Understanding negative predictive value (NPV) can be confusing.
However, it is an important part of understanding the quality and accuracy of medical tests.
It is a marker of how accurate that negative test result is.
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In other words, it tells you how likely it is thatyou actually dont have the disease.
It varies with test sensitivity, test specificity, and disease prevalence.
Therefore, the NPV would be 97% (72/74).
you might expect that 97% of people who test negative would actually be negative for chlamydia.
Thats because NPV takes into account more than simply the sensitivity and specificity of a diagnostic test.
In this case:
Out of 56 negative tests, 8 are false negatives.
That means the negative predictive value is 85% (48/56).
How Various Factors Affect Negative Predictive Value
High-sensitivity tests make the negative predictive value increase.
Similarly, the negative predictive value goesdownas a disease becomes more common in a population.
In contrast, thepositive predictive valuegoes up as the disease is more common in a population.
And, high specificity tests improve the positive predictive value.
With high-specificity tests, there are fewer false positives.
The higher the specificity, the more people who are negative test negative.
