These values are also called post-test probabilities. You guessed it – we have another video that explains these values with the help of the 2×2 table and shows you how to calculate these values.
Now if you’ve watched the video, PPV and NPV are a great tool if you have a good idea about the prevalence of your patient group and if this prevalence is identical with the prevalence of the RCT, where you have gotten your statistical values from for a specific test in the first place. If this is not the case, using PPV and NPV cannot be used. Imagine how the pre-test probability of an anterior cruciate ligament (ACL) rupture changes in different settings: For example, the prevalence of patients with an ACL tear in a general practice will be much lower than in a sports clinic that is specialized in knee injuries. The higher the prevalence, the higher your PPV and the lower your NPV will be.
To solve this dilemma, we use likelihood ratios, which are covered in the next section.