An easy way to understand NPV is to dissect the name. In Negative Predictive Value, we are looking at how good a Negative test is at predicting Negative disease (ie. that the person doesn’t have a disease).
The negative predictive value of a test is the proportion of patients with negative test results who do not have the disorder.
Among patients who test negative for a disease, the NPV measures how likely it is that the patient is healthy. That is, the probability of not having the disease, given a negative test. Just like PPV, PPV depends on the prevalence of the disease in the population and the sensitivity/specificity of the test. If prevalence increases, NPV decreases. Mathematically, NPV is the number of true negatives divided by the number of people with a negative test. NPV = D / ( C + D)