Positive & Negative Predictive Value (PPV & NPV)


Before you watch this video you should really watch the previous videos which cover Two by Two Tables, TP, TN, FP & FN as well as Sensitivity & Specificity. Those videos lay the foundation for this video so it may be tough to watch this one by itself.

 Positive Predictive Value & Negative Predicative Value

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 & Negative Predictive Value NPV PPV

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.

Mnemonic for Sensitivity, Specificity, PPV & NPV

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.

Double Hump Graph

 

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  

 

 

37 thoughts on “Positive & Negative Predictive Value (PPV & NPV)”

  1. I think the best way for me to remember PPV, NPV, sens. and spec. is by drawing out a 2×2 table then adding the third column/row. Then putting whatever is in line with the boxes gets put into x/x+y. That way there is no memorizing, if you understand the 2×2 you understand everything else. Plus it solidifies 2×2 table, I think you hinted at this method in the 2×2 video.

    1. That is a great point Chris. I never realized that. I will probably end up remaking these videos once I get better equipment and more experience edit/recording so I will consider adding your mnemonic

  2. Please Keep making the videos. They are very helpful!!! I have one request if u can please have an example questions at the end of the video for that topic it would be even better.
    Thank you for all your hard work and time.

    1. Thanks so much for commenting! Yes example questions is something a couple other people have suggested and I think it is a great idea. My long term goal is to have a full sized Q Bank that has like 20 questions for each video, but that will probably be years in the future so it would be good to do something smaller in the meantime.

    2. Oh and I am definitely going to keep making videos. I had to put the project on pause for the last few months because I was studying for Step 2, getting my residency application together and finishing up a couple research projects. But my schedule is going to lighten up a bunch in fall and I plan to start cranking out more videos soon

    1. Thanks so much for commenting! Helping people study more efficiently is my main goal so it is great to hear that that is what you got out of the material

  3. Great videos! very helpful. For this particular video, the way I remember SEN and Spec is that in the general formula TP/TP+F(?) , just pick seN (N) for sen and sPec (P) for specificity. great job!

  4. thanks for all of your videos. especially for tackling the crap the usually gets taught in first year of med school and never sticks. I think there are a lot of resources out there that tackle the pathophysiology of organ systems, micro, pharm, etc. and not no much that tackles this stuff, so you have a niche. these high yield explanations and lists are really saving me during my dedicated time so once again, thx.

    1. Thanks for the feedback UNU! Getting comments like these makes all of the hard work on this project worth it 🙂

    1. Thanks for the feedback Mariam! I’m actually currently working on a set of biostats/epidemiology videos with practice question and answers. It will be a while before they are done but it is definitely coming

  5. Thank you so much for taking the time to make these videos. I’ve read books related to biostats and always get confused but your videos put across the information in a clear concise way that makes it really easy to understand and remember. Thanks again!

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