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Determination of ultrafilterable prolactin: elimination of macroprolactin interference with a monomeric prolactin-selective sample pretreatment.

CONTEXT
Macroprolactin (macroPRL), present in as many as 25% of serum specimens with elevated serum prolactin concentrations, can cause apparent hyperprolactinemia in the absence of clinical features and lead to unnecessary clinical, laboratory, and neuroradiological workups.


OBJECTIVE
To develop an ultrafiltration method that eliminates macroPRL interference from PRL immunoassays.


DESIGN
The method involves centrifugation of undiluted serum in a Centricon-100 filter device followed by a PRL assay of the serum ultrafiltrate.


RESULTS
Ultrafiltrates prepared by this technique are devoid of gamma globulins and contain (mean +/- SE) 19% +/- 7% of the albumin concentration of the original serum. These ultrafiltrates contain 85% +/- 7% of the total PRL immunoreactivity of serum spiked with 23 kd recombinant human prolactin (rHuPRL) and less than 2% of the 50 kd big PRL (bPRL) of whole serum. The fractional recovery of ultrafilterable PRL (uPRL) from serum samples of 54 female patients was 0.78 (confidence interval 0.73-0.83) of the total. The run-to-run coefficient of variation of the uPRL assay was 4.3%. The uPRL concentration (mean +/- SD) in a group of healthy female controls was 8.0 +/- 3.1 ng/mL.


CONCLUSIONS
Ultrafiltration is a rapid and simple method for eliminating analytical interference by macroPRL. Ultrafiltrates can be analyzed by most, if not all, currently available PRL immunoassays and represent a practical and precise alternative to gel filtration chromatography for the estimation of the monomeric prolactin concentration of serum.