A 38-year-old woman with resistant hypertension (BP 160/100 mmHg on three antihypertensive agents) presents with hypokalemia (K⁺ 3.1 mEq/L) and metabolic alkalosis (HCO₃⁻ 32 mEq/L). Which single investigation would be most helpful in confirming the suspected diagnosis?

Explanation

## Primary Hyperaldosteronism — Diagnosis **Key Point:** The aldosterone-to-renin ratio (ARR) is the **screening test of choice** for suspected primary hyperaldosteronism in a patient with resistant hypertension, hypokalemia, and metabolic alkalosis. ### Clinical Presentation Clues - **Resistant hypertension** — inadequate control on ≥3 agents at optimal doses - **Hypokalemia** — aldosterone promotes renal K⁺ wasting - **Metabolic alkalosis** — aldosterone drives H⁺ excretion and HCO₃⁻ reabsorption - **Metabolic syndrome** — often coexists with primary aldosteronism ### Diagnostic Algorithm for Primary Hyperaldosteronism ```mermaid flowchart TD A[Resistant HTN + Hypokalemia + Metabolic Alkalosis]:::outcome --> B[Measure ARR<br/>PAC and PRA]:::action B --> C{ARR > 20-30?}:::decision C -->|Yes| D[Confirmatory test:<br/>Saline suppression or<br/>Captopril challenge]:::action C -->|No| E[Primary aldosteronism<br/>unlikely]:::outcome D --> F{Aldosterone suppressed?}:::decision F -->|No| G[Confirmed primary<br/>hyperaldosteronism]:::outcome F -->|Yes| H[Test negative]:::outcome G --> I[CT/MRI abdomen<br/>to localize adenoma]:::action ``` ### ARR Interpretation | ARR Value | Interpretation | Next Step | |-----------|----------------|----------| | >20–30 | Suggestive of primary aldosteronism | Proceed to confirmatory test | | <10 | Primary aldosteronism unlikely | Consider other causes | | 10–20 | Borderline; clinical context matters | Repeat or proceed to confirmatory test | **High-Yield:** ARR = (PAC in ng/dL) / (PRA in ng/mL/hr). A **suppressed renin** (low PRA) with **elevated aldosterone** is the hallmark of primary aldosteronism. **Mnemonic:** **ALDO** — **A**ldosterone-to-**R**enin ratio is the **first-line screening** test; **L**ow renin + **D**ominant aldosterone = primary hyperaldosteronism. **Clinical Pearl:** Hypokalemia should be corrected before measuring ARR, as low K⁺ suppresses renin and may falsely elevate the ratio. Diuretics and ACE inhibitors should ideally be withdrawn 4 weeks before testing, though this is often impractical in resistant hypertension.