A 38-year-old woman presents with hypertension (BP 158/96 mmHg), hypokalemia (K⁺ 3.1 mEq/L), and metabolic alkalosis (pH 7.48, HCO₃⁻ 32 mEq/L). Which single finding best discriminates primary hyperaldosteronism from essential hypertension?

## Discriminating Feature: Aldosterone-to-Renin Ratio **Key Point:** The aldosterone-to-renin ratio (ARR) is the gold-standard screening test that distinguishes primary hyperaldosteronism from essential hypertension. In primary hyperaldosteronism, aldosterone is **inappropriately elevated despite suppressed renin**, whereas in essential hypertension, the renin-aldosterone axis remains physiologically intact. ### Pathophysiology Comparison | Feature | Primary Hyperaldosteronism | Essential Hypertension | |---------|---------------------------|------------------------| | **Plasma Renin Activity** | Suppressed (< 1 ng/mL/hr) | Normal to elevated | | **Plasma Aldosterone** | Elevated (> 15 ng/dL) | Normal | | **ARR** | > 20–30 (highly specific) | < 10 | | **Hypokalemia** | Common (60–70%) | Uncommon unless on diuretics | | **Metabolic Alkalosis** | Present | Absent | | **Mechanism** | Autonomous aldosterone secretion | Multifactorial (genetic, environmental) | **High-Yield:** An ARR > 20 with suppressed renin is pathognomonic for primary hyperaldosteronism and is the **single best discriminator** from essential hypertension. This ratio is far more specific than individual aldosterone or renin values alone. **Clinical Pearl:** The combination of **hypokalemia + metabolic alkalosis + hypertension + suppressed renin + elevated aldosterone** in this patient is classic for Conn syndrome (aldosterone-producing adenoma), a subtype of primary hyperaldosteronism. ### Why Other Findings Are Non-Discriminatory - **Elevated renin activity** would actually argue *against* primary hyperaldosteronism (renin is suppressed in that condition). - **LVH** occurs in both conditions due to chronic hypertension and is not discriminatory. - **Elevated 24-hour urinary sodium** is non-specific and occurs in both essential and secondary hypertension. **Mnemonic:** **ARR** = **A**ldosterone-to-**R**enin **R**atio — if > 20 with low renin, think **primary aldosteronism**.