Negative & Positive Predictive Value Equation & Calculation

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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 (www.stomponstep...)

[@stomponstep1989]

If you liked this video and want me to make more please let me know by commenting, liking this video or by subscribing to my KZbin channel. If you have a question, please don’t hesitate to ask and I’ll try to answer it ASAP.

[@CamariiZida]

Thank you for the tips in memorizing the formulas~ It's very practical and easy to remember. :D

[@petermokaya7908]

I'm currently doing epidemiology 1 unit at the University of Nairobi Kenya and i find your explanations easy to understand. Thank you a million times

[@stomponstep1989]

+PETER MOKAYA Thanks for the feedback and good luck with the rest of epidemiology!

[@cassandracolemanlautredou5429]

Thank you for making this video. It really helped me sort out PPV/NPV and sensitivity and specificity in my head.

[@stomponstep1989]

Cassandra Coleman No problem. I'm glad it helped you with the material!

[@SisMandy]

Thanks Daniel, this is very helpful. All your videos are excellent. God bless you!

[@stomponstep1989]

Mandisa Mtwesi Thank you for the kind words!

[@carlosdamas5949]

Your trick for memorizing the equations is great. I always jumble them up.

[@rachngim978]

Thank you so much, your videos are super clear and easy to understand.

[@RaviGupta-mh2hw]

Hello sir, I think it would be better to correlate sensitivity with negative predictive value and specificity with positive predictive value because when the cutoff for a test is lowered, due to less fn --> sensitivity increases and the corresponding fn rate (negative predictive value) also increases due to decreased fn. I love your videos. They have helped me so much.

[@yogeshsabnis8507]

Good and simple explanation of PPV and NPV. What I was lacking is why & when one would use it. cheers!

[@stomponstep1989]

Thanks for the feedback! Those things are definitely worth knowing, but this video series is focused on the highest yield material for the USMLE Step 1 exam and they don't tend ask questions like that. It is usually just a simple question based on the definition of PPV/NPV or they just ask you to calculate it after giving you the values. But at some point I may loop back and dive into some topics in more depth and I'll keep your suggestions in mind

[@MsBabygenius]

Brillant video. Thanks for sharing.

[@dr.brentcompton5147]

Daniel, Just my $0.02 worth. I love the video but wonder if it would be helpful if I shared how I learned to use the 2x2 tables. Basically make the standard table and fill in TP, FP, FN, and TN as you said. Then to remember sensitivity just circle TP and draw and arrow going down to FN (arrow means you ad both). Similar to find Specificity, circle TN and draw arrow up to FP. PPV is as simple as circling TP and drawing arrow to the right and NPV is circling TN and draw arrow to the left to FN. Just was for me to remember this way and might help others.

[@stomponstep1989]

Dr. Brent Compton Thanks for adding your mnemonic!

[@danielloyoj]

I liked the advice to memorize the formulas. You might want to put that in the title to attract more viewers :)

[@stomponstep1989]

Daniel Enrique Loyo Jáuregui Thanks for the comment!

[@michelleharvey6840]

Very useful keep posting

[@amjadibrahim8433]

God bless you

[@Manjit133]

Great .... Thanks !!

[@ranganawadugedara6672]

Thank u!

[@DrTay-zn5re]

GOOD JOB

[@stomponstep1989]

MeltingSunRays Thanks :)

[@Faidah-z7b]

thanxxxxxxxxxxxx

[@sshnao]

Awsome

[@stomponstep1989]

Mustafa Kerem thanks!

[@levert.gomellewis.8306]

forshee pious