Plasma Metanephrine Unit Converter
Plasma Metanephrine Unit Converter
Convert plasma free metanephrine between pg/mL, ng/L and pmol/L, and see why the sample must be drawn supine and off interfering drugs before it can be trusted.
Plasma Metanephrine converter
Mass ⇄ molarPlasma free metanephrine 65 pg/mL, supine sample
Formula and conversion factor
pg/mL = pmol/L ÷ 5.07022
- 5.07022
- derived from the molecular weight of metanephrine, 197.23 Da
- ng/L
- numerically identical to pg/mL
- normetanephrine
- a different molecule, 183.20 Da, with its own conversion factor — do not apply this factor to it
Worked example
Plasma free metanephrine 65 pg/mL, supine sample
65 × 5.07022 = 330 pmol/L
= 65 ng/L
Before the sample is drawn
| Requirement | Reason |
|---|---|
| Supine, after 30 minutes’ rest | Upright posture raises catecholamine metabolites substantially and is a common cause of a false positive |
| Withhold tricyclic antidepressants and SNRIs | Both raise plasma metanephrines independently of any tumour |
| Withhold levodopa and sympathomimetics | Raise catecholamine metabolites through their own pathway |
| Check the assay for paracetamol interference | Interferes with some chromatographic assays |
Why metabolites, not catecholamines, are measured
Plasma free metanephrines are the most sensitive test available for phaeochromocytoma and paraganglioma, at close to 99%. The reason is mechanistic rather than simply a better assay: tumours metabolise catecholamines continuously inside the tumour cell, producing a steady output of metanephrine and normetanephrine, whereas catecholamine secretion itself is episodic and can be entirely normal between surges. Measuring the metabolites therefore outperforms measuring adrenaline or noradrenaline directly, which can miss a tumour sampled between episodes.
The trade-off is specificity, and false positives are common enough that sampling technique matters as much as the number itself. The sample should be drawn supine after at least 30 minutes’ rest, since upright posture raises the metabolites substantially through sympathetic activation. Interfering medication should be withheld where clinically possible: tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors, levodopa, phenoxybenzamine and other sympathomimetics all raise the result, and paracetamol interferes with some chromatographic assays specifically.
Magnitude helps separate a genuine finding from noise. A result above three to four times the upper limit is close to diagnostic on its own. Smaller elevations are much more often explained by posture, stress or medication than by a tumour, and the sensible next step is to repeat the sample supine, after the required drug washout, before pursuing imaging.
Frequently asked questions
How do I convert plasma metanephrine to pmol/L?
Multiply the pg/mL (or ng/L, which is numerically identical) value by 5.070. A metanephrine of 65 pg/mL is 330 pmol/L.
Why are metanephrines measured rather than adrenaline itself?
Tumours metabolise catecholamines continuously inside the tumour cell, so metanephrine and normetanephrine are produced steadily. Catecholamine secretion is episodic and can be normal between surges, so measuring it directly can miss a tumour.
Why must the sample be drawn supine?
Upright posture raises plasma metanephrines substantially through sympathetic activation, and is one of the commonest causes of a false-positive result. Rest supine for at least 30 minutes before the draw.
Which drugs should be stopped before testing?
Tricyclic antidepressants, SNRIs, levodopa, phenoxybenzamine and other sympathomimetics where clinically feasible. Paracetamol interferes with some chromatographic assays specifically.
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References
- Lenders JWM et al. Pheochromocytoma and paraganglioma: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2014;99(6):1915–42.
- Eisenhofer G et al. Biochemical diagnosis of pheochromocytoma. Clin Chem. 2003;49(10):1739–52.