Medical test

A medical test is a medical procedure performed to detect, diagnose, or monitor 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 to clinical chemistry and molecular diagnostics, are typically performed in a medical setting.

• Accuracy of a laboratory test is its correspondence with the true value. Accuracy is maximized by calibrating laboratory equipment with reference material and by participating in external quality control programs.
• Precision of a test is its reproducibility when it is repeated on the same sample. An imprecise test yields widely varying results on repeated measurement. Precision is monitored in laboratory by using control material.

Tests performed in a physical examination are usually aimed at detecting a symptom or sign, and in these cases, a test that detects a symptom or sign is designated a positive 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, most blood tests. This is not only answering if a target entity is present or absent, but also how much is present. In blood tests, the quantification is relatively well specified, such as given in mass concentration, while most other tests may be quantifications as well although less specified, such as a sign of being "very pale" rather than "slightly pale". Similarly, radiologic images are technically quantifications of radiologic opacity of tissues.

Especially in the taking of a medical history, there is no clear limit between a detecting or quantifying test versus rather descriptive 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.

In the finding of a pathognomonic sign or symptom it is almost certain that the target condition is present, and in the absence of finding a sine 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 a post-test probability of a condition or other entity.

Most diagnostic tests basically use a reference group to establish performance data such as predictive values, likelihood ratios and relative 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.

Algunos procedimientos de pruebas médicas conllevan riesgos para la salud e incluso requieren anestesia general , como la mediastinoscopia . ^{[ 17 ]} Otras pruebas, como el análisis de sangre o la citología vaginal, presentan pocos o ningún riesgo directo. ^{[ 18 ]} Las pruebas médicas también pueden tener riesgos indirectos , como el estrés que genera la prueba, y pueden requerirse pruebas más riesgosas como seguimiento de un resultado (potencialmente) falso positivo . Consulte al profesional de la salud (incluidos médicos , asistentes médicos y enfermeros especializados ) que le prescriba cualquier prueba para obtener más información.

Cada prueba tiene sus propias indicaciones y contraindicaciones. Una indicación es una razón médica válida para realizar la prueba. Una contraindicación es una razón médica válida para no realizarla. Por ejemplo, una prueba básica de colesterol puede estar indicada (médicamente apropiada) para una persona de mediana edad. Sin embargo, si a esa persona se le realizó la misma prueba muy recientemente, entonces la existencia de la prueba anterior constituye una contraindicación (una razón médicamente válida para no realizarla).

El sesgo informativo es el sesgo cognitivo que lleva a los profesionales sanitarios a solicitar pruebas que proporcionan información que, en realidad, no esperan ni pretenden utilizar para tomar una decisión médica. Las pruebas médicas están indicadas cuando la información que proporcionan será útil. Por ejemplo, una mamografía de cribado no está indicada (no es médicamente apropiada) para una mujer en fase terminal, porque incluso si se detecta cáncer de mama, fallecerá antes de que pueda comenzar cualquier tratamiento oncológico.

De forma simplificada, la indicación de una prueba para un individuo depende en gran medida de su beneficio neto para ese individuo. Las pruebas se eligen cuando el beneficio esperado es mayor que el daño esperado. El beneficio neto se puede estimar aproximadamente mediante: ^{[ 19 ]}

${\displaystyle b_{n}=\Delta p\times r_{i}\times (b_{i}-h_{i})-h_{t}}$

, dónde:

• b _n es el beneficio neto de realizar una prueba
• Λp is the absolute difference between pre- and posttest probability of conditions (such as diseases) that the test is expected to achieve. A major factor for such an absolute difference is the power of the test itself, such as can be described in terms of, for example, sensitivity and specificity or likelihood ratio. Another factor is the pre-test probability, with a lower pre-test probability resulting in a lower absolute difference, with the consequence that even very powerful tests achieve a low absolute difference for very unlikely conditions in an individual (such as rare diseases in the absence of any other indicating sign), but on the other hand, that even tests with low power can make a great difference for highly suspected conditions. The probabilities in this sense may also need to be considered in context of conditions that are not primary targets of the test, such as profile-relative probabilities in a differential diagnostic procedure.
• r_i is the rate of how much probability differences are expected to result in changes in interventions (such as a change from "no treatment" to "administration of low-dose medical treatment"). For example, if the only expected effect of a medical test is to make one disease more likely compared to another, but the two diseases have the same treatment (or neither can be treated), then, this factor is very low and the test is probably without value for the individual in this aspect.
• b_i is the benefit of changes in interventions for the individual
• h_i is the harm of changes in interventions for the individual, such as side effects of medical treatment
• h_t is the harm caused by the test itself.

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 an abdominal palpation potentially inducing intestinal activity whose sounds interfere with a subsequent abdominal 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 give informed 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]

• World Health Organization (2019). First WHO Model List of Essential In Vitro Diagnostics. Geneva: World Health Organization. hdl:10665/311567. ISBN 978-92-4-121026-3. ISSN 0512-3054. WHO Technical Report Series, No. 1017. License: CC BY-NC-SA 3.0 IGO.