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Negative Predictive Value (NPV)

## What factors affect positive predictive value?

Positive and negative predictive values are influenced by **the prevalence of disease in the population that is being tested**. If we test in a high prevalence setting, it is more likely that persons who test positive truly have disease than if the test is performed in a population with low prevalence..

## What increases negative predictive value?

How Various Factors Affect Negative Predictive Value. **High sensitivity tests** make the negative predictive value increase. That’s because more people who are actually positive have a positive test result on a high sensitivity test and there are fewer false negatives.

## How does increased prevalence affect positive predictive value?

Prevalence thus impacts the positive predictive value (PPV) and negative predictive value (NPV) of tests. As the prevalence increases, **the PPV also increases but the NPV decreases**. Similarly, as the prevalence decreases the PPV decreases while the NPV increases.

## Why does PPV increase with prevalence?

For any given test (i.e. sensitivity and specificity remain the same) as prevalence decreases, the PPV decreases because there will be more false positives for every true positive.

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Negative predictive value (NPV)

Prevalence | PPV | NPV |
---|---|---|

50% | 90% | 90% |

## What is a good positive predictive value?

Positive predictive value focuses on subjects with a positive screening test in order to ask the probability of disease for those subjects. Here, the positive predictive value is 132/1,115 = 0.118, or **11.8%**. Interpretation: Among those who had a positive screening test, the probability of disease was 11.8%.

## Is a high negative predictive value good?

The more sensitive a test, the less likely an individual with a negative test will have the disease and thus the greater the negative predictive value. The more specific the test, the less likely an individual with a positive test will be free from disease and the greater the positive predictive value.

## What is the negative predictive value?

Negative predictive value:

It is **the ratio of subjects truly diagnosed as negative to all those who had negative test results** (including patients who were incorrectly diagnosed as healthy). This characteristic can predict how likely it is for someone to truly be healthy, in case of a negative test result.

## What is the difference between positive predictive value and positive likelihood ratio?

LR is one of the most clinically useful measures. LR shows how much more likely someone is to get a positive test if he/she has the disease, compared with a person without disease. Positive LR is **usually a number greater than one** and the negative LR ratio usually is smaller than one.

## Is positive predictive value a percentage?

The predictive value of a test is a measure (%) of the times that the value (positive or negative) is the true value, i.e. the **percent of all positive tests that are true positives is** the Positive Predictive Value.

## Does sensitivity depend on prevalence?

They are dependent on **the prevalence of the disease in the population of interest**. The sensitivity and specificity of a quantitative test are dependent on the cut-off value above or below which the test is positive. In general, the higher the sensitivity, the lower the specificity, and vice versa.