Can Brain Natriuretic Peptide (BNP) levels be falsely low in patients with obesity?

BNP Levels in Obesity: Diagnostic Implications

Yes, BNP levels are falsely lower in patients with obesity, which can lead to misdiagnosis of heart failure and requires adjusted diagnostic thresholds for accurate clinical assessment. 1

Mechanism and Evidence

The relationship between obesity and BNP levels is well-established in clinical guidelines:

• In patients with obesity, BNP and NT-proBNP levels are significantly lower than in non-obese patients with the same degree of heart failure 1
• For any given left ventricular diastolic pressure, BNP levels are lower in obese patients 1
• When examining patients with heart failure, those with BMI > 30 kg/m² had significantly lower BNP levels than those with BMI < 20 kg/m² (median 332 pg/mL vs. 747 pg/mL, p = 0.0001) 1

This inverse relationship between BMI and natriuretic peptide levels persists even after adjusting for other variables known to affect BNP 2.

Clinical Impact

The suppression of BNP in obesity has important diagnostic implications:

• Using standard BNP cutoff of 100 pg/mL, false negative results occur in up to 20% of overweight and obese patients with acute heart failure 2
• For NT-proBNP (using 900 pg/mL cutoff), false negatives occur in 10% of overweight patients and 15% of obese patients 2
• In some obese patients with heart failure with preserved ejection fraction (HFpEF), NT-proBNP levels can be below the diagnostic threshold of 125 pg/mL despite having significant cardiac dysfunction 3

Diagnostic Threshold Adjustments

Guidelines recommend adjusting diagnostic thresholds for BNP in obese patients:

• For patients with BMI ≥ 30 kg/m², BNP cutoff points should be adjusted to 342 pg/mL (compared to standard 100 pg/mL) 1
• NT-proBNP appears to retain better diagnostic utility in obesity, with standard cutoff points maintaining reasonable exclusion utility 1
• Some obese patients with symptomatic heart failure may have BNP levels of only 60-100 pg/mL 1

Special Considerations

• The "obesity paradox" in heart failure refers to the observation that obesity is associated with improved survival in patients with established heart failure, despite being a risk factor for developing heart failure 1
• Bariatric surgery and weight loss may actually increase BNP levels in obese patients with heart failure 1
• Diagnostic algorithms for heart failure in obese patients should rely more heavily on clinical assessment and imaging findings rather than absolute BNP values 1

Practical Approach to BNP Interpretation in Obesity

1. Recognize that BMI > 30 kg/m² significantly impacts BNP levels
2. Consider using higher diagnostic thresholds (342 pg/mL for BNP) in obese patients
3. NT-proBNP may be slightly more reliable than BNP in obese patients but still requires cautious interpretation
4. Do not exclude heart failure based solely on "normal" BNP levels in obese patients
5. Integrate BNP results with clinical assessment, echocardiography, and other diagnostic modalities
6. Consider that even modestly elevated BNP levels (60-100 pg/mL) may indicate heart failure in very obese patients

Pitfalls to Avoid

• Assuming normal BNP levels rule out heart failure in obese patients
• Failing to adjust diagnostic thresholds based on BMI
• Overlooking clinical signs of heart failure when BNP levels are only mildly elevated in obese patients
• Not considering obesity as a factor when interpreting seemingly normal BNP results in patients with symptoms suggestive of heart failure

The mechanism behind lower BNP levels in obesity remains incompletely understood but may involve altered proBNP processing, with decreased concentrations of proBNP not glycosylated at threonine 71 in obese patients 4.