Plasma‐Free Metanephrine and Free Normetanephrine Measurement for the Diagnosis of Pheochromocytoma in Dogs

Background Measurement of plasma‐free metanephrines is the test of choice to identify pheochromocytoma in human patients. Objectives To establish the sensitivity and specificity of plasma‐free metanephrine ( fMN ) and free normetanephrine ( fNMN ) concentrations to diagnose pheochromocytoma in dogs. Animals Forty‐five client‐owned dogs (8 dogs with pheochromocytoma, 11 dogs with adrenocortical tumors, 15 dogs with nonadrenal disease, and 11 healthy dogs.) Methods A prospective study. EDTA plasma was collected from diseased and healthy dogs and submitted for fMN and fNMN measurement by liquid chromatography‐tandem mass spectrometry (LC‐MS/MS). Results Free MN concentration (median [range]) was significantly higher in dogs with pheochromocytoma (8.15 [1.73–175.23] nmol/L) than in healthy dogs (0.95 [0.68–3.08] nmol/L; P  < .01) and dogs with adrenocortical tumors (0.92 [0.25–2.51] nmol/L; P  < .001), but was not different from dogs with nonadrenal disease (1.91 [0.41–6.57] nmol/L; P  ≥ .05). Free NMN concentration was significantly higher in dogs with pheochromocytoma (63.89 [10.19–190.31] nmol/L) than in healthy dogs (2.54 [1.59–4.17] nmol/L; P  < .001), dogs with nonadrenal disease (3.30 [1.30–10.10] nmol/L; P  < .001), and dogs with adrenocortical tumors (2.96 [1.92–5.01] nmol/L); P  < 0.01). When used to diagnose pheochromocytoma, a fMN concentration of 4.18 nmol/L had a sensitivity of 62.5% and specificity of 97.3%, and a fNMN concentration of 5.52 nmol/L had a sensitivity of 100% and specificity of 97.6%. Conclusions and Clinical Importance Plasma fNMN concentration has excellent sensitivity and specificity for the diagnosis of pheochromocytoma in dogs, whereas fMN concentration has moderate sensitivity and excellent specificity. Measurement of plasma‐free metanephrines provides an effective, noninvasive, means of identifying dogs with pheochromocytoma.