CD4 and pulmonary emphysema: Given that CD4+ T cells isolated from peripheral blood samples in ever-smokers with emphysema (irrespective of airflow obstruction) are functionally distinguishable from controls, and the maximal Youden index (Youden, 1950) on the IFN-γ ROC, we defined the threshold for a positive increase in T cell cytokine responses to EFs at 1.5-fold increase over nil stimulation.