Spirometry Interpreter Calculator

The fixed FEV1/FVC ratio of 0.70 is the threshold GOLD criteria use because it is simple to apply consistently across patients and settings, though it is known to overdiagnose obstruction in older adults (whose FEV1/FVC ratio declines with age even without disease) and underdiagnose it in younger adults. Some guidelines instead use a lower limit of normal (LLN) that adjusts for age -- typically the 5th percentile of a reference population, from equations this calculator does not implement -- but the fixed 0.70 ratio remains the more commonly cited cutoff in clinical practice and trial eligibility criteria, and is the only obstruction threshold this calculator applies.

Not definitively -- a reduced FVC with a preserved FEV1/FVC ratio is suggestive of restriction, but spirometry cannot distinguish true restrictive lung disease from reduced effort, chest wall or neuromuscular limitation, or severe air trapping in an obstructive disease that lowers FVC without a genuinely restrictive process. Confirming restriction requires measuring total lung capacity by body plethysmography or gas dilution, which is why this calculator's restrictive classification explicitly needs confirmation, not outright diagnosis.

Under the ATS/ERS severity bands used here, moderate spans 60-69% of predicted FEV1 while severe spans 35-49% -- meaning a patient graded severe is functioning at roughly half or less of the expected lung capacity for their age, height, and sex, compared to a moderate patient who retains close to two-thirds. The moderately severe band (50-59%) sits between the two, reflecting how the ATS/ERS system subdivides what many older classification schemes lumped into a single moderate category.

Lung size scales strongly with height because taller people generally have larger thoracic cavities and airway diameters, so reference equations weight height heavily -- often as a squared term -- alongside age (which reduces predicted values as elastic recoil and respiratory muscle strength decline) and sex (reflecting average differences in lung and airway size). Comparing a short patient's raw FEV1 to a tall patient's would misclassify one of them as abnormal purely due to body size, not lung disease.