ABSTRACT
BACKGROUND: Large amounts of antigen may produce false low values on immunoradiometric assays due to the so-called high-dose hook effect. The physicians' awareness of the possibility of the "high-dose hook effect" will prevent preoperative misdiagnosis. The study was designed to identify the frequency and clinical features of patients with pituitary macroadenomas in whom a high-dose PRL hook effect was documented. METHODS: Our retrospective study involved 42 patients with non-functioning pituitary adenomas (tumor diameter >30mm) who underwent transsphenoidal microsurgery from between Jan 1999 to Aug 2004, and 6 patients with non-functioning pituitary adenoma(tumor diameter>30mm) were selected for prospective study from Sep 2003 to Feb 2004. Our retrospective study also involved 13 patients with macroprolactinoma for the comparison of the clinical features. RESULTS: 1) The presence of a high-dose hook effect was retrospectively suggested when the PRL levels increased in 4 out of the 42 patients with non- functioning adenomas(tumor diameter >30mm) after surgery. Post-operative immunohistochemical staining of their pituitary specimens revealed the tumors to be prolactinoma. 2) Prospectively, dilution testing of the specimens obtained before surgery was done in the 6 patients, and one patient presented with a case of the hook effect. The patient's prolactin level was measured at 53.1ng/mL before dilution and this was increased up to 22,600ng/mL upon the 1:1000 dilution. 3) Conclusively, the hook effect was seen in 5 of the 48 patients(10.4%) with non-functioning pituitary adenoma(tumor diameter >30mm) 4) Compared with other 2 patient groups(the macroprolactinoma(N=13) group, and the non-functioning pituitary tumor(N=43) group), the high-dose PRL hook effect is more likely to be observed in male patients with large pituitary tumors. CONCLUSION: In order to avoid the high-dose hook effect, PRL should be assayed at 1:100~1:200 or even higher dilutions of serum from all patients(and especially the male patients) with large pituitary tumors