USMLE purpose: Convert diagnostic test word problems into a 2×2 table and calculate PPV/NPV, sensitivity, or specificity.

How to use this page

  1. Draw the box immediately.
  2. Put the real disease status across the top and test result down the side.
  3. Fill totals first, then calculate missing cells.
  4. For “test positive and actually has disease,” think positive predictive value.

One-liner

If a patient tests positive and the question asks the probability they actually have disease, the metric is PPV = TP / (TP + FP).

2×2 Diagnostic Box

Disease + Disease -
Test + True positive False positive
Test - False negative True negative

Formula table

Metric Formula Question wording
Sensitivity TP / (TP + FN) Among disease+, how many test positive?
Specificity TN / (TN + FP) Among disease-, how many test negative?
PPV TP / (TP + FP) Positive test → truly diseased?
NPV TN / (TN + FN) Negative test → truly disease-free?

Worked example

If 100 patients have HSV2 and 90 test positive, then:

PPV = 90 / (90 + 20) = 90 / 110 ≈ 82%

Anki-style rapid recall