# Positive predictive value

Positive predictive value

The positive predictive value, or precision rate, or post-test probability of disease, is the proportion of patients with positive test results who are correctly diagnosed.It is the most important measure of a diagnostic method as it reflects the probability that a positive test reflects the underlying condition being tested for. Its value does however depend on the prevalence of the disease, which may vary.

Worked example

Definition

The Positive Predictive Value can be defined as

$PPV = frac\left\{ m number of True Positives\right\}$ m number of True Positives}+{ m number of False Positives

or, alternatively,

$PPV = frac\left\{\left(\left\{ m sensitivity\right\}\right) \left(\left\{ m prevalence\right\}\right)\right\}\left\{\left(\left\{ m sensitivity\right\}\right) \left(\left\{ m prevalence\right\}\right) + \left(1 - \left\{ m specificity\right\}\right) \left(1-\left\{ m prevalence\right\}\right)\right\}$

Problems with positive predictive value

Note that the PPV is not intrinsic to the test--it depends also on the prevalence. PPV is directly proportional to the prevalence of the disease /condition. In the above example, if the group of people tested had included a higher proportion of people with bowel cancer, then the PPV would probably come out higher and the NPV lower. If "everybody" in the group had bowel cancer, the PPV would be 100% and the NPV 0%.

Predictive values are often used in medical research to evaluate the usefulness of a diagnostic test. Hence the PPV is used to indicate the probability that in case of a positive test, that the patient really has the specified disease. However there may be more than one cause for a disease and any single potential cause may not always result in the overt disease seen in a patient.

An example is the microbiological throat swab used in patients with a sore throat. Usually publications stating PPV of a throat swab are reporting on the probability that this bacteria is present in the throat, rather than that the patient is ill from the bacteria found. If presence of this bacteria always resulted in a sore throat, then the PPV would be very useful. However the bacteria may colonise individuals in a harmless way and never result in infection or disease. Sore throats occurring in these individuals is caused by other agents such as a virus. In this situation the gold standard used in the evaluation study represents only the presence of bacteria (that might be harmless) but not a causal bacterial sore throat illness. It can be proven that this problem will affect positive predictive value far more than negative predictive value. To evaluate diagnostic tests where the gold standard looks only at potential causes of disease, one may use an extension of the predictive value termed the [http://www.infovoice.se/fou/epv Etiologic Predictive Value] . [cite journal |author=Gunnarsson RK, Lanke J |title=The predictive value of microbiologic diagnostic tests if asymptomatic carriers are present |journal=Statistics in medicine |volume=21 |issue=12 |pages=1773–85 |year=2002 |pmid=12111911 |doi=10.1002/sim.1119]

ee also

* Sensitivity and specificity
* Negative predictive value
* Relevance (information retrieval)

References and notes

*

Wikimedia Foundation. 2010.

### См. также в других словарях:

• positive predictive value — prognozinė teigiamojo testo vertė statusas T sritis biomedicinos mokslai apibrėžtis Tikimybė, kad žmogus serga, jei diagnostinis testas yra teigiamas, t. y. sergančių asmenų, kurių testas teigiamas, ir visų tirtų asmenų, kurių testas teigiamas,… …   Lithuanian dictionary (lietuvių žodynas)

• positive predictive value — see predictive v …   Medical dictionary

• Predictive value of tests — can refer to:* Positive predictive value * Negative predictive value …   Wikipedia

• predictive value — the conditional probability that a clinical test result correctly identifies a patient as having or not having a disease, i.e., the predictive value of a positive test (positive predictive v.) is the probability that a person with a positive test …   Medical dictionary

• Negative predictive value — In statistics and diagnostic testing, the negative predictive value (NPV) is a summary statistic used to describe the performance of a diagnostic testing procedure. It is defined as the proportion of subjects with a negative test result who are… …   Wikipedia

• value — A particular quantitative determination. For values not given below, see the specific name. SEE ALSO: index, number. [M.E., fr. O.Fr., fr. L. valeo, to be of v.] acetyl v. the milligrams of KOH required to neutralize the acetic acid produced by… …   Medical dictionary

• Predictive analytics — encompasses a variety of techniques from statistics and data mining that analyze current and historical data to make predictions about future events. Such predictions rarely take the form of absolute statements, and are more likely to be… …   Wikipedia

• Option time value — Finance Financial markets Bond market …   Wikipedia

• Expectancy-value theory — Introduction Expectancy value theory was originally created in order to explain and predict individual s attitudes toward objects and actions. Originally the work of psychologist Martin Fishbein, the theory states that attitudes are developed and …   Wikipedia

• HIV test — Randall L. Tobias, former U.S. Global AIDS Coordinator, being publicly tested for HIV in Ethiopia in an effort to reduce the stigma of being tested.[1] HIV tests are used to detect the presence of the human immunodeficiency virus (HIV), the virus …   Wikipedia

### Поделиться ссылкой на выделенное

##### Прямая ссылка:
Нажмите правой клавишей мыши и выберите «Копировать ссылку»