HIGH PROLACTIN LEVELS MAY BE MISSED BY IMMUNORADIOMETRIC ASSAY IN PATIENTS WITH MACROPROLACTINOMAS

OBJECTIVE Large amounts of antigen may produce fallow values in immuno radiometric assays due to the so-called high dose, hook effect. The st udy was designed to identify the clinical and biochemical features of patients with pituitary macroadenomas in whom a high dose PRL hook eff ect was documented. DESIGN The clinical and biochemical features of fo ur patients with the high dose PRL hook effect were compared with thos e of 54 patients with pituitary non-functioning adenomas and 11 with m acroprolactinomas who underwent transsphenoidal microsurgery between 1 989 and 1994. MEASUREMENTS The presence of the high dose PRL hook effe ct was confirmed by an increase in the initial PRL concentration when the immunoradiometric assay was processed after dilutions of the serum . This phenomenon was observed in 5.8% (4/69) of patients with pituita ry macroadenomas. Undiluted median (range) PRL levels were 9140 (1530- 83850), 1530 (162-3210) and 2110 mU/I (1470-45000) In patients with ma croprolactinoma, non-functioning macroadenoma and the hook effect, res pectively. In patients with the hook effect, the median PRL levels inc reased to 384 720 (317 520-950 000) mU/I when the assay was performed after serum dilution. The proportion of males was 9.9% (1/11) in the m acroprolactinoma group, 46.3% (25/54) in the non-functioning macroaden oma group and 100% (4/4) in patients with the hook effect. Patients wi th prolactinoma and nonfunctioning adenoma had mean tumour sizes of 20 +/- 9 and 27 +/- 11 mm (SD), respectively, while in the hook effect g roup it was 51 +/- 10 mm. CONCLUSION This study suggests that the high dose PRL hook effect is observed particularly in patients with very l arge tumours. The immunoradiometric PRL assay must be performed with s erum dilution in order to overcome the high dose PRL hook effect in al l new patients with pituitary macroadenomas who may have a prolactinom a.