Objective To explore the predictive value of early changes in platelet counts in the prognosis of severe pneumonia in aged patients.Methods This retrospective study included elderly patients with severe pneumonia,who were ≥65 years old and whose length of ICU stay ≥72 hours,admitted to the intensive care unit(ICU)of NO.2 People's Hospital of Changzhou from January 2014 to January 2017.They were divided into a survival group and a death group according to the 28-day outcome.General information and serum platelet levels at 0,24,36,and 72 hours after admission were collected.Receiver operating characteristic curve (ROC)was plotted according to platelet counts,changes in platelet counts and rates of change in platelet counts to evaluate their predictive value for 28-day prognosis.Kaplan-Meier survival curve was used to analyze the 28-day cumulative survival rate between different groups of patients,who were further divided according to platelet counts at 0 and 72 hours after admission to ICU,changes in platelet counts and rates of change in platelet counts at 72 hours after admission.Results (1)One hundred elderly patients with severe pneumonia were enrolled,among whom 41 cases were in the death group,thus with a mortality of 41.0％.The acute physiology and chronic health evaluation Ⅱ (APACHE Ⅱ),sequential organ failure assessment (SOFA),C-reactive protein (CRP),and procalcitonin(PCT)in the death group were significantly higher than those in the survival group.2)Serum platelet levels showed a downward trend in both the death group and the survival group.The level of serum platelets at 72 hours after admission to ICU in the death group was significantly lower than that in the survival group(80.00 × 109/L vs.171.00 × 109/L,Z=5.786,P＜0.05);changes in platelet counts and rates of change in platelet counts in the death group were significantly higher than those in the survival group,especially at 72 hours after admission to ICU(/△PLT72:-79.00 × 109/L vs.-38.00 × 109/L,Z =4.938,P ＜0.05;△PLT 72％:43.6％ vs.-17.7％,Z=6.816,P＜0.05).(3)ROC curve analysis showed that platelet levels,changes in platelet counts,and rates of change in platelet counts could predict 28-day mortality in aged patients with severe pneumonia.The largest area under ROC curve was 0.902 when plotted with the rate of platelet counts at 72 hours after admission to ICU.Using the cut-off value of-36.14％ in the rate of change at 72 hours after admission to evaluate the predictive value in 28 day mortality,the sensitivity and specificity were 89.8％ and 75.6％,respectively.(4)Kaplan-Meier survival analysis showed that the 28 day survival rate was significantly higher and the length of survival was significantly greater when platelet counts at 0 and 72 hours after admission to ICU were higher than the cut-off value,and this also occurred in changes in platelet count and rates of change at 72 hours after admission to ICU.Conclusions Continuous decline in serum p[atelet levels indicates poor prognosis.When combined with platelet counts,changes in platelet counts and rates of change in platelet counts at 72 hours after admission to ICU,it may play an important role in assessing the prognosis of aged patients with severe pneumonia.