From the Guidelines
A normal plasma metanephrines test is typically considered to be less than 2 times the upper limit of the reference range, with exact values varying by laboratory, and requires careful interpretation based on clinical context and potential interfering factors. When interpreting plasma metanephrines results, it's crucial to consider the clinical context and potential interfering factors, as outlined in a study published in Clinical Cancer Research in 2017 1. The study provides guidance on the interpretation of plasma metanephrines results, including:
- If levels are greater than 4 times the upper limit of the reference range, it is consistent with disease, and imaging should be performed to localize the lesion.
- If levels are between 2-4 times the upper limit of the reference range, testing should be repeated in 2 months.
- If levels are marginally elevated, testing should be repeated in 6 months or a clonidine suppression test should be considered to exclude false positivity. Key factors to consider when interpreting plasma metanephrines results include:
- Avoiding interfering agents prior to testing, such as certain medications and substances that can affect catecholamine levels.
- Ensuring proper sample handling and testing procedures to minimize the risk of false positives.
- Considering alternative testing methods, such as 24-hour urine fractionated metanephrines, in certain situations, as noted in the study 1.
From the Research
Normal Plasma Metanephrines Test
- The normal range for plasma metanephrines is not explicitly stated in the provided studies, but it is mentioned that an elevation of fractionated plasma or urinary metanephrine (MN) or nor-metanephrine (NMN) >4-fold above the upper limit of normal (ULN) is usually considered to be diagnostic for pheochromocytoma (PHEO) 2.
- Plasma free metanephrines are relatively independent of renal function and are more suitable for diagnosis of pheochromocytoma among patients with renal failure than measurements of deconjugated metanephrines 3.
- The use of antihypertensive agents such as β-blockers, thiazide diuretics, and angiotensin-converting enzyme (ACE) inhibitors does not affect the measurement of plasma free metanephrines 4.
- The studies do not provide a specific normal range for plasma metanephrines, but they suggest that modest elevations (<4-fold ULN) of metanephrines may be associated with false positive results, especially in patients with adrenal masses >5 cm 2.
- In patients with renal failure, plasma concentrations of free metanephrines may be up to two-fold higher than in healthy individuals, but this increase is relatively small compared to the increase in deconjugated metanephrines 3.
Factors Affecting Plasma Metanephrines
- Renal function: Plasma free metanephrines are relatively independent of renal function, but deconjugated metanephrines are strongly affected by renal function 3.
- Antihypertensive agents: The use of antihypertensive agents such as β-blockers, thiazide diuretics, and ACE inhibitors does not affect the measurement of plasma free metanephrines 4.
- Adrenal masses: Modest elevations (<4-fold ULN) of metanephrines may be associated with false positive results, especially in patients with adrenal masses >5 cm 2.