What is the proper protocol for drawing plasma metanephrines?

Proper Protocol for Drawing Plasma Metanephrines

Plasma metanephrines should be collected from an indwelling venous catheter after the patient has been lying supine for 30 minutes to minimize false-positive results. 1

Patient Preparation

• Fasting status: Patient should fast overnight before the test
• Positioning: Patient must be supine for at least 30 minutes before blood collection
• Location: Ideally performed as an inpatient test (reduces false positives by 3.4-fold compared to outpatient testing) 2
• Avoid interfering substances:
  • Several foods and medications can interfere with metanephrine analysis
  • Review and temporarily discontinue interfering medications when possible
  • Provide patients with a list of foods/medications to avoid prior to testing

Collection Procedure

1. Cannula placement: Use an indwelling venous catheter rather than direct venipuncture 2
2. Rest period: Ensure patient remains supine for 30 minutes before drawing blood
3. Environmental conditions: Maintain warm room temperature during collection (cold temperatures increase false positives) 2
4. Collection method: Draw blood from the indwelling catheter without having the patient change position

Important Considerations

• Reference intervals: Use appropriate reference intervals based on collection position

  • Using supine reference intervals for seated samples leads to decreased specificity (74.7% vs 98.9%) 3
  • Pediatric reference intervals should be used when testing children 1

• Follow-up for borderline results:

  • If results are ≥4× upper limit of reference range: consistent with disease, proceed with imaging
  • If results are 2-4× upper limit of reference range: repeat testing in 2 months
  • If results are marginally elevated: repeat testing in 6 months or consider clonidine suppression test 1

Common Pitfalls to Avoid

1. Seated sampling: Leads to 25% higher plasma concentrations and increased false positives 1
2. Direct venipuncture: Increases stress response and false positives 2
3. Cold environment: Low temperature during collection increases false positives, particularly for outpatient sampling 2
4. Incorrect reference intervals: Using supine reference intervals for seated samples decreases specificity 3
5. Failure to account for medications: Many medications can interfere with results

Alternative Testing Options

• 24-hour urine fractionated metanephrines are an acceptable alternative to plasma metanephrines, especially for patients who are continent of urine 1
• Urinary testing has higher specificity (98% vs 85%) but slightly lower sensitivity (90% vs 97%) compared to plasma metanephrines 4
• Plasma testing may be preferred for high-risk patients with familial endocrine syndromes, while urinary testing may be better for low-risk patients due to fewer false positives 4

By following this protocol, you can minimize false-positive results and improve the diagnostic accuracy of plasma metanephrine testing for pheochromocytoma and paraganglioma detection.