Epidemiology: predictive value at low prevalence: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Epidemiology: predictive value at low prevalence

Sample item

A test with 90 percent sensitivity and 90 percent specificity is used in a population where the disease prevalence is 1 percent. About what proportion of positives are truly diseased?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. 8 percent

In 1000 people, 10 are diseased and 9 test positive; of 990 healthy, 99 test false positive. So 9 of 108 positives, about 8 percent, are true. Low prevalence lowers the positive predictive value.

Option by option

  • A. 8 percent (correct)

    Correct. 9 of 108 positives are true.

  • B. 90 percent

    90 percent is the sensitivity.

  • C. 1 percent

    1 percent is the prevalence.

  • D. 50 percent

    50 percent would need a much higher prevalence.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More PSM questions

Questions about this PSM question

What is the answer to this PSM question?

The best answer is A, 8 percent. In 1000 people, 10 are diseased and 9 test positive; of 990 healthy, 99 test false positive.

Why is "90 percent" not the answer?

90 percent is the sensitivity.

Why is "1 percent" not the answer?

1 percent is the prevalence.

Why is "50 percent" not the answer?

50 percent would need a much higher prevalence.

What concept does this question test?

It tests Epidemiology: predictive value at low prevalence in PSM. So 9 of 108 positives, about 8 percent, are true. Low prevalence lowers the positive predictive value.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Epidemiology: predictive value at low prevalence?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.