Positive Predictive Value

Show notes

Sensitivity and specificity describe the test. Predictive values describe your patient. PPV answers the question they actually asked, which is that the test came back positive, so do they have it. NPV answers the other side, which is that the test came back negative, so are they in the clear. Both move with prevalence, and that's what separates them from sensitivity and specificity. The same test performs differently depending on who's sitting in front of you, which is why a positive screening result in a low-risk population so often turns out to be nothing. We build both on the same 2×2 table, walk through why reading across the rows answers a different question than reading down the columns, and go through the traps the exam builds around it.