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ATS / ERS guidelines recommend using % predicted FEV1 as the metric to define severity of obstructive, restrictive, and mixed ventilatory disorders. The main driver: FEV1 is the most robust variable as an overall measure of health (epidemiologically speaking). Critics charge that this overestimates severity of obstruction when restriction is also present, causing confusion, misdiagnosis, and over-treatment.
Back in 2002, Balfe and friends used the Intermountain guidelines to propose a grading system based on the FEV1/FVC ratio -- which reclassified most patients with mixed disorders as having mild obstructive disease (due to increased airflow, from the restrictive disease). Adopting this method while correcting for its apparent biases would have required complex recalculations / normalization of existing data sets, and was thus rejected. A proposed hedge has been to call the combined defect a "ventilatory impairment" (rather than obstruction or restriction) with severity based on FEV1.
Enter Gardner et al in this month's CHEST. They analyzed pulmonary function test data on 199 patients with combined restrictive/obstructive disease from St. Louis University Hospital, 2003-2009. They simply divided FEV1 % predicted by total lung capacity % predicted, and plugged the resulting FEV1% into ATS/ERS guidelines to grade obstructive severity. Their method downgrades the severity of obstruction in most patients, but not to the extent of Balfe et al's. They argue their resulting data better reflects reality, with a "more even distribution of severity grades."
Gardner ZS et al. Grading the Severity of Obstruction in Mixed Obstructive-Restrictive Lung Disease. CHEST 2011;140:598-603.