Investigations
1st investigations to order
serum prolactin
Test
Blood sample should be collected at any time of day with minimal venepuncture stress.
In asymptomatic patients with a high serum prolactin, macroprolactin should be measured. The majority (85%) of circulating prolactin is monomeric. Macroprolactin is a polymeric form, also known as 'big prolactin', which represents less than 5% of circulating prolactin. It consists of an antigen-antibody complex of monomeric prolactin and immunoglobulin G (IgG). Macroprolactin has limited bioavailability and bioactivity. Standard laboratory prolactin immunoassays do not reliably detect macroprolactin, and its presence needs to be confirmed by other methods, such as polyethylene glycol precipitation. In prolactinomas, serum prolactin concentration usually correlates with tumour size. In large (giant) macroprolactinomas, serum prolactin concentrations may be so high that the antibodies used in immunoradiometric assays to measure prolactin are saturated, preventing formation of the prolactin antibody sandwich. The resultant loss of labelled antibody leads to falsely low values of prolactin. This assay artefact is termed the 'hook effect' and can be overcome by performing a 1:100 serum sample dilution. Elimination of a possible hook effect by dilution of serum prolactin samples is recommended where there is a discrepancy between tumour size and serum prolactin concentration.[11]
Result
elevated
pituitary MRI
Test
A gadolinium-enhanced MRI of the pituitary is able to detect small microadenomas, and define the extension of invasive macroadenomas. Because approximately 12% of the normal population have asymptomatic pituitary adenomas, it is important to confirm a pathological elevation of prolactin prior to performing pituitary imaging.[11]
Result
characteristic features of pituitary adenoma
computerised visual-field examination
Test
Performed in all patients with macroprolactinomas, particularly those with suprasellar extension and chiasmal compression.
Result
may reveal unilateral or bi-temporal hemianopia
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