A 38-year-old woman presents with hypertension, hypokalemia, and metabolic alkalosis. Laboratory investigations show elevated aldosterone (45 ng/dL) with suppressed plasma renin activity (0.3 ng/mL/hr). Which single feature best distinguishes primary aldosteronism from secondary aldosteronism?

Explanation

## Distinguishing Primary from Secondary Aldosteronism ### The Key Discriminator: Plasma Renin Activity (PRA) **High-Yield:** The cardinal difference between primary and secondary aldosteronism lies in the **renin-aldosterone relationship**. In primary aldosteronism, aldosterone is autonomously elevated *despite* suppressed renin; in secondary aldosteronism, both aldosterone AND renin are elevated together. ### Comparison Table | Feature | Primary Aldosteronism | Secondary Aldosteronism | |---------|----------------------|------------------------| | **Plasma Renin Activity** | **Suppressed (<0.5 ng/mL/hr)** | **Elevated (>1.5 ng/mL/hr)** | | Aldosterone Level | Elevated | Elevated | | Aldosterone/Renin Ratio | >20–30 (high) | <10 (low) | | Blood Pressure | Hypertension | Variable (often low in cirrhosis) | | Hypokalemia | Common | May be absent | | Metabolic Alkalosis | Present | Present | | Cause | Adrenal tumor or bilateral hyperplasia | Volume depletion, renal disease, cirrhosis, heart failure | ### Key Point: **Suppressed PRA in the presence of elevated aldosterone is pathognomonic for primary aldosteronism.** This reflects loss of the normal feedback inhibition of renin by aldosterone, which occurs only when aldosterone production is autonomous (not dependent on the renin-angiotensin system). ### Clinical Pearl: In secondary aldosteronism (e.g., cirrhosis, heart failure, renal artery stenosis), the body perceives volume depletion or low perfusion pressure, so the renin-angiotensin system is activated. Both renin AND aldosterone rise together. The aldosterone/renin ratio remains low (<10), which helps rule out primary aldosteronism. ### Mechanism Diagram ```mermaid flowchart TD A[Elevated Aldosterone]:::outcome --> B{Is Renin Suppressed?}:::decision B -->|Yes| C[Primary Aldosteronism]:::outcome B -->|No| D[Secondary Aldosteronism]:::outcome C --> E[Autonomous adrenal production]:::action D --> F[RAS activation due to volume loss]:::action ``` ### Why Other Features Are Not Discriminators: - **Elevated aldosterone:** Present in BOTH primary and secondary forms. - **Hypokalemia:** Common in primary but may be absent in secondary; not a reliable discriminator. - **Metabolic alkalosis:** Occurs in both due to H^+^ loss in urine from aldosterone excess; not specific. [cite:Harrison 21e Ch 297]