Macroprolactinaemia in patients with hyperprolactinaemia: composition of macroprolactin and stability during long-term follow-up.

OBJECTIVE

Macroprolactin, which comprises immunoglobulin G (IgG) or anti-prolactin (PRL) autoantibody-bound PRL, is one of the causes of hyperprolactinaemia. This study evaluated the composition and stability of macroprolactin and determined whether the condition was long lasting. We also investigated whether we could predict the composition of macroprolactin based on the ratio of polyethylene glycol (PEG)-precipitable PRL.

PATIENTS: Two hundred and ninety-two patients with hyperprolactinaemia (252 women and 40 men).

DESIGN

We used PEG precipitation to screen for macroprolactinaemia in 625 serum samples from 292 patients with hyperprolactinaemia visiting a city hospital over the last 5·5 years. Macroprolactinaemia was defined by PEG-precipitable PRL ratio greater than 60%. IgG-bound PRL and anti-PRL autoantibodies were measured by protein G and (125) I-hPRL binding studies, respectively. We also examined the autoantibody specificity and binding characteristics.

RESULTS

In 39 patients with macroprolactinaemia, IgG-bound PRL was present in 87% and anti-PRL autoantibodies in 67%. A non-IgG-bound form of macroprolactin was found mainly in sera with marginally elevated PEG-precipitable PRL. The higher the PEG-precipitable PRL ratio, the greater the likelihood that autoantibodies were involved in the composition of macroprolactin. The autoantibodies were IgG type and specific to human PRL and had a low affinity and high capacity. Long-term follow-up (2-17 years) revealed that the ratios of PEG-precipitable PRL, IgG-bound PRL and anti-PRL autoantibody-bound PRL were relatively stable.

CONCLUSIONS

This study showed that higher PEG-precipitable PRL ratio in macroprolactinaemic sera might preferentially indicate the presence of anti-PRL autoantibodies and that macroprolactinaemia might be a long-lasting condition.