Prolactin, Pituitary Macroadenoma, Serum

Serum (preferred) or plasma

1.5 mL

1 mL (Note: This volume does not allow for repeat testing.)

Red-top tube, gel-barrier tube, or green-top (lithium heparin) tube. Do not use oxalate, EDTA, or citrate plasma.

Useful for quantifying prolactin in serum specimens where the high-dose hook effect is suspected (e.g., presence of pituitary tumor with symptoms of prolactinoma and lower than expected serum prolactin concentration).

Modestly elevated prolactin may occur in patients with large nonfunctioning adenomas due to decreased dopamine, which inhibits prolactin secretion due to hypothalamic stalk dysfunction.^1,2

In patients receiving therapy with high biotin doses (i.e., >5 mg/day), no sample should be taken until at least eight hours after the last biotin administration.³

As with all tests containing monoclonal mouse antibodies, erroneous findings may be obtained from samples taken from patients who have been treated with monoclonal mouse antibodies or who have received them for diagnostic purposes.³

In rare cases, interference due to extremely high titers of antibodies to ruthenium can occur.³ Extremely high titers of antibodies to streptavidin can occur in isolated cases and cause interference.³

Electrochemiluminescence immunoassay (ECLIA)

Prolactin is measured by electrochemiluminescence immunoassay (ECLIA) with Roche Cobas Prolactin II assay. The Roche Cobas Prolactin II assay demonstrates no high-dose hook effect at prolactin concentrations up to approximately 12,690 ng/mL.³

Samples are tested directly and after 100-fold with sample diluent. A high-dose hook effect is ruled out if the result generated from the diluted sample (after correcting for dilution) is not significantly greater than the result obtain from the undiluted sample.^1,2 The 100-fold dilution will overcome a potential hook effect and can help to distinguish between a large prolactinoma and a large nonfunctioning adenoma.^1,2

• Male: 4.0−15.2 ng/mL

• Female: 4.8−23.3 ng/mL

In general, serum prolactin concentrations parallel tumor size in patients with prolactinomas. Macroadenomas greater than 10 mm in diameter are typically associated with serum prolactin concentrations in excess of 250 ng/mL.^1,2,4-15 Prolactin-secreting macroadenomas can sometimes produce exceedingly high serum prolactin concentrations that may paradoxically result in falsely low prolactin concentrations when measured by immunometric assays.^1,2,4-15 In such situations, very high concentrations of prolactin saturate both the capture and signal antibodies in the assay, block formation of the capture antibody-prolactin-signal antibody "sandwich," and result in falsely decreased prolactin results (referred to as the high-dose hook effect). With such tumors, serum prolactin levels may be falsely decreased into the normal reference interval, potentially resulting in inappropriate patient management. Since the magnitude of analyte concentration in the serum is as a rule proportionate to the size of a secreting tumor, only large prolactinomas and malignant and widely metastatic tumors are likely to present this problem.^1,2,4-15 Dilution of the specimen eliminates the analytic artifact in these cases.^1,2,4-15