A ratio of CSF to plasma prolactin was determined for patients with pituitary tumors and for pregnant women. The highest ratio occurred in patients with suprasellar extension (mean, 0.46). Patients with prolactin-secreting pituitary tumors but without suprasellar extension had a much lower mean ratio of 0.125. Pregnant subjects had an intermediate mean ratio of 0.211. Serial plasma and CSF specimens were also obtained during pneumoencephalographic stress from patients with pituitary tumors. The CSF prolactin concentration remained constant in the four patients tested without suprasellar extension despite an increase of the plasma prolactin level. In contrast, two of three patients with suprasellar extension demonstrated an increase of CSF prolactin that occurred with the increase of the plasma prolactin. These findings suggest that prolactin is secreted directly into CSF of patients with suprasellar extension of a pituitary tumor. A CSF to plasma prolactin ratio of 0.2 or greater in a nonpregnant patient or an increase in the CSF prolactin concentration during stimulatory testing is strongly suggestive of suprasellar extension of a pituitary tumor.
(Arch Intern Med 139:208-211, 1979)