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Step 1 · Biostats & Epi · EP 01

Sensitivity and Specificity from Any 2×2 Table

Learn to pull sensitivity and specificity out of any 2×2 table without mixing them up, and why a 90% sensitive, 90% specific test can still leave a positive result at a coin flip. Includes an original practice question and a bedside twist on rapid flu tests.

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Practice question

A new rapid blood assay for bacteremia is evaluated in 500 adults admitted to the intensive care unit with suspected septic shock. Blood culture, the reference standard, is positive in 200 of them. The assay is positive in 160 of the culture-positive patients and negative in 270 of the culture-negative patients.

Which of the following is the sensitivity of the assay?

  1. A.80%
  2. B.84%
  3. C.86%
  4. D.87%
  5. E.90%
Show answer and explanations

Correct: A. 80% — sensitivity = TP ÷ (TP + FN) = 160 ÷ 200 = 0.80.

Full table: TP 160, FN 40 (200 culture-positive); TN 270, FP 30 (300 culture-negative).

  • B. 84% — positive predictive value: 160 ÷ (160 + 30) = 160 ÷ 190 = 0.842. Uses the test-positive row.
  • C. 86% — overall accuracy: (160 + 270) ÷ 500 = 0.86. Uses every cell.
  • D. 87% — negative predictive value: 270 ÷ (270 + 40) = 270 ÷ 310 = 0.871. Uses the test-negative row.
  • E. 90% — specificity: 270 ÷ 300 = 0.90. Uses the healthy (culture-negative) column.

Sources

  1. Parikh R, et al. Understanding and using sensitivity, specificity and predictive values. Indian J Ophthalmol. 2008;56(1):45–50. https://pmc.ncbi.nlm.nih.gov/articles/PMC2636062/
  2. Pewsner D, et al. Ruling a diagnosis in or out with "SpPIn" and "SnNOut": a note of caution. BMJ. 2004;329:209–213. https://pmc.ncbi.nlm.nih.gov/articles/PMC487735/
  3. Šimundić AM. Measures of diagnostic accuracy: basic definitions. EJIFCC. 2009;19(4):203–211. https://pmc.ncbi.nlm.nih.gov/articles/PMC4975285/
  4. USMLE® Content Outline (2026). Federation of State Medical Boards and National Board of Medical Examiners. https://www.usmle.org/pdfs/usmlecontentoutline.pdf
  5. Centers for Disease Control and Prevention. Rapid Influenza Diagnostic Tests (information for health care providers), page dated June 22, 2026. https://www.cdc.gov/flu/hcp/testing-methods/clinician_guidance_ridt.html

Graphics drawn in code. Narration: AI clone of my own voice.

For exam study only, not medical advice. Not affiliated with USMLE, NBME or FSMB.

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