Background: Urinary catecholamine measurement has been the mainstay for diagnosis of a phaeochromocytoma. Plasma metanephrine estimation has been introduced more recently although its precise position remains unclear. We have undertaken a retrospective analysis of patients who have undergone this test to assess its usefulness in the diagnostic process.
Patients and methods: We evaluated all patients who have had plasma metanephrine estimation over the past 4 years. The pre-test clinical and biochemical information and patient outcomes at a minimum of 2-year follow up was correlated with the test result. We also evaluated clinical and biochemical profile of the last 10 consecutive patients who underwent surgery for suspected phaeochromocytoma.
Results: Twenty-three patients had plasma metanephrine estimation. Four patients had typical clinical features with normal multiple urinary catecholamines and 19 patients had raised one or both catecholamines of <3 times upper limit of reference range (ULRR). 22/23 had normal plasma metanephrines. All patients have been stable over a 2-year follow-up period excluding phaeochromocytoma. 1/23 had raised plasma metanephrines with unequivocally positive imaging studies but was unfit for surgery. During the same period, 10 patients had surgery for excision of a suspected phaeochromocytoma. None of these patients had plasma metanephrines estimation. 9/10 had a strong clinical suspicion and urinary adrenaline or noradrenaline >3 times ULRR in at least one urinary sample. Phaeochromocytoma was confirmed histologically in all patients. 10th patient had surgery without biochemical confirmation due to an increase in size of adrenal incidentaloma. Histological examination excluded phaeochromocytoma.
Conclusion: Our results confirm that plasma metanephrines estimation has a high negative predictive value in the diagnosis of a phaeochromocytoma. This test does not appear to have a significant role in patients with high clinical suspicion and unequivocally raised urinary catecholamines. This selective approach would avoid unnecessary use of this expensive and logistically difficult test.