M09.01.006 Measurement of screening test

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Learning Objectives

By the end of this lesson, the medical student will:

  • Define Sensitivity and Specificity using the 2×2 table.
  • Apply the SnNout and SpIn mnemonics to clinical scenarios.
  • Differentiate between the “True Positive Rate” and the “True Negative Rate.”
  • Analyze the trade-off between sensitivity and specificity in screening.

I. Sensitivity (The True Positive Rate)

Sensitivity is the probability that a test will correctly identify those with the disease.

Formula:

Sensitivity = TP / (TP + FN)

*Uses the Left Column (All Diseased Persons) of the 2×2 table.

Clinical Mnemonic: SnNout

A Sensitive test, when Negative, rules out the disease. High sensitivity means there are very few False Negatives (1 – Sensitivity).

Example: ESR for Temporal Arteritis (TA) is nearly 100% sensitive. If the ESR is normal, you can safely rule out TA.

II. Specificity (The True Negative Rate)

Specificity is the probability that a test will correctly identify those without the disease.

Formula:

Specificity = TN / (TN + FP)

*Uses the Right Column (All Healthy Persons) of the 2×2 table.

Clinical Mnemonic: SpIn

A Specific test, when Positive, rules IN disease. High specificity means there are very few False Positives (1 – Specificity).

Example: CT Angiogram for Pulmonary Embolism (PE) is 97% specific. If the CT is positive, the patient almost certainly has a PE.

III. High-Yield Comparison

Feature Sensitivity Specificity
Goal Catch all the sick Exclude all the healthy
Minimize… False Negatives False Positives
Best for… Screening (Initial) Confirmation (Final)

Screening tests are designed to “cast a wide net.” We want to catch every single person who might have the disease, which requires high sensitivity. Specificity and PPV are more important for confirmatory testing.

Activity