Observational studies on macroprolactin in a routine clinical laboratory

Background: It is now recommended that all samples with raised prolactin should be examined for the presence of macroprolactin. We performed a retrospective review of our experience of macroprolactin to determine the incidence and the natural history of macroprolactin. Methods: A retrospective study of macroprolactin was made in a large clinical laboratory. Macroprolactin was measured on those samples where it is requested and where the total prolactin is >1000 mIU/L. Prolactin was measured using the Siemens Centaur and macroprolactin was measured following polyethylene glycol (PEG)-precipitation. Results: The incidence of macroprolactin in samples where the total prolactin was >1000 mIU/L was 36/670 (5.4%). During this period, 12,064 samples were received for prolactin analysis. Over the period since 2006, 22 subjects had a sample with an isolated macroprolactin measurement followed by another sample without macroprolactin after a median period of 0.46 years. Twenty-five subjects had multiple consecutive measurements of macroprolactin lasting a median period of 2.1 years. Fourteen subjects had more than six samples which had been subjected to PEG precipitation. In these subjects, the reproducibility of PEG precipitation over a median of 6 years was 1.1% CV (recovery 75% [26- 110] (median [range])). Conclusions: The presence of macroprolactin can change over time and we cannot advise that once a test for macroprolactinemia has been performed that it is not necessary to repeat the investigation if a subsequent sample is hyperprolactinemic; nor can one assume that macroprolactin will not develop even if it has been excluded previously.