A 38-year-old woman with newly diagnosed hypertension (BP 165/105 mmHg) is found to have hypokalaemia (K⁺ 3.1 mEq/L) and metabolic alkalosis on routine blood work. Renal function is normal. Which investigation is most appropriate as the next step to confirm the underlying mechanism of hypertension?

## Screening for Primary Hyperaldosteronism **Key Point:** The combination of hypertension, hypokalaemia, and metabolic alkalosis is the classic presentation of **primary hyperaldosteronism (Conn syndrome)**, which accounts for 5–10% of secondary hypertension cases. **High-Yield:** The **aldosterone-to-renin ratio (ARR)** is the **first-line screening test** for primary hyperaldosteronism. An elevated ARR (>20–30, depending on assay units) in the setting of suppressed plasma renin activity indicates autonomous aldosterone secretion. ### Diagnostic Approach to Primary Hyperaldosteronism | Step | Investigation | Interpretation | |------|---|---| | **Screening** | Plasma aldosterone-to-renin ratio (ARR) | ARR >20–30 suggests primary hyperaldosteronism | | **Confirmation** | Saline suppression test OR captopril challenge test | Failure to suppress aldosterone confirms autonomy | | **Subtype** | Adrenal CT or adrenal vein sampling (AVS) | Distinguishes unilateral adenoma from bilateral hyperplasia | ### Proper Specimen Collection for ARR 1. **Patient position:** Supine for ≥30 minutes before blood draw (upright posture increases renin) 2. **Time of day:** Morning (8–10 AM) — aldosterone shows diurnal variation 3. **Medications:** Discontinue ACE inhibitors, ARBs, beta-blockers, and diuretics 4–6 weeks prior (if safe) to avoid false negatives 4. **Posture during test:** Some protocols use upright posture; consistency is key **Clinical Pearl:** A normal ARR essentially excludes primary hyperaldosteronism; however, a single elevated ARR requires **confirmatory testing** (saline suppression or captopril challenge) before imaging, as ARR can be falsely elevated in secondary hyperaldosteronism (renal artery stenosis, diuretic use). ### Why This Patient Needs ARR Testing ```mermaid flowchart TD A[Hypertension + Hypokalaemia + Alkalosis]:::outcome --> B[Suspect primary hyperaldosteronism]:::outcome B --> C[Measure ARR]:::action C --> D{ARR elevated?}:::decision D -->|Yes| E[Confirmatory test:<br/>Saline suppression or<br/>Captopril challenge]:::action D -->|No| F[Primary hyperaldosteronism<br/>unlikely]:::outcome E --> G{Aldosterone suppressed?}:::decision G -->|No| H[Confirmed primary<br/>hyperaldosteronism]:::outcome G -->|Yes| I[Consider secondary<br/>hyperaldosteronism]:::outcome H --> J[Adrenal imaging:<br/>CT or AVS]:::action ``` [cite:Harrison 21e Ch 297]