Introduction: Macroprolactin is a physiologically inactive form of prolactin, usually composed of a prolactin monomer and an IgG or anti-prolactin antibody molecule. Whilst clinically non-reactive, it interferes with immunological assays used for prolactin detection. It is identified by polyethylene glycol (PEG) precipitation, and levels of macroprolactin are generally believed to remain stable over time.
Case: We present the case of a 36-year-old gentleman who was referred for hyperprolactinaemia and investigation of possible secondary hypogonadism. He reported symptoms of fatigue and erectile dysfunction, which prompted his general practitioner to measure testosterone, prolactin, and macroprolactin on three occasions. Macroprolactin estimations are carried out routinely in our department on samples with a raised prolactin level (>350 mIU/l in men), unless two previous estimations have already been carried out within the last year. As the blood tests were requested via primary care, a macroprolactin estimation was carried out on each sample. An initial prolactin level was mildly elevated at 832 mIU/l, with a negative macroprolactin and a testosterone level at the lower end of the normal range. However, on repeat testing 1 month later, his prolactin was mildly raised at 432 mIU/l, but with ~80% of the prolactin immunoreactivity in the sample being due to the presence of macroprolactin. Seven months later his tests were again repeated and his macroprolactin was again found to be negative.
Discussion: This case highlights that macroprolactin titres can vary over time in some individuals, which could be significant in determining whether a prolactin level is genuinely raised. Furthermore, it is important to consider that macroprolactin can co-exist with genuine hyperprolactinaemia.2 Thus some authors have suggested that PEG precipitation be considered prior to every prolactin measurement.3 Nevertheless this case highlights that a repeat macroprolactin estimation should be considered if measured prolactin levels do not correlate well with a patients clinical presentation.