The urinary metanephrine-to-creatinine ratio for the diagnosis of pheochromocytoma

Objective: To compare the operating characteristics of two tests for diagnosing pheochromocytoma: 1) measurement of the ratio between urinary metanephrine and creatinine levels and 2) measurement of urinary metanephrine levels alone. A second objective was to ascertain the reasons for false-positive test results. Design: Cross-sectional study. Setting: Hypertension referral center. Patients: 1013 patients referred for hypertension and tested for pheochromocytoma. Measurements: 24-hour urinary levels of metanephrine (measured using liquid chromatography) and creatinine. The presence of pheochromocytoma was confirmed at surgery. In patients with positive test results, the absence of pheochromocytoma was documented by negative results of retests and imaging procedures. Results: Of 58 patients with increased metanephrine levels or increased metanephrine-to-creatinine ratios, 20 had pheochromocytoma and 38 did not. Of the 38 patients without pheochromocytoma, 15 had high metanephrine levels but normal metanephrine-to-creatinine ratios. The respective operating characteristics of measurement of urinary metanephrine levels and measurement of the metanephrine-to-creatinine ratio were as follows: sensitivity, 95% and 100%; specificity, 98% and 98%; positive predictive value, 46% and 47%; and negative predictive value, 100% and 100%. In 13 of the 23 patients who had a high metanephrine-to-creatinine ratio, various acute events may have caused hypersecretion of catecholamines. Conclusions: Measurement of the metanephrine-to-creatinine ratio is a sensitive and specific test for pheochromocytoma, However, acute events may increase urinary metanephrine excretion to the level that occurs with tumors.