Sarah L Pitkin, Francis Lam, Sachith Abhayaratna, Stephanie E Baldeweg & Anne Dawnay
UCL Hospitals NHS Foundation Trust
The utility of reporting the dopamine metabolite 3-methoxytyramine (3-MT) in plasma free metanephrines profiles has been questioned due to analytical and clinical concerns. We developed and validated an LC-MS/MS assay for plasma free metanephrines, including 3-MT, and reviewed the analytical and clinical utility of reporting 3-MT over a 1 year period. Performance of the assay in an external quality assurance scheme was excellent. Clinically, 3-MT was found to be a valuable addition to the profile, providing a useful tumour marker for long term monitoring in one patient. Cabergoline therapy was found to decrease plasma 3-MT, whereas levodopa raised it.
Plasma free metanephrines profiles usually include normetanephrine and metanephrine, but may also include 3-methoxytyramine (3-MT). The utility of reporting this analyte has been called in to question by some, but encouraged by others. We have reviewed the analytical quality and clinical utility of 3-MT.
Blood was collected (K2-EDTA tubes on ice) and plasma separated and frozen within 2 hours. Internal standards (normetanephrine-d3, metanephrine-d3 and 3-methoxytyramine-d3) were added and samples solid phase extracted using Oasis WCX uElution plates on a Tecan automated liquid handling system. The eluent was injected on to a Waters ACQUITY UPLC-TQD mass spectrometer. Gradient elution on an ACQUITY BEH amide column was followed by quantification by electrospray ionisation mass spectrometry in multiple reaction monitoring mode, with a run time of 5 minutes.
Over two cycles (12 months), the assay was consistently within the acceptable performance limits of the RCPAQAP plasma metanephrines EQA scheme for 3-MT. The total error was consistently below the total allowable error [(2SD + bias)/allowable limit <1]. The coefficient of variation was 5.6-8.8% with an average bias of 47.1-111.0 pmol/L.
Between 01/01/14-31/12/14 301 samples (258 patients) were analysed. 3-MT was elevated in 8 patients (14 samples), 5 of whom also had a raised normetanephrine and subsequently had phaeochromocytoma (n=3) or extra-adrenal paraganglioma (n=2) confirmed by histology. One patient had borderline raised 3-MT and normetanephrine, which normalised on repeat. One patient had an isolated raised 3-MT due to levodopa therapy. The remaining patient had an isolated raised 3-MT of 1014 pmol/L (< 180), with a history of SDHC mutation, glomus jugulare tumour and prolactin-secreting macroadenoma. Drug therapy was cabergoline (dopamine D2 receptor agonist) only. 3-MT increased 1.9-fold after cabergoline therapy withdrawal and returned to previous levels once cabergoline was re-started. It was concluded that residual glomus jugulare tumour in this patient was secreting 3-MT, which could be used to monitor the tumour subsequently.
3-MT was found to be a valuable addition to the plasma metanephrines profile, with robust analytical performance. It provided a useful tumour marker for long term monitoring in one patient. Cabergoline therapy was found to decrease plasma 3-MT. Care must be taken to exclude levodopa as a cause of raised 3-MT.