The utility of rested prolactin sampling in the evaluation of hyperprolactinaemia

BACKGROUND AND AIMS: Serum prolactin levels may be elevated by venepuncture stress. We investigated the utility of a rested prolactin sample, obtained through an indwelling venous cannula, in preventing the overdiagnosis of hyperprolactinaemia. METHODS: Patients at our institution undergo serial prolactin sampling, usually over 40 min, when investigating hyperprolactinaemia. We retrospectively reviewed all serial prolactin sampling performed during a 3-year period. Patients with possible medication-induced hyperprolactinaemia and macroprolactin interference were excluded. We assessed the effect of venepuncture-associated stress on hyperprolactinaemia with the main outcome being normalisation of serum prolactin at the end of serial sampling. RESULTS: Ninety-three patients with documented hyperprolactinaemia (range 360-1690 mU/L) were included in the analysis. Prolactin decreased during serial sampling in 73 patients (78%), suggesting a prevalent effect of venepuncture stress. The final prolactin sample was normal in 50 patients (54%), consistent with stress hyperprolactinaemia rather than pathological hyperprolactinaemia. Patients with a referral prolactin result greater than two times the upper reference limit (URL) were less likely (15%) to have a normal prolactin result on serial sampling. Measurement of a single rested prolactin sample from an indwelling cannula showed the same diagnostic utility as serial sampling. CONCLUSION: Serum prolactin results are frequently elevated by the stress of venepuncture. Confirmation of pathological hyperprolactinaemia in a rested sample obtained from an indwelling venous cannula is recommended in patients with mild hyperprolactinaemia, particularly when the referral prolactin is less than two times the URL.