornegative: this has nothing to do with a badprognosis, but rather means that the test worked or not, and a certain parameter that was evaluated was present or not. For example, a negativescreening test forbreast cancer means that no sign of breast cancer could be found (which is in fact very positive for the patient).[citation needed]
The classification of tests into either positive or negative results in abinary classification, allowing for the application ofbayesian probability and the calculation ofdiagnostic test accuracy measures, such assensitivity, specificity,likelihood ratios, and thediagnostic odds ratio.[14][15] These metrics are commonly used insystematic review of diagnostic test accuracy andmeta-analyses ofdiagnostic accuracy studies.[16]
Tests whose results are of continuous values, such as mostblood values, can be interpreted as they are, or they can beconverted to a binary ones by defining acutoff value, with test results being designated as positive or negative depending on whether the resultant value is higher or lower than the cutoff.
In the finding of apathognomonic sign or symptom it is almost certain that the target condition is present, and in the absence of finding asine qua non sign or symptom it is almost certain that the target condition is absent. In reality, however, the subjective probability of the presence of a condition is never exactly 100% or 0%, so tests are rather aimed at estimating apost-test probability of a condition or other entity.
Most diagnostic tests basically use areference group to establish performance data such aspredictive values,likelihood ratios andrelative risks, which are then used to interpret the post-test probability for an individual.
In monitoring tests of an individual, the test results from previous tests on that individual may be used as a reference to interpret subsequent tests.
Some medical testing procedures have associated health risks, and even requiregeneral anesthesia, such as themediastinoscopy.[17] Other tests, such as theblood test orpap smear have little to no direct risks.[18] Medical tests may also haveindirect risks, such as the stress of testing, and riskier tests may be required as follow-up for a (potentially)false positive test result. Consult the health care provider (includingphysicians,physician assistants, andnurse practitioners) prescribing any test for further information.
Each test has its ownindications and contraindications. Anindication is a valid medical reason to perform the test. Acontraindication is a valid medical reason not to perform the test. For example, a basiccholesterol test may beindicated (medically appropriate) for a middle-aged person. However, if the same test was performed on that person very recently, then the existence of the previous test is a contraindication for the test (a medically valid reason to not perform it).
Information bias is thecognitive bias that causes healthcare providers to order tests that produce information that they do not realistically expect or intend to use for the purpose of making a medical decision. Medical tests are indicated when the information they produce will be used. For example, a screening mammogram is not indicated (not medically appropriate) for a woman who is dying, because even if breast cancer is found, she will die before any cancer treatment could begin.
In a simplified fashion, how much a test is indicated for an individual depends largely on itsnet benefit for that individual. Tests are chosen when the expected benefit is greater than the expected harm. The net benefit may roughly be estimated by:[19]
, where:
Some additional factors that influence a decision whether a medical test should be performed or not included: cost of the test, availability of additional tests, potential interference with subsequent test (such as anabdominal palpation potentially inducing intestinal activity whose sounds interfere with a subsequentabdominal auscultation), time taken for the test or other practical or administrative aspects. The possible benefits of a diagnostic test may also be weighed against the costs of unnecessary tests and resulting unnecessary follow-up and possibly even unnecessary treatment of incidental findings.[20]
In some cases, tests being performed are expected to have no benefit for the individual being tested. Instead, the results may be useful for the establishment of statistics in order to improve health care for other individuals. Patients may giveinformed consent to undergo medical tests that will benefit other people.
In addition to considerations of the nature of medical testing noted above, other realities can lead to misconceptions and unjustified expectations among patients. These include: Different labs have different normal reference ranges; slightly different values will result from repeating a test; "normal" is defined by a spectrum along a bell curve resulting from the testing of a population, not by "rational, science-based, physiological principles"; sometimes tests are used in the hope of turning something up to give the doctor a clue as to the nature of a given condition; and imaging tests are subject to fallible human interpretation and can show"incidentalomas", most of which "are benign, will never cause symptoms, and do not require further evaluation," although clinicians are developing guidelines for deciding when to pursue diagnoses of incidentalomas.[21]
The QUADAS-2 revision is available.[22]
