Converting Direct Renin Concentration to Plasma Renin Activity
To convert Direct Renin Concentration (DRC) to Plasma Renin Activity (PRA), use a ratio of approximately 8.2 mIU/L of DRC for each 1 ng/mL/h of PRA, though this conversion should be validated locally as assay-specific variations exist. 1
Understanding the Two Measurement Methods
- Direct Renin Concentration (DRC) measures the actual concentration of renin protein in the blood, typically reported in mIU/L or pg/mL 2
- Plasma Renin Activity (PRA) measures the enzymatic activity of renin by quantifying angiotensin I generation rate, reported in ng/mL/h 3
- Many laboratories are transitioning from PRA to DRC measurements due to DRC being faster, simpler, and more reproducible 2
Conversion Factors and Considerations
- The relationship between DRC and PRA is generally linear, with strong correlations (r = 0.92) observed in both supine and standing positions 2
- When converting between the two measurements, population-derived cutoffs are more appropriate than using simple conversion factors 4
- A commonly used conversion factor suggests:
- 1 ng/mL/h of PRA ≈ 8.2 mIU/L of DRC 1
- Alternatively: DRC (mIU/L) ÷ 8.2 = PRA (ng/mL/h)
Clinical Implications of Conversion
- Using direct conversion factors may result in significantly higher cutoffs and potential missed cases of conditions like primary aldosteronism 4
- The aldosterone-to-renin ratio (ARR) calculated with DRC (ADRR) tends to have fewer false positives than ARR calculated with PRA (ARRP) 2
- For primary aldosteronism screening, an ADRR ≥30 provides acceptable specificity (>80%) and sensitivity (100%) 1
Important Considerations When Measuring Renin
- All medications that affect renal renin secretion should ideally be discontinued for at least 2 weeks before testing, including antihypertensives, diuretics, and NSAIDs 5
- Patients should maintain a controlled dietary sodium intake of 100-200 mmol per day during testing 5
- If stopping all antihypertensive medications is unsafe, calcium channel blockers or alpha-1 adrenergic blockers can be continued as they minimally affect renin measurements 5
- Sample processing should occur at room temperature to avoid irreversible cryoactivation of prorenin, which occurs when plasma is liquid at temperatures below 6°C 3
Laboratory-Specific Validation
- Each laboratory should establish its own conversion factor between DRC and PRA based on local population data 4
- When transitioning between assays, using large population data to establish historically consistent cutoffs is recommended rather than relying on simple conversion factors 4
- For critical diagnostic decisions, consider measuring both PRA and DRC during a transition period to establish reliable correlation in your specific patient population 1