Screening: combining two tests: 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 · Screening: combining two tests

Sample item

Two independent tests for the same disease have sensitivities of 80% and 70%. In a screening programme both tests are done on every person, and anyone positive on either test is called positive. What is the effect on the combined sensitivity and specificity?

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

The answer and the reasoning

C. Sensitivity 94%, and overall specificity falls below that of either test

Interpreting either positive result as positive is parallel testing. A diseased person is missed only if both tests are negative: 0.2 x 0.3 = 0.06, so the combined sensitivity is 94%. More false positives accrue, so specificity falls. Series testing (both must be positive) would give a sensitivity of 0.8 x 0.7 = 56% with higher specificity.

Option by option

  • A. Sensitivity 56%, and overall specificity rises above that of either test

    A sensitivity of 56% and higher specificity describes series testing, where both tests must be positive.

  • B. Sensitivity 94%, and overall specificity rises above that of either test

    Parallel testing increases the chance of a false-positive result from either test, so specificity cannot rise.

  • C. Sensitivity 94%, and overall specificity falls below that of either test (correct)

    Parallel testing gains sensitivity (1 - 0.06 = 94%) at the cost of specificity.

  • D. Sensitivity 75%, and overall specificity is unchanged

    Sensitivity is not the average of the two tests; independent tests in parallel combine multiplicatively for the misses.

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.

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Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, Sensitivity 94%, and overall specificity falls below that of either test. Interpreting either positive result as positive is parallel testing.

Why is "Sensitivity 56%, and overall specificity rises above that of either t…" not the answer?

A sensitivity of 56% and higher specificity describes series testing, where both tests must be positive.

Why is "Sensitivity 94%, and overall specificity rises above that of either t…" not the answer?

Parallel testing increases the chance of a false-positive result from either test, so specificity cannot rise.

Why is "Sensitivity 75%, and overall specificity is unchanged" not the answer?

Sensitivity is not the average of the two tests; independent tests in parallel combine multiplicatively for the misses.

What concept does this question test?

It tests Screening: combining two tests in PSM. A diseased person is missed only if both tests are negative: 0.2 x 0.3 = 0.06, so the combined sensitivity is 94%. More false positives accrue, so specificity falls. Series testing (both must be positive) would give a sensitivity of 0.8 x 0.7 = 56% with higher specificity.

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 Screening: combining two tests?

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.