Screening: combining two tests in parallel: 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 in parallel

Sample item

A programme screens for a serious treatable disease with two independent tests. Test A has sensitivity 80% and specificity 90%. Test B has sensitivity 70% and specificity 80%. A person is labelled positive if either test is positive. What are the net sensitivity and specificity?

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

The answer and the reasoning

B. Sensitivity 94%, specificity 72%

With parallel testing the result is positive when either test is positive. A diseased person is missed only if both tests miss, so net sensitivity = 1 - (0.2 x 0.3) = 94%. A healthy person is correctly negative only if both are negative, so net specificity = 0.9 x 0.8 = 72%. Parallel testing raises sensitivity and lowers specificity.

Option by option

  • A. Sensitivity 75%, specificity 85%

    These are simple averages of the two tests and do not follow from combining independent results.

  • B. Sensitivity 94%, specificity 72% (correct)

    Correct. Parallel use gives 1 - 0.06 = 94% sensitivity and 0.72 = 72% specificity.

  • C. Sensitivity 56%, specificity 98%

    These are the values for series testing, where both tests must be positive: 0.8 x 0.7 and 1 - 0.1 x 0.2.

  • D. Sensitivity 94%, specificity 98%

    Sensitivity is right, but specificity cannot rise when a positive in either test counts as positive.

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 B, Sensitivity 94%, specificity 72%. With parallel testing the result is positive when either test is positive.

Why is "Sensitivity 75%, specificity 85%" not the answer?

These are simple averages of the two tests and do not follow from combining independent results.

Why is "Sensitivity 56%, specificity 98%" not the answer?

These are the values for series testing, where both tests must be positive: 0.8 x 0.7 and 1 - 0.1 x 0.2.

Why is "Sensitivity 94%, specificity 98%" not the answer?

Sensitivity is right, but specificity cannot rise when a positive in either test counts as positive.

What concept does this question test?

It tests Screening: combining two tests in parallel in PSM. A diseased person is missed only if both tests miss, so net sensitivity = 1 - (0.2 x 0.3) = 94%. A healthy person is correctly negative only if both are negative, so net specificity = 0.9 x 0.8 = 72%. Parallel testing raises sensitivity and lowers 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 in parallel?

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.