What is the most likely diagnosis and recommended work‑up and treatment for a patient presenting with unilateral or bilateral lower‑leg edema and an elevated B‑type natriuretic peptide?

Lower Leg Swelling with Elevated BNP: Diagnosis and Management

Heart failure is the most likely diagnosis when a patient presents with lower leg edema and elevated BNP, and the immediate priority is to obtain transthoracic echocardiography within 2 weeks to determine left ventricular ejection fraction and guide treatment. 1, 2

Most Likely Diagnosis

Bilateral lower leg edema with elevated BNP strongly suggests heart failure (either HFrEF or HFpEF), as BNP is released from cardiac ventricles in response to ventricular wall stress from volume expansion and pressure overload. 3, 4 The degree of BNP elevation determines the likelihood:

• BNP <100 pg/mL: Heart failure highly unlikely (90% sensitivity, 94% negative predictive value) 1
• BNP 100-400 pg/mL: Indeterminate "gray zone" requiring echocardiography and evaluation of confounders 1, 2
• BNP >400 pg/mL: Heart failure highly likely (positive likelihood ratio >10) 1, 2

For NT-proBNP, age-adjusted thresholds apply: <50 years >450 pg/mL, 50-75 years >900 pg/mL, >75 years >1800 pg/mL. 1, 2

Critical Confounders That Elevate BNP Without Heart Failure

Before confirming heart failure, you must exclude these alternative causes of elevated BNP:

• Renal dysfunction (eGFR <60 mL/min/1.73 m²): Reduced clearance elevates BNP independent of cardiac status; adjust thresholds upward (rule-out BNP ≈200-225 pg/mL) 2, 5
• Atrial fibrillation: Raises BNP by 20-30% independent of ventricular function 2, 5
• Pulmonary embolism: Causes right ventricular strain and BNP elevation 2, 6
• Sepsis: Markedly elevated BNP (>1000 pg/mL) can occur without volume overload or LV dysfunction, and confers 2.71-fold increased mortality risk 7
• Obesity (BMI ≥30 kg/m²): Paradoxically lowers BNP by 20-30%, potentially causing false negatives 2, 5

Recommended Immediate Work-Up

1. Urgent Echocardiography (Within 2 Weeks if BNP >400 pg/mL)

Echocardiography is mandatory to define the cardiac abnormality and distinguish HFrEF from HFpEF, as this determines treatment. 1, 2 The study must assess:

• Left ventricular ejection fraction (LVEF ≤40% = HFrEF; >40% = HFpEF) 1, 2
• Diastolic function parameters (E/A ratio, E/e′, left atrial volume) 1
• Valvular abnormalities (especially mitral regurgitation) 1, 2
• Right ventricular function and pulmonary artery pressure 1, 6
• Inferior vena cava assessment for elevated central venous pressure 1

Thoracic ultrasound findings supporting cardiac origin include presence of interstitial syndrome (B-lines), simple pleural effusion, and signs of elevated central venous pressure. 1

2. Concurrent Laboratory Assessment

Obtain these tests immediately while scheduling echocardiography:

• Serum creatinine and eGFR: Renal impairment elevates BNP independent of heart failure 2, 5
• Serum electrolytes (sodium, potassium): Essential before initiating diuretics 2
• 12-lead ECG: Identify atrial fibrillation (raises BNP 20-30%), prior MI, or LV hypertrophy 1, 2
• Thyroid-stimulating hormone: Thyroid dysfunction affects BNP levels 2
• Complete blood count: Anemia may contribute to symptoms 2

3. Chest X-Ray

A normal chest X-ray does not exclude heart failure, particularly early-stage or diastolic dysfunction. 2 However, it helps identify pulmonary edema, pleural effusions, or alternative pulmonary causes. 1

Treatment Based on Echocardiography Results

If HFrEF (LVEF ≤40%)

Immediately initiate quadruple guideline-directed medical therapy (GDMT): 2

1. ACE inhibitor/ARB or ARNI (sacubitril-valsartan)
2. Beta-blocker (carvedilol, metoprolol succinate, or bisoprolol)
3. Mineralocorticoid receptor antagonist (spironolactone or eplerenone)
4. Loop diuretic (furosemide or torsemide) titrated to relieve congestion

BNP-guided titration of GDMT leads to higher medication doses and improved outcomes; target a >30-50% reduction in BNP with treatment. 2, 8

If HFpEF (LVEF >40%)

Focus on aggressive comorbidity management: 2

• Blood pressure control (target <130/80 mmHg) using ACE inhibitors/ARBs and diuretics
• Diabetes management with SGLT2 inhibitors (provide combined cardiac-renal protection)
• Atrial fibrillation management (rate control and anticoagulation if present)
• Loop diuretics for symptom relief when fluid overload is present

Critical caveat: BNP levels are substantially lower in HFpEF compared to HFrEF; many symptomatic HFpEF patients have BNP levels of only 60-100 pg/mL, so a mildly elevated or even normal BNP does not exclude HFpEF. 2, 5

Common Pitfalls to Avoid

• Over-reliance on BNP alone: Specificity in the gray zone (100-400 pg/mL) is only 60-76%; always confirm with echocardiography 1, 2
• Ignoring renal function: Even modest creatinine elevations (1.2-1.5 mg/dL) raise BNP independent of cardiac disease 2
• Missing obesity effect: Severe obesity (BMI >35 kg/m²) reduces BNP sensitivity for detecting heart failure; consider lowering thresholds by 20-30% 2, 5
• Assuming bilateral edema = heart failure: Lymphedema, venous insufficiency, and medication side effects (calcium channel blockers) can cause bilateral leg swelling with normal or mildly elevated BNP 9
• Delaying echocardiography: LVEF determination is critical for selecting appropriate GDMT; do not delay imaging 1, 2

Prognostic Implications

Elevated BNP predicts increased mortality and morbidity: Each 100 pg/mL increase in BNP associates with approximately 35% higher relative risk of death over 1.5-3 years. 1, 2 For every 500 pg/mL increase in NT-proBNP above baseline, mortality risk increases by 3.8%. 2, 8

Serial BNP monitoring can track treatment response: a ≥30% reduction from baseline indicates adequate response and improved prognosis, while rising levels signal treatment failure or disease progression. 2, 8