A 27-year-old man present with complaints of loss of libido and erectile dysfunction since a year. On questioning, he gives history of headache, dull aching, global, not associated with nausea or vomiting, usually relieved by painkillers, present on 5 days a week. He has also noticed that sometimes he bumped into objects on his sides while walking. He has also noticed constipation and weight gain. On examination, he is 160 cm tall with a BMI 25 kg/m2. Blood pressure 110/70 mm of Hg, no postural drop. Confrontation test reveals bilateral temporal hemianopia. Testicular volume 15 mL soft bilateral. No galactorrhea is elicited. Ankle jerks delayed. Laboratory investigations are as follows: Hemoglobin 10.5 g/dL, normocytic normochromic anemia. Testosterone 100 ng/dL, LH 0.01 IU/L, FSH 0.01 IU/L, TSH 2 microIU/L, free T4 0.68 ng/dL. Cortisol 2.8 mg/dL. Prolactin 47 ng/mL from one lab and 1780 ng/mL from another lab. Automated perimetry reveal bitemporal hemianopia. Magnetic resonance imaging of the hypothalamic pituitary region reveals 2.4 cm tall, pituitary mass with suprasellar extension with compression of the optic chiasm without parasellar extension. A diagnosis of pituitary macroadenoma probably macroprolactinoma with hypopituitarism is made.
Why is there a discrepancy in the prolactin assay from the laboratory?
Medicine
The probable reason for the discrepancy is the hook effect, an assay artifact that may be seen when high serum prolactin concentrations saturate antibodies in the two-site immunoassay.
The presence of excess amounts of the prolactin in the unbound state, binds to the signaling antibody alone and is washed out without binding to the capture antibody, leading to a false low result. This can be averted by either diluting the sample or by allowing a washout after adding the serum to remove excess unbound prolactin before adding signaling antibody. Many newer assays have higher upper limits of detection, thereby reducing chances of a high dose hook effect. Thus, when prolactin values are not as high as expected, the assay should be repeated after a 1:100 serum sample dilution to overcome a potential hook effect.
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