Study Population
During the 6-year study period, 2,503 patients enrolled into the VALID study were considered for inclusion in the current study (Fig. 1). We excluded patients who transferred from other hospitals without thorough medical information and excluded patients with an ICU stay less than 48 hours. Patients admitted to the trauma ICU were excluded since overall this younger group of patients was less likely to be taking prehospital aspirin and preadmission medication histories were often not available. To avoid the confounding effects of cardiovascular diseases that might be associated with both higher rates of aspirin use and lower rates of ARDS, we excluded patients admitted to the cardiovascular ICU and patients with an acute cardiac diagnosis. We also excluded patients admitted for elective surgery because antiplatelet agents are frequently discontinued before surgery and medication history may be inaccurate. Finally, we excluded patients less than 40 years old because prehospital aspirin was rarely prescribed in that age group.