Body plethysmography is generally superior for COPD patients because it measures all gas within the thorax, including air trapped behind obstructed airways. Helium dilution often underestimates total lung capacity in COPD cases by up to 15% because the tracer gas cannot reach poorly ventilated areas. We recommend plethysmography to ensure 100% of your lung volume is accounted for, providing the most reliable data for managing chronic conditions.