How to collect plasma metanephrine?

From the Guidelines

To collect plasma metanephrine, blood should be drawn into EDTA or heparin tubes, filled until the vacuum is exhausted, and then gently inverted/mixed 8-10 times, before being placed upright on a rack until centrifugation, as recommended by the most recent guidelines 1. When collecting plasma metanephrine, it is essential to follow proper procedures to ensure accurate results.

• The blood collection tubes should be filled until the vacuum is exhausted to prevent hemolysis and ensure adequate sample volume.
• The tubes should be gently inverted/mixed 8-10 times to prevent clotting and ensure proper mixing of the blood with the anticoagulant.
• The tubes should be placed upright on a rack until centrifugation to prevent hemolysis and ensure proper separation of the plasma from the blood cells. It is also crucial to consider other factors that may affect the results, such as:
• Avoiding interfering agents, such as certain foods and medications, that can affect metanephrine levels 1.
• Ensuring the patient has been lying supine for 30 minutes before blood collection to establish baseline levels 1.
• Using an indwelling venous catheter for blood collection to minimize stress and physical exertion, which can affect metanephrine levels 1. By following these guidelines and considering these factors, healthcare providers can ensure accurate and reliable plasma metanephrine results, which are essential for diagnosing and managing conditions like pheochromocytoma or paraganglioma.

From the Research

Blood Sampling Methods

• Blood sampling for plasma metanephrines can be done using either venipuncture or an indwelling intravenous cannula, with the latter being the preferred method 2
• Studies have shown that venipuncture can result in higher concentrations of plasma free metanephrines, normetanephrine, and epinephrine compared to sampling using an indwelling cannula 2
• However, the differences in concentrations between the two methods are small, and venipuncture may be justifiable for most patients 2

Patient Position and Sampling Conditions

• Sampling of blood in the supine position is recommended to minimize false-positive results for plasma metanephrines 3, 4
• Outpatient sampling can result in higher plasma concentrations and false-positive results for normetanephrine compared to inpatient sampling 3
• Low temperature, semi-recumbent position, and direct venipuncture can also lead to higher plasma concentrations and false-positive results for plasma normetanephrine 3

Preanalytical Considerations

• Reference intervals for plasma metanephrines should be established for blood samples collected after 30 minutes of supine rest and after an overnight fast 4
• Age-adjusted reference intervals can help minimize false-positive results 4
• The use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) is recommended for the analysis of plasma metanephrines due to its high sensitivity, specificity, and throughput 5

Diagnostic Protocols

• Standardized diagnostic protocols for plasma-free metanephrines can ensure the highest diagnostic accuracy for the investigation of pheochromocytoma and paraganglioma 6
• The use of supine reference intervals for result interpretation with fasted/seated-sampling protocols can improve diagnostic performance and reduce false-positive results 6