This estimates what a person's forced vital capacity should be based on age, height, and sex, so an actual spirometry result can be compared against an expected normal.
Result in liters. GLI-2012 regression equations are the clinical standard.
Dividing the measured value by the predicted value gives percent predicted.
This reflects an ongoing, actively discussed shift in pulmonary medicine toward race-neutral spirometry reference equations (GLI Global 2022). It's best understood as a current, evolving practice area rather than a settled fact.
It expresses a measured value as a percentage of what's expected for someone of the same age, height, and sex — so 70% predicted means a value 30% below the population norm.
Generally 80% predicted or above is considered within normal limits, though the lower limit of normal (about the 5th percentile) is the more rigorous cutoff.
GLI-2012 (and the newer race-neutral GLI Global) are the guideline-recommended standards. The simplified formula here is illustrative for teaching only.