A mammography screening programme is applied to a population with breast cancer prevalence of 1%. The test has a sensitivity of 80% and specificity of 95%. If a woman tests positive, what is the approximate probability that she actually has breast cancer (positive predictive value)?

• A About 80%
• B About 50%
• C About 62%
• D About 14% ✓

Explanation

In 10,000 women screened, 100 have cancer and 80 test positive (true positives). Of the 9,900 without cancer, 5% or 495 test positive (false positives). Positive predictive value equals 80 divided by 575, approximately 14%. The common error is choosing 80%, which confuses sensitivity with predictive value. Low disease prevalence means most positives are false positives despite good specificity.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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