Acute chest syndrome (ACS) is similar to severe pneumonia and is a common cause of hospitalizations for people with sickle cell disease (SCD). Blood transfusions are one treatment option for ACS. High levels of an enzyme called secretory phospholipase A2 (sPLA2) may be present in people before they develop ACS. This study will determine how well sPLA2 levels can predict the onset of ACS and whether identifying high sPLA2 levels allows enough time to prevent ACS with blood transfusions. Results from this study will help to determine the feasibility of conducting a larger study that would further examine the use of sPLA2 levels and blood transfusions to prevent ACS in people with SCD.