Clinical biochemistry: prevalence and predictive values: practice question

A Biochemistry 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.

Try it

Biochemistry · Clinical biochemistry: prevalence and predictive values

Sample item

A screening test of fixed sensitivity and specificity is moved from a general clinic to a referral clinic where disease prevalence is much higher. What happens to its predictive values?

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

The answer and the reasoning

C. Positive predictive value rises and negative predictive value falls

Sensitivity and specificity are properties of the test, but predictive values depend on prevalence. As prevalence rises, a larger share of positive results are true positives, so positive predictive value increases, while negative results are more often false negatives, so negative predictive value decreases.

Option by option

  • A. Both predictive values rise

    Both cannot rise together, because a higher prevalence makes false negatives relatively more common.

  • B. Both predictive values stay unchanged

    Predictive values change with prevalence even when sensitivity and specificity are fixed.

  • C. Positive predictive value rises and negative predictive value falls (correct)

    Correct. Higher prevalence raises PPV and lowers NPV.

  • D. Positive predictive value falls and negative predictive value rises

    This is the pattern when prevalence falls, as in population screening.

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 Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Positive predictive value rises and negative predictive value falls. Sensitivity and specificity are properties of the test, but predictive values depend on prevalence.

Why is "Both predictive values rise" not the answer?

Both cannot rise together, because a higher prevalence makes false negatives relatively more common.

Why is "Both predictive values stay unchanged" not the answer?

Predictive values change with prevalence even when sensitivity and specificity are fixed.

Why is "Positive predictive value falls and negative predictive value rises" not the answer?

This is the pattern when prevalence falls, as in population screening.

What concept does this question test?

It tests Clinical biochemistry: prevalence and predictive values in Biochemistry. As prevalence rises, a larger share of positive results are true positives, so positive predictive value increases, while negative results are more often false negatives, so negative predictive value decreases.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Clinical biochemistry: prevalence and predictive values?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.