How do I interpret the results of plasma free metanephrines in the diagnosis of phaeochromocytoma?

Interpreting plasma free metanephrines for diagnosing phaeochromocytoma involves assessing both the absolute levels and the clinical context. Elevated plasma free metanephrines are highly sensitive markers for phaeochromocytoma, as these metabolites reflect continuous catecholamine production by the tumour, making them superior to direct catecholamine measurement for initial screening ¹.

Results significantly above the upper reference limit strongly suggest phaeochromocytoma, warranting further confirmatory testing or imaging. Mild or borderline elevations require cautious interpretation, as false positives can occur due to stress, medications, or other conditions; thus, clinical correlation and repeat testing under controlled conditions may be necessary ¹.

Analytical methods such as liquid chromatography tandem mass spectrometry (LC-MS/MS) provide high specificity and sensitivity for plasma metanephrines, reducing false positives compared to older assays. This method is now considered the gold standard for measurement, improving diagnostic accuracy (Davies and Davison, 2019).

In summary, plasma free metanephrine results should be interpreted as follows: markedly elevated levels are diagnostic of phaeochromocytoma in the appropriate clinical setting; borderline elevations require careful evaluation of pre-analytical factors and possible repeat testing; and normal levels effectively exclude the diagnosis in most cases ¹ (Davies and Davison, 2019).