Negative & Positive Predictive Value Equation & Calculation

http://www.stomponstep1.com/negative-...

PPV & NPV are used to interpret test results once you have them. For example, if your patient just received a positive HIV test result you would use the PPV to evaluate what that test result means to your patient (what the percentage is that this person actually has HIV). It is important to remember that the PPV & NPV change as prevalence changes. This makes sense, because if the prevalence of a disease increases you are going to automatically get more TPs and less TNs just based on the fact that more people have the disease. Both measurements are given as a percentage ranging from 0% to 100%.

Positive Predictive Value (PPV) is the percentage chance that a positive test result is a true positive or the percentage chance that a patient with a positive result actually has the disease. It is used when determining how to proceed after a patient gets a positive result. PPV increases with increases in prevalence. PPV decreases with decreases in prevalence.

Negative Predictive Value (NPV) is the Percentage chance that a negative test result is a true negative or the percentage chance that a patient with a negative result is actually disease free. It is used when determining how to proceed after a patient gets a negative result. NPV decreases with an increase in prevalence. NPV increases with a decrease in prevalence.

This is how I remember the formulas for Sen, Spec, PPV & NPV. First I think that the top value (numerator) is always a positive value and the bottom "left" value always matches the top value. The value of the bottom "right" is always false. Then I think that the term with positive in the name (PPV) has "all positives" & the term with negative in the name (NPV) has "all negatives." Next I think of Sen looking sort of like PPV & Spec looking sort of like NPV. You just swap out the value on the bottom "right" value. To remember which set of values are affected by prevalence I think that increasing Prevalence increases the formula with the most Ps in it. That lets you know PPV is directly proportional with prevalence and it is intuitive that NPV is the opposite because those two are an obvious pair. So in my head I'm seeing something like this.

Occasionally, you will get these types of questions in graphical form. These questions with a picture are much less common than questions that test the definition of a term or ask you to make a calculation based on a two-by-two table. However, I am going to spend some time on this question format as I believe it helps to solidify the overall concept.

These questions will have a graph that looks something like the above picture. That can be a little confusing to start with so let's break it down piece by piece.

Now that you are done with this video you should check out the next video in the Biostatistics & Epidemiology section which covers the definitions & calculations of Prevalence & Incidence (http://www.stomponstep1.com/incidence...)