Gynaecological endocrinology: hook effect in prolactin assay: practice question

A OBG 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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OBG · Gynaecological endocrinology: hook effect in prolactin assay

Sample item

A woman with amenorrhoea, headache and bitemporal hemianopia has a 3 cm pituitary macroadenoma, yet serum prolactin is only mildly raised. What is the most appropriate next step?

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

The answer and the reasoning

A. Repeat prolactin on a diluted serum sample

Very high prolactin can saturate both antibodies in a two-site assay and read falsely low (hook effect). Serial dilution of the sample unmasks the true level and a prolactinoma, which is treated medically with a dopamine agonist rather than surgery.

Option by option

  • A. Repeat prolactin on a diluted serum sample (correct)

    Correct. Dilution removes assay saturation and shows the true, very high prolactin.

  • B. Repeat after a fasting overnight sample

    Fasting does not alter an assay artefact caused by antibody saturation.

  • C. Plan surgery for a non-functioning adenoma

    Surgery is premature because a prolactinoma would respond to medical treatment.

  • D. Start oral contraceptives

    Oral contraceptives do not address a macroadenoma threatening the optic chiasm.

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 OBG question

What is the answer to this OBG question?

The best answer is A, Repeat prolactin on a diluted serum sample. Very high prolactin can saturate both antibodies in a two-site assay and read falsely low (hook effect).

Why is "Repeat after a fasting overnight sample" not the answer?

Fasting does not alter an assay artefact caused by antibody saturation.

Why is "Plan surgery for a non-functioning adenoma" not the answer?

Surgery is premature because a prolactinoma would respond to medical treatment.

Why is "Start oral contraceptives" not the answer?

Oral contraceptives do not address a macroadenoma threatening the optic chiasm.

What concept does this question test?

It tests Gynaecological endocrinology: hook effect in prolactin assay in OBG. Serial dilution of the sample unmasks the true level and a prolactinoma, which is treated medically with a dopamine agonist rather than surgery.

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 Gynaecological endocrinology: hook effect in prolactin assay?

Learn management algorithms as steps: PPH, eclampsia, ectopic pregnancy. Then use the flashcards and the other OBG 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.