At this time, only a few clinical variables or inflammatory mediators have been shown to be associated with COPD exacerbations and their related morbidity and mortality. Some of those include: age [18] [19] [20] ; FEV 1 , forced vital capacity and peak expired flow [19, 21, 22] ; body mass index [20] ; albumin [20, 22, 23] ; sodium [23] ; pH [24, 25] ; eosinophils [26] [27] [28] [29] ; interleukins 6 and 8 [29] [30] [31] [32] ; fibrinogen [31] ; and C-reactive protein [33] . Significant clinical events such as the number of exacerbations per year, the number of hospital admissions per year, time to relapses, and days in hospital have been regarded as useful measures in clinical studies designed to assess drug efficacy and cost-effectiveness as well as to standardize existing hospital support programs for COPD [34] [35] [36] [37] [38] [39] . However, it is not known how these measures change with increasing severity of COPD exacerbations. Therefore, we have surveyed the medical literature to identify which of the commonly accepted variables and suggested markers for COPD exacerbations change according to the ATS/ERS' levels of exacerbation severity. The long-term aim of our work is to assess the sensitivity and specificity of potential markers for use in future COPD studies as well as to determine how such markers can be further studied and fully integrated into the development of new drugs for COPD.