Macroprolactinemia in patients with hyperprolactinemia: an experience from a single tertiary center

Macroprolactinemia frequently causes misdiagnosis, unnecessary investigation and inappropriate treatment in hyperprolactinemic patients. Aim of this study is to investigate the prevalence and clinical characteristics of Thai patients with macroprolactinemia. We performed a cross-sectional study in 56 hyperprolactinemic patients (51 women and 5 men) whose sera were subsequently tested for the presence of macroprolactin. Recovery of less than 40% of serum prolactin after polyethylene glycol (PEG) precipitation was indicative of macroprolactinemia. Our study revealed 19.64% (11/56) of patients with hyperprolactinemia were found to have a preponderance of macroprolactin. All patients with macroprolactinemia were women, of which eight of them were initially diagnosed as idiopathic hyperprolactinemia and mistreated with dopamine agonist medications. Interestingly, neuroradiological abnormalities were reported in three patients with macroprolactinemia, 2 cases with prolactinoma and one case with stalk effect hyperprolactinemia. In conclusion, nearly one-fifth of our patients with hyperprolactinemia have macroprolactinemia. This finding suggests that the diagnostic algorithm of all patients with hyperprolactinemia should include the PEG precipitation test as the initial step. Domain: Endocrinology


Introduction
Hyperprolactinemia is a common endocrine problem in general practice. Clinically, it manifests mainly as reproductive and sexual dysfunction including amenorrhea or oligomenorrhea, galactorrhea and infertility in women or loss of libido in men [1,2]. Etiologies of hyperprolactinemia can be classified into three categories: physiologic, pharmacologic, and pathologic. Prolactin (PRL) circulates in human sera in three major forms according to their molecular size; monomeric (23 kDa), big or dimeric (45-60 kDa) and big-big (150-170 kDa) or macroprolactin. Monomeric form is the most prominent form (85-95%) of the circulating PRL and is known to be biologically and immunologically active. Macroprolactin accounts for less than 5% of circulating PRL and can interfere with all currently available commercial PRL immunoassays leading to falsely elevated PRL levels in terms of macroprolactinemia1. Several recommendations include the determination of macroprolactin as the first in the differential diagnosis of hyperprolactinemia [2][3][4][5]. A simple and inexpensive method using polyethylene glycol (PEG), precipitation has been accepted as a screening test to identify the presence of macroprolactin in serum instead of gel filtration chromatography which is too timeconsuming and expensive [6,7]. The aim of this study is to investigate the prevalence of macroprolactinemia in hyperprolactinemic Thai patients by using PEG precipitation, and to determine the clinical and neuroradiological features of affected individuals.

Methods
Patients with hyperprolactinemia or PRL levels of more than 25 ng/mL were followed by the Endocrine Clinic, King Chulalongkorn Memorial Hospital, Chulalongkorn University, Bangkok, Thailand were retrospectively evaluated. Patients with lactation or pregnancy were excluded. Their sera were centrifuged and stored at -80°C for subsequent determination of macroprolactin. Complete medical data of the patients were reviewed including presenting signs and symptoms related to hyperprolactinemia such as menstrual irregularities, infertility or galactorrhea in women or impotence in men, the presence of headache and visual loss, physical examination, hormonal and imaging study and their treatment. Imaging data were studied by maximum diameter and invasion of tumor from magnetic resonance imaging (MRI) study. Tumor size was classified as microadenoma (< 1 cm) and macroadenoma. The study was approved by the Ethical Committee of Chulalongkorn University and written informed consent was obtained from each subject. Serum PRL was measured by the Abbott Architect LN 7K76 prolactin chemiluminescent microparticle immunoassay (Abbott Laboratories, USA) on an Abbott Architect i2000 SR platform. PEG precipitation tests were used as method for detection of macroprolactin [8]. In brief, to perform PEG precipitation, equal volumes (200 µL) of a 25% solution of PEG (molecular weight 6,000 kDa) and patient´s serum were mixed and centrifuged at 1,500g for 30 min. Immunoreactive PRL was measured in the supernatant, and the results after correction for dilution were compared with those obtained from unprecipitated serum. The results were expressed as the percent of PRL recovered. Recovery less than or equal to 40% of initial PRL value was taken as evidence that a significant level of macroprolactin was present in the serum [7,8].
The within-assay and inter-assay CV for PRL recoveries were 2.7% and 3.7%, respectively.
Their serum PRL levels in untreated sera ranged from 35.5 to 7.350 ng/mL. Clinical characteristics of the patients are shown in Table 1.
Menstrual abnormalities and galactorrhea were common presentations in women while mass effect and loss of libido were most common in male patients. Prolactinoma (37 cases) and idiopathic hyperprolactinemia -hyperprolactinemia with normal imaging study ( (Table   3). Interestingly, all patients with macroprolactinemia which were initially diagnosed as idiopathic hyperprolactinemia and treated with dopamine agonist medications.

Discussion
PRL measurement is one common laboratory test used in clinical practice, particularly in the reproductive field [5]. Errors in PRL measurement from falsely lowered levels or hook effect and falsely elevated levels or macroprolactinemia result in unnecessary investigation and inappropriate treatment in hyperprolactinemic patients [1,9]. Our study demonstrates about at least one-fifth of

Conclusion
Our study reveals that nearly 20% of our patients with

Competing interests
The authors declare no competing interests.
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Authors' contributions
All the authors contributed to the conduct of this work. All authors also state that they have read and approved the final version of the manuscript.