USMLE purpose: Work backward from sensitivity, specificity, and prevalence to build a 2×2 diagnostic table.

How to use this page

  1. If no sample size is given, choose an easy number like 1000.
  2. Use prevalence to split disease+ vs disease−.
  3. Use sensitivity to fill the disease+ column.
  4. Use specificity to fill the disease− column.
  5. Calculate PPV or NPV from the completed table.

One-liner

When prevalence is given but sample size is missing, choose N = 1000, build the diagnostic table, then calculate PPV.

Worked PPV setup

Example: sensitivity 95%, specificity 80%, prevalence 20%, N = 1000.

Disease + Disease -
Test + 190 160
Test - 10 640
Total 200 800

PPV = 190 / (190 + 160) = 190 / 350 ≈ 54%

Board Exam Buzzwords

Given Use it to fill Formula
Prevalence Disease+ column total Prevalence × N
Sensitivity True positives among disease+ TP / disease+
Specificity True negatives among disease− TN / disease−
Positive test actually infected? PPV TP / TP+FP

Anki-style rapid recall