pft calculator

Pulmonary function tests measure how effectively the lungs move air in and out. The two core spirometry values are forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). Clinicians compare a patient's measured results to predicted equations adjusted for age, height, and sex. This calculator uses the commonly cited NHANES III equations to estimate predicted values and returns the percentage of predicted for both FEV1 and FVC as well as the measured FEV1/FVC ratio to screen for obstruction or restriction patterns.

How the Equations Work

NHANES III regression formulas estimate predicted lung volumes from large population data sets:

• Male predicted FEV1 = 0.0414 × height(cm) – 0.0244 × age – 2.190
• Female predicted FEV1 = 0.0342 × height(cm) – 0.0255 × age – 1.578
• Male predicted FVC = 0.0523 × height(cm) – 0.0287 × age – 3.210
• Female predicted FVC = 0.0407 × height(cm) – 0.0239 × age – 2.620

The percentage of predicted is calculated as (measured ÷ predicted) × 100. Percentages near 100 indicate normal lung volume, while lower values may suggest obstruction, restriction, or a mixed pattern. The FEV1/FVC ratio below 0.70 typically indicates airflow obstruction after bronchodilation.

Example Interpretation

Example: A 45-year-old male, 178 cm tall, records a measured FEV1 of 2.80 L and FVC of 3.70 L. The calculator estimates predicted FEV1 ≈ 3.67 L (so 2.80/3.67 = 76%) and predicted FVC ≈ 4.84 L (3.70/4.84 = 76%). The measured FEV1/FVC ratio is 0.76, which borders the obstruction threshold, prompting clinicians to compare against lower limit of normal tables or repeat post-bronchodilator testing.