Patients were divided into 2 subgroups:

1. No infection group (including patients with procalcitonin<0.1 ng/ml at enrollment, and patients with 0.1 ng/ml ≤procalcitonin ≤0.25 ng/ml at enrollment who are with stable respiratory and hemodynamics, and without serious complications) : the group is not given the application of antibiotics.
2. Infection group (including patients with procalcitonin>0.25 ng/ml at the time of enrollment, and patients with 0.1 ng/ml ≤ procalcitonin≤0.25 ng/ml at enrollment who have severe disease, unstable hemodynamics, and severe complications or intensive care unit treatment): the group is given the application of antibiotics. According to the guidelines for severe sepsis / septic shock treatment, the standard for discontinuation of antimicrobial agents: If the procalcitonin level falls below <0.1 ng/ml or more than 80%, compared with the baseline before enrollment, it is recommended to discontinue the antimicrobial agent.

The application of antibiotics is given to patiens according to the doctor's experience.

The application of antimicrobial agents is determined by PCT results.