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Which term describes the proportion of people who truly have a disease and are correctly identified as positive by a test?
- A Sensitivity of the test You picked ✓
- B Positive predictive value You picked ✕
- C Specificity of the test You picked ✕
- D Negative predictive value You picked ✕
- E Prevalence of the disease You picked ✕
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The answer is A: Sensitivity of the test
Explanation
Why A is correct
See Key Concepts for Studies. Option A is correct. Sensitivity is the proportion of people with the disease who test positive, in other words the proportion of true positives. It answers the question: of everyone who really has the disease, how many does the test pick up? It is calculated as true positives divided by the sum of true positives and false negatives (people with the disease whom the test misses). A highly sensitive test rarely misses disease, so a negative result is reassuring, which is why sensitive tests are valued for screening. Specificity is the mirror image: the proportion of people without the disease who test negative. The predictive values answer a different question for the patient: given this result, how likely is it that I have, or do not have, the disease? They depend on prevalence as well as the test. A documented memory aid is SnNOUT: a highly Sensitive test, when Negative, rules OUT the disease. Its partner is SpPIN: a highly Specific test, when Positive, rules IN the disease.
- Sensitivity: the proportion of people with the disease who test positive, in other words the proportion of true positives.
- Calculation: true positives divided by the sum of true positives and false negatives.
- SnNOUT: a highly sensitive test, when negative, rules out the disease, which is why sensitive tests are valued for screening.
Why the other options are incorrect
- B. Positive predictive value: This is the probability that a person who tests positive has the disease. It starts from the positive results, not from the people who have the disease, and it changes with prevalence. Your answer
- C. Specificity of the test: Specificity is the proportion of people without the disease who correctly test negative. It starts from healthy people, whereas the stem starts from people who have the disease. Your answer
- D. Negative predictive value: This is the probability that a person with a negative result does not have the condition. It starts from the negative results and also varies with prevalence. Your answer
- E. Prevalence of the disease: Prevalence is how common the disease is in a population at a given time. It is not a measure of how well a test performs. Your answer
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1
Which concept describes the tendency for healthcare resources to be more readily available to people who are more affluent, and so less available to those with greater need?
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2
A community survey reports that 8% of adults in a town have a current diagnosis of asthma. Which epidemiological measure does this describe?
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