Negative predictive value

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 correctly diagnosed. A high NPV means that when the test yields a negative result, it is most likely correct in its assessment. In the familiar context of medical testing, a high NPV means that the test only rarely misclassifies a sick person as being healthy. Note that this says nothing about the tendency of the test to mistakenly classify a healthy person as being sick.

Contents

Definition

The Negative Predictive Value is defined as:

 
{\rm NPV} = \frac{\rm number\ of\ True\ Negatives}{{\rm number\ of\ True\ Negatives}+{\rm number\ of\ False\ Negatives}} =
\frac{\rm number\ of\ True\ Negatives}{{\rm number\ of\ Negative\ calls}}

where a "true negative" is the event that the test makes a negative prediction, and the subject has a negative result under the gold standard, and a "false negative" is the event that the test makes a negative prediction, and the subject has a positive result under the gold standard.

The following diagram illustrates how the positive predictive value, negative predictive value, sensitivity, and specificity are related.

Condition
(as determined by "Gold standard")
Positive Negative
Test
outcome
Positive True Positive False Positive
(Type I error)
Positive predictive value
= \,\!\tfrac{\Sigma\text{ True Positive}}{\Sigma\text{ Test outcome Positive}}
Negative False Negative
(Type II error)
True Negative Negative predictive value
= \,\!\tfrac{\Sigma\text{ True Negative}}{\Sigma\text{ Test outcome Negative}}

Sensitivity
= \,\!\tfrac{\Sigma\text{ True Positive}}{\Sigma\text{ Condition Positive}}

Specificity
= \,\!\tfrac{\Sigma\text{ True Negative}}{\Sigma\text{ Condition Negative}}

Note that the positive and negative predictive values can only be estimated using data from a cross-sectional study or other population-based study in which valid prevalence estimates may be obtained. In contrast, the sensitivity and specificity can be estimated from case-control studies.

If the prevalence, sensitivity, and specificity are known, the negative predictive value can be obtained from the following identity:

 
{\rm NPV} = \frac{({\rm specificity}) ({\rm 1 - prevalence})}{({\rm specificity}) ({\rm 1 - prevalence}) + (1 - {\rm sensitivity}) ({\rm prevalence})}.

Worked example

Suppose that a fecal occult blood (FOB) screen test is used in 2030 people to detect bowel cancer:

Patients with bowel cancer
(as confirmed on endoscopy)
Positive Negative
Fecal
occult
blood
screen
test
outcome
Positive True Positive
(TP) = 20
False Positive
(FP) = 180
→ Positive predictive value
= TP / (TP + FP)
= 20 / (20 + 180)
= 20 / 200
= 10%
 Negative  False Negative
(FN) = 10
True Negative
(TN) = 1820
→ Negative predictive value
= TN / (FN + TN)
= 1820 / (10 + 1820)
= 1820 / 1830
≈ 99.5%

Sensitivity
= TP / (TP + FN)
= 20 / (20 + 10)
= 20 / 30
≈ 66.67%

Specificity
= TN / (FP + TN)
= 1820 / (180 + 1820)
= 1820 / 2000
= 91%

In this setting, with NPV = 99.5%, a negative test result may provide some reassurance that the subject is unlikely to have cancer. This high NPV value would be particularly notable if the cancer were relatively common. For example, if 5% of people in the population had bowel cancer, then a NPV of 99.5% would indicate that a person with a negative test result has much lower than the average population risk for bowel cancer. However if the prevalence of bowel cancer were 0.5%, a negative test result in this setting would be uninformative.

Relation to negative post-test probability

Although sometimes used synonymously, a negative predictive value generally refers to what is established by control groups, while a negative post-test probability rather refers to a probability for an individual. Still, if the individual's pre-test probability of the target condition is the same as the prevalence in the control group used to establish the negative predictive value, then the two are numerically equal.

See also

References


Wikimedia Foundation. 2010.

См. также в других словарях:

  • negative predictive value — prognozinė neigiamojo testo vertė statusas T sritis biomedicinos mokslai apibrėžtis Tikimybė, kad žmogus sveikas, jei diagnostinis testas yra neigiamas, t. y. sveikų asmenų, kurių testas neigiamas, ir visų tirtų asmenų, kurių testas neigiamas,… …   Lithuanian dictionary (lietuvių žodynas)

  • negative 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

  • 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 …   Wikipedia

  • Negative value — may refer to: Negative predictive value in statistics Negative ethic or philosophic value insolvency This disambiguation page lists articles associated with the same title. If an internal link …   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


Поделиться ссылкой на выделенное

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