FEV1/FVC ratio and index values below 70% are typical for chronic obstructive pulmonary disease - COPD (see: the BODE index calculator).
We can distinguish a few stages of COPD, designated by the Tiffeneau index and FEV1 values:
I — Mild, with FEV1/FVC < 70% and FEV1 > 80% predicted;
II — Moderate, with FEV1/FVC < 70% and 50% < FEV1 < 80% predicted;
III — Severe, with FEV1/FVC < 70% and 30% < FEV1 < 50% predicted; and
IV — Very severe, with FEV1/FVC < 70% and FEV1 < 30% predicted.
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