Determination of Ultrafilterable Prolactin: Elimination of Macroprolactin Interference With a Monomeric Prolactin-Selective Sample Pretreatment

Comparison of PEG precipitation and ultrafiltration treatment for serum macroprolactin in Chinese patients with hyperprolactinemia

BACKGROUND

Macroprolactinemia is a common cause of hyperprolactinemia (HPRL), with an average worldwide incidence of 18.9 %. This study aimed to explore the feasibility of ultrafiltration (UF) and polyethylene glycol (PEG) precipitation for macroprolactin screening, as well as the incidence and clinical characteristics of Chinese patients with macroprolactinemia.

METHODS

In this study, 94 patients with HPRL and 206 healthy individuals were included. Gel filtration chromatography (GFC), PEG precipitation, and UF were used to screen for macroprolactin, and chemiluminescence was used to determine the prolactin levels.

RESULTS

The detected incidence of macroprolactinemia in the patients with HPRL was 7.45% (7/94, GFC) and 5.32% (5/94, PEG precipitation). Patients with macroprolactinemia usually present with atypical clinical symptoms, moderately increased prolactin levels, and negative or microadenoma-positive pituitary images. In addition, the recovery of monomeric prolactin by PEG precipitation and UF was significantly correlated to that of GFC (r _PEG=0.493, P<0.001; r _UF=0.226, P=0.014), with a higher correlation coefficient between PEG precipitation and GFC. Furthermore, PEG precipitation had a smaller variation (95% confidence interval [CI]: -35.77% to 18.34%) than UF in monomeric prolactin recovery and substantial diagnostic consistency with GFC (Cohen's kappa coefficient=0.647). The proportion of monomeric prolactin in patients with HPRL did not change significantly between the two visits within one year (P >0.05).

CONCLUSION

The incidence of macroprolactinemia in Chinese patients with HPRL is low in the present study. Based on our analysis, we recommend that only patients who are clinically suspected of having macroprolactinemia should be screened using PEG precipitation.

Ultrafiltration - an alternative method to polyethylene glycol precipitation for macroprolactin detection

INTRODUCTION

The aim of the study was to evaluate two methods of macroprolactin (MaPRL) detection - precipitation with polyethylene glycol (PEG) and ultrafiltration and to compare these techniques with "gold standard" - gel filtration chromatography (GFC).

MATERIAL AND METHODS

The study was conducted on 245 patients - 45 with organic and 200 with functional hyperprolactinaemia. In all the subjects MaPRL was detected by precipitation with PEG and ultrafiltration. Additionally, gel filtration chromatography was performed in some of the serum samples.

RESULTS

Macroprolactinaemia was detected in 27 patients - 8 with prolactinoma and 19 with functional hyperprolactinaemia. Assessing positive and negative results for MaPRL, we observed high diagnostic agreement (95.9%) and positive correlation (r = 0.506, p < 0.001) between the methods. The results of precipitation and ultrafiltration positive for MaPRL were concordant in 63%. The dominance of MaPRL detected with precipitation and/or ultrafiltration was confirmed by GFC in 76% of cases (all patients with functional hyperprolactinaemia). Among 6 examined patients with prolactinoma, GFC showed four false-positive results - 1 case of precipitation and 3 cases of ultrafiltration.

CONCLUSIONS

Efficacy of MaPRL detection with precipitation and ultrafiltration is comparable especially in cases of functional hyperprolactinaemia. In patients with prolactinoma, precipitation seems to be a more efficient separation method.

Macroprolactinemia: new insights in hyperprolactinemia

Hypersecretion of prolactin by lactotroph cells of the anterior pituitary may lead to hyperprolactinemia in physiological, pathological and idiopathic conditions. Most patients with idiopathic hyperprolactinemia may have radiologically undetected microprolactinomas, but some may present other causes of hyperprolactinemia described as macroprolactinemia. This condition corresponds to the predominance of higher molecular mass prolactin forms (big-big prolactin, MW > 150 kDa), that have been postulated to represent prolactin monomer complexed with anti-prolactin immunoglobulins or autoantibodies. The prevalence of macroprolactinemia in hyperprolactinemic populations between 15–46% has been reported. In the pathophysiology of macroprolactinemia it seems that pituitary prolactin has antigenicity, leading to the production of anti-prolactin autoantibodies, and these antibodies reduce prolactin bioactivity and delay prolactin clearance. Antibody-bound prolactin is big enough to be confined to vascular spaces, and therefore macroprolactinemia develops due to the delayed clearance of prolactin rather than increased production. Although the clinical symptoms are less frequent in macroprolactinemic patients, they could not be diff erentiated from true hyperprolactinemic patients, on the basis of clinical features alone. Although gel filtration chromatography (GFC) is known to be the gold standard for detecting macroprolactin, the polyethylene glycol precipitation (PEG) method has off ered a simple, cheap, and highly suitable alternative. In conclusion, macroprolactinemia can be considered a benign condition with low incidence of clinical symptoms and therefore hormonal and imaging investigations as well as medical or surgical treatment and prolonged follow-up are not necessary.

Hyperprolactinemia and prolactinomas

Any process interfering with dopamine synthesis, its transport to the pituitary gland, or its action at the level of lactotroph dopamine receptors can cause hyperprolactinemia. As described in this article, considering the complexity of prolactin regulation, many factors could cause hyperprolactinemia, and hyperprolactinemia can have clinical effects not only on the reproductive axis. Once any drug effects are excluded, prolactinomas are the most common cause of hyperprolactinemia. The most frequent symptom is hypogonadism in both genders. Medical and surgical therapies generally have excellent results, and most prolactinomas are well controlled or even cured in some cases.