A 58-year-old man with chronic heart failure (NYHA Class III) on lisinopril and metoprolol presents with worsening dyspnea and peripheral edema. His serum creatinine is 1.8 mg/dL and potassium is 5.2 mEq/L. Before initiating an aldosterone antagonist, which investigation is most appropriate to assess safety?

## Investigation Rationale **Key Point:** Aldosterone antagonists (spironolactone, eplerenone) carry significant risk of life-threatening hyperkalemia and worsening renal function, especially in patients already on ACE inhibitors or ARBs. ### Why Serum Electrolytes & Renal Function? Before adding an aldosterone antagonist, baseline assessment of: - **Potassium level** — must be ≤5.0 mEq/L (ideally <5.5 mEq/L) - **Serum creatinine and eGFR** — contraindicated if eGFR <30 mL/min/1.73 m² - **Risk of dual RAAS blockade** — patient is already on ACE inhibitor; combination increases hyperkalemia risk **High-Yield:** This is a **safety screen**, not a diagnostic test. It determines whether the drug can be safely given at all. ### Clinical Pearl The RALES trial (spironolactone in HF) and EMPHASIS-HF trial (eplerenone in HFrEF) both required baseline K⁺ <5.0 mEq/L and eGFR >30. Monitoring must occur at 3 days, 1 week, and 4 weeks post-initiation. ### Comparison of Options | Investigation | Purpose in HF | Role Here | |---|---|---| | Transthoracic echo | Assess EF, structure, diastolic function | Diagnostic; not for drug safety | | Serum K⁺ & creatinine | Detect hyperkalemia risk, renal impairment | **Safety prerequisite** | | Cardiac catheterization | Hemodynamics, CAD exclusion | Invasive; not indicated for drug initiation | | 6-minute walk test | Functional capacity assessment | Prognostic; not for drug safety | **Mnemonic — RAAS Antagonist Safety Check: K-REF** - **K** — Potassium <5.0 mEq/L - **R** — Renal function (eGFR >30) - **E** — Electrolytes (Na⁺, Cl⁻ baseline) - **F** — Follow-up labs at day 3–7