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The hook effect in prolactin immunoassays.

Saud N Al Sifri¹, Hussein Raef

¹Department Medicine, King Faisal Specialist Hospital and Research Center, Riyadh, Kingdom of Saudi Arabia.

Saudi Medical Journal

|May 13, 2004

Summary

A rare hook effect in prolactin testing can mask giant pituitary tumors, leading to delayed diagnosis. Diluting serum samples helps identify this phenomenon for accurate pituitary adenoma management.

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Area of Science:

Endocrinology
Oncology
Laboratory Medicine

Background:

Giant sellar and suprasellar tumors can cause modest prolactin elevation due to stalk effect.
Pituitary adenomas are common tumors, but giant invasive ones are rare.

Observation:

A young female patient with a giant invasive pituitary tumor initially presented with mildly elevated prolactin (165 ng/ml).
Two years later, she developed vision deterioration with markedly elevated prolactin (>16000 ng/ml), and tissue staining confirmed prolactinoma.

Findings:

The initial low prolactin level was likely due to a hook effect, a common interference in immunoassays.
This hook effect masked the true extent of the prolactinoma, delaying appropriate treatment.

Implications:

Clinicians should suspect a hook effect in patients with large pituitary/parasellar masses and seemingly low prolactin levels.
Diluting serum samples is crucial for accurate prolactin measurement, enabling timely diagnosis and management of pituitary tumors.

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