Utility of Cannulated Prolactin to Exclude Stress Hyperprolactinemia in Patients with Persistent Mild Hyperprolactinemia

BACKGROUND: Stress-induced hyperprolactinemia can be difficult to differentiate from true hyperprolactinema and may result in patients having unnecessary investigations and imaging. We report the results of cannulated prolactin tests with serial prolactin measurements from an indwelling catheter to differentiate true from stress-induced hyperprolactinemia in patients with persistently mildly elevated prolactin levels in both referral and repeat samples. METHODS: Data were collected for 42 patients who had a cannulated prolactin test between January 2017 and May 2018. After cannula insertion, prolactin was measured at 0, 60, and 120 minutes. Normalization is defined as a decline in prolactin to gender-defined normal ranges. RESULTS: The mean age was 33.8 years (SD ± 9.9), and 37 (88%) were female. Menstrual irregularities were the main presenting symptom in 28.57% of the patients. Prolactin normalized in 12 (28.6%) patients of whom cannulated prolactin test was done. Repeat random prolactin levels were significantly higher in patients whose prolactin did not normalize during the cannulated prolactin test. MRI of the pituitary gland showed an abnormality in 23 out of 28 (82%) patients who did not normalize prolactin, a microadenoma in the majority of patients (18 patients). CONCLUSION: The cannulated prolactin test was useful in excluding true hyperprolactinemia in 28.6% of patients with previously confirmed mildly elevated random prolactin on two occasions, thus avoiding over-diagnosis and unnecessary imaging.