The Endocrine Society recommends urinary or plasma metanephrines as first choice screening tests for phaeochromocytoma due to their high sensitivity. Test specificity is limited by the influence of many commonly prescribed medications resulting in false positive results and additional investigations. The aims of this retrospective study were to (1) Determine the diagnostic accuracy of urinary metanephrines using current cut-off values and (2) Evaluate if alternative diagnostic thresholds would improve test performance.
Patients who underwent a clonidine suppression test, or had confirmed phaeochromocytoma on histology or imaging were included. All pre-operative urinary metanephrine (MAO) and normetanephrine (NMAO) results were obtained. 168 cases were identified (148 normal, 18 phaeochromocytoma (11 NMAO raised, 7 NMAO + MAO raised). In those with no phaeochromocytoma, MAO was elevated in 15.5% and NMAO in 57.4%. 119 (71.7%) were known to be taking interfering medications. Sensitivity + specificity for MAO at the upper level of normal (ULN) and two-fold elevation beyond ULN were 100% + 84.8% and 100% and 97.9% respectively. Sensitivity + specificity for NMAO at ULN and two-fold elevation beyond ULN were 100% + 41.8% and 66.7% + 95.2% respectively. ROC curve analysis of NMAO results (Area under curve- 0.908 (P<0.001)) identified an alternative higher cut-off with sensitivity + specificity 100% + 62.3% respectively.
Our data demonstrate excellent diagnostic accuracy of MAO using our current reference range but less accuracy in the more commonly elevated NMAO. Application of a higher diagnostic threshold will help reduce excessive investigation attributed to false positive results.