Endocrine Abstracts

Background: The diagnosis of pheochromocytoma and paraganglioma (PPGL) requires the measurement of plasma metanephrines; however, several pre-analytical parameters can lead to false positive test results. During venipuncture, pain perception may activate the sympathetic nervous system, increasing catecholamine metabolite levels. Furthermore, hypertension patients exhibit sympathetic hyperactivity compared to normotensive subjects. The purpose of this study was to compare the plasma levels of metanephrines obtained by direct venipuncture to those obtained by an indwelling intravenous cannula in hypertensive patients, as well as the effect of pain intensity on plasma levels of metanephrines.

Methods: Sixty-eight hypertensive patients, 17 men and 51 women, with a mean age of 55 years (range 31-77 years) were enrolled in the study. After at least 30 minutes of supine rest, we collected blood samples through an indwelling intravenous cannula and then promptly via direct venipuncture on the contralateral arm. A visual analogue scale was used to gauge the level of pain experienced during blood collection. Plasma concentrations of free metanephrines, which included plasma free metanephrine, normetanephrine, and 3-methoxytyramine, were determined by the liquid chromatography-tandem mass spectrometry method.

Results: Plasma normetanephrine levels obtained by direct venipuncture were significantly higher than those obtained by an indwelling intravenous cannula (mean difference 4.14 pg/mL; 95% CI 0.22 to 8.06; p-value 0.039 and mean percentage difference 8.06%; 95% CI 2.44 to 13.69%; p-value 0.006). However, the sample results were within the reference range. On the contrary, plasma levels of 3-methoxytyramine and metanephrine did not differ substantially between the two sampling techniques (mean difference 1.26 ng/L, 95%CI -0.44 to 2.95 and -0.98 pg/mL; 95%CI -2.52 to 0.56, respectively). Furthermore, there was no correlation between pain score and variation in plasma metanephrine levels.

Conclusions: This study proposes an optimal sample collection route using an indwelling intravenous cannula for the diagnosis of PPGL among hypertensive patients. Despite the minimal differences, clinicians should be aware that this pre-analytical component could cause incorrect diagnoses and unnecessary investigations.