RELIABILITY OF SERUM PROLACTIN MEASUREMENTS IN WOMEN

Prolactin, a hormone indispensable for milk secretion, has been shown to enhance the development and growth of mammary tumors in rodents; ho wever, its importance in human breast cancer is uncertain. Serum prola ctin levels are known to fluctuate considerably under normal condition s, and lack of precision in the hormone measurements may have contribu ted to the largely negative findings in humans to date. The purpose of this study was to investigate the reliability of prolactin measuremen ts in women using stored serum from an ongoing prospective study of br east cancer. Separate groups of postmenopausal and premenopausal women who donated multiple blood samples at approximately 1-year intervals were studied. The reliability of a single log prolactin determination, as measured by the intraclass correlation coefficient, was 0.76 for t he postmenopausal women (95% confidence interval, 0.66-0.85) and 0.48 for the premenopausal women (95% confidence interval, 0.31-0.62). Thes e findings suggest that a single measurement is sufficient to characte rize the serum prolactin level of postmenopausal women for epidemiolog ical research. For premenopausal women, however, multiple samples are desirable. Controlling for phase of the menstrual cycle does not appea r to substantially improve the reliability of premenopausal measuremen ts.