Spirometry Predicted Values
FEV₁, FVC, FEV₁/FVC & PEFR using validated Indian (Jindal) reference equations
Patient Parameters
Measured Values (enter if available)
Leave blank to see predicted values only without % predicted comparison.
ATS/ERS Severity Classification of Ventilatory Defects
| Severity | FEV₁ % Predicted | Description | Common Causes |
|---|---|---|---|
| Normal | ≥ 80% | Within normal limits | — |
| Mild | 70–79% | Mildly reduced airflow | Early COPD, mild asthma |
| Moderate | 60–69% | Moderate airflow limitation | Moderate COPD, occupational lung disease |
| Moderately Severe | 50–59% | Significant limitation | Advanced occupational asthma, silicosis |
| Severe | 35–49% | Severely reduced | Severe COPD, advanced fibrosis |
| Very Severe | < 35% | Very severely reduced | End-stage lung disease |
Why Indian Reference Equations (Jindal) Are Used
Lung function reference values vary significantly between ethnic populations. Indians have smaller thoracic cage dimensions relative to their standing height compared to Caucasians — a well-documented finding across multiple population studies spanning decades. The practical implication is that using Caucasian or NHANES III predicted equations for Indian patients consistently overestimates predicted values by approximately 15–25%.
Jindal et al. published regression equations derived from healthy, non-smoking Indian adults across multiple states, providing the most widely cited and validated reference for Indian spirometry. The equations used in this calculator are the Jindal equations from the Indian Journal of Chest Diseases, adapted for clinical use in occupational health, pre-employment, and periodic medical examinations across India.
Obstructive Ventilatory Defect
Defined as FEV₁/FVC < 70% (fixed ratio method). The FVC may be normal or reduced. This pattern indicates narrowing of the airways, reducing the rate of airflow. Common occupational causes include occupational asthma (isocyanates, latex, grain dust, wood dust), byssinosis (cotton dust), and COPD from coal or silica exposure. Reversibility testing with bronchodilator is essential — improvement of ≥12% AND ≥200 mL in FEV₁ suggests asthma.
Restrictive Ventilatory Defect
FEV₁/FVC ≥ 70% with FVC < 80% predicted (spirometric restriction — note: true restriction requires TLC measurement by body plethysmography or helium dilution). Common occupational causes: silicosis, asbestosis, coal worker's pneumoconiosis, byssinosis stage 2+. Non-occupational causes include pulmonary fibrosis, pleural disease, chest wall deformity, and obesity.
Mixed Ventilatory Defect
Both FEV₁/FVC < 70% AND FVC < 80% predicted simultaneously. Seen in advanced silicosis with COPD, complicated pneumoconiosis, and severe occupational asthma with air trapping. Requires further workup including full lung volume assessment.
Spirometry in Pre-Employment Medical Examinations
Under the Factories Act 1948 and specific Hazardous Processes Rules, spirometry is mandatory for workers exposed to notifiable occupational lung hazards. Industries requiring baseline and periodic PFT include cotton textile mills, stone quarries, coal mines, asbestos processing, isocyanate manufacturing, grain handling facilities, and chemical industries with respiratory sensitisers. AFIH (Associate Fellow of Industrial Health) doctors are trained to perform and interpret occupational spirometry.
Quality Criteria for Acceptable Spirometry (ATS/ERS)
- Start of test: back-extrapolated volume < 5% of FVC or < 0.15L
- End of test: exhalation ≥6 seconds or plateau (change <25 mL in 1s)
- No cough, glottis closure, or early termination
- At least 3 acceptable manoeuvres with best FVC and FEV₁ within 150 mL
- Use the highest FVC and highest FEV₁ from acceptable manoeuvres (may come from different blows)
Frequently Asked Questions
What is the difference between FEV₁ and FVC?
FVC (Forced Vital Capacity) is the total volume of air forcefully exhaled from full inspiration to complete exhalation. FEV₁ (Forced Expiratory Volume in 1 second) is the volume exhaled in the first second of that manoeuvre. The ratio FEV₁/FVC tells you how much of the total lung capacity you can empty in 1 second — reduced when airways are narrowed (obstructive pattern).
Can this calculator replace a spirometry machine?
No. This tool calculates predicted reference values based on age, sex, and height using validated Indian equations. The actual spirometry test must be performed with a calibrated spirometer that generates flow-volume and volume-time curves. This calculator helps you compare measured values against Indian predicted normals and classify the ventilatory pattern.
At what FEV₁ is a worker considered unfit for dusty work?
There is no single universal FEV₁ cutoff for fitness to work in dusty environments in India. DGMS (Directorate General of Mines Safety) and DGFASLI (Directorate General Factory Advice Service) guidelines specify industry-specific criteria. Generally, a baseline PFT within normal limits is required for placement in high-dust areas, and progressive decline (>15% from baseline over periodic examinations) triggers review, job modification, or transfer.
Why does PEFR vary with time of day?
PEFR shows diurnal variation of up to 20% in normal individuals — lowest in the early morning, highest in the afternoon. In asthma, this variation exceeds 20% and is a diagnostic criterion. For occupational asthma, serial peak flow monitoring at and away from work (using a mini peak flow meter) over 4 weeks is the standard diagnostic approach recommended by BOHRF and SIGN guidelines.