EpidemiologyLesson 1 of 2 in the course Differential diagnosis and clinical reasoning

Diagnostic Test Accuracy: Sensitivity, Specificity, and Interpretation

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

This lesson introduces sensitivity and specificity through the four possible outcomes of a diagnostic test. These measures matter because a test result is useful only when we understand how it compares with a person's actual condition.

The words true and false indicate whether the test classification agrees with the actual status; positive and negative describe the test's classification.

All flashcards

What is a true positive?
A true positive (TP) occurs when a person who has the condition receives a positive test result.
What is a false negative?
A false negative (FN) occurs when a person has the condition but the test classifies them as negative.
How is sensitivity calculated?
Sensitivity = TP/(TP+FN)TP/(TP+FN). It is the proportion of people who truly have the condition who test positive.
How is specificity calculated?
Specificity = TN/(TN+FP)TN/(TN+FP). It is the proportion of people who truly do not have the condition who test negative.
What does a false positive mean?
A false positive (FP) is a positive test result in someone who does not actually have the condition.

Quiz questions

  1. When a condition is uncommon, how does its low prevalence generally affect the positive predictive value of a test, all else being equal?Answer: It tends to lower the positive predictive value.
  2. For a test where lowering the numerical cutoff makes a positive result easier to obtain, what is the usual effect of lowering the cutoff?Answer: Sensitivity tends to rise while specificity tends to fall.
  3. What does a receiver operating characteristic (ROC) analysis compare across possible test cutoffs?Answer: Sensitivity against the false-positive rate.
  4. How is the positive likelihood ratio calculated from sensitivity and specificity?Answer: LR+=sensitivity/(1−specificity)LR^+=\text{sensitivity}/(1-\text{specificity}).
  5. Why should diagnostic-test estimates based on small samples be interpreted cautiously?Answer: They may be unstable and uncertain estimates of performance.

Based on the Wikipedia article Sensitivity and specificity, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.

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