What formula expresses Negative Predictive Value (NPV)?

Negative Predictive Value (NPV) is the probability that subjects with a negative screening test truly don’t have the disease. It measures the proportion of true negatives among all negative test results.

The formula for NPV is given by the number of true negatives (TN) divided by the total number of negative test results, which consists of true negatives and false negatives (FN). Therefore, the formula can be written as:

NPV = TN / (TN + FN)

This formula indicates that to assess how trustworthy a negative test result is, one must consider not just how many tests came back negative but also how many of those were actually true negatives (correctly identifying individuals without the disease) as opposed to false negatives (individuals who have the disease but tested negative).

Understanding this formula is crucial in interpreting diagnostic tests and determining their reliability in clinical settings, as a high NPV indicates that a negative result can be trusted to mean that a patient does not have the disease.