Background: Airflow limitation in diffuse parenchymal lung disease (DPLD) needs a better spirometric understanding.

Objective: To appreciate airflow obstruction in DPLD.

Methods: Serial-diagnosed DPLD subjects based on HRCT Chest were examined for airflow limitations with the GOLD-guideline forwarded criteria of FEV1/FVC<0.7. Thus two groups ?DPLD-unmixed? and ?DPLD-airflow limitation overlap? were formed. The two groups were compared with possible spirometric markers of airflow obstruction as %-predicted FEF25-75, FEV1- FEF25-75 distance, FEV1 reversibility(>100 ml), and FEF25-75(>30%-predicted value). Those showing a highly significant difference (P<0.0001) were further addressed by performing ROC (Receiving Operator characteristic) curves and determination of cut-off values to avail them as surrogates of FEV1/FVC<0.7. These criteria were further applied to the ?unmixed-DPLD? population.

Results: 26 of 201 patients met the classical FEV1/FVC<0.7 criteria with all of them having a ratio well below the lower limit of normal. The %-predicted FEF25-75 and FEV1-FEF25-75 distance qualified to be surrogates of FEV/FVC<0.7 with 84.62% and 73.77% as specificity, 81.15% and 80.77% as sensitivity respectively. These two parameters identified 29 subjects each and reversibility of FEV1 and FEF25-75 identified 11 each as having airflow obstruction in the ?DPLD-unmixed? group. Thus, a revelation of 80(45.45%) covert airflow obstruction is apparent in ?unmixed? DPLD subjects. 

Conclusion: The covert airflow limitation has even unleashed, a significant number of patients from the supposedly ?unmixed-DPLD? population apart from the conventional airflow obstruction criteria and that demands further validation.