Clinical biochemistry: hook effect in immunoassays: 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.

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Biochemistry · Clinical biochemistry: hook effect in immunoassays

Sample item

A woman with a large pituitary mass and headache has a serum prolactin reported as mildly raised. Her symptoms suggest a much higher level. The laboratory repeats the assay after diluting the serum 1 in 100 and now reports a very high value. What explains the first result?

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

The answer and the reasoning

C. The high-dose hook effect, in which excess analyte saturates both antibodies of a two-site assay

In a sandwich immunoassay, a very high analyte concentration saturates capture and detection antibodies separately, so sandwiches fail to form and the signal falls. Dilution brings the analyte into the working range and reveals the true level. This is important in giant prolactinomas, where the first result may be falsely low.

Option by option

  • A. Haemolysis of the sample

    Haemolysis does not cause a falsely low prolactin that normalises on dilution.

  • B. Biotin interference causing falsely raised values

    Biotin interference lowers results in sandwich assays but does not give a rise on dilution.

  • C. The high-dose hook effect, in which excess analyte saturates both antibodies of a two-site assay (correct)

    Correct. Antibody saturation by excess analyte lowers the signal, and dilution unmasks the true high value.

  • D. Heterophile antibody interference giving a falsely high result

    Heterophile antibodies usually give falsely high results in sandwich assays and do not explain a falsely low one that rises on dilution.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, The high-dose hook effect, in which excess analyte saturates both antibodies of a two-site assay. In a sandwich immunoassay, a very high analyte concentration saturates capture and detection antibodies separately, so sandwiches fail to form and the signal falls.

Why is "Haemolysis of the sample" not the answer?

Haemolysis does not cause a falsely low prolactin that normalises on dilution.

Why is "Biotin interference causing falsely raised values" not the answer?

Biotin interference lowers results in sandwich assays but does not give a rise on dilution.

Why is "Heterophile antibody interference giving a falsely high result" not the answer?

Heterophile antibodies usually give falsely high results in sandwich assays and do not explain a falsely low one that rises on dilution.

What concept does this question test?

It tests Clinical biochemistry: hook effect in immunoassays in Biochemistry. Dilution brings the analyte into the working range and reveals the true level. This is important in giant prolactinomas, where the first result may be falsely low.

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 Clinical biochemistry: hook effect in immunoassays?

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.