Inclusion Criteria:  critically ill hospitalized males or nonpregnant females aged 18 years with severe infections which the treating clinician was treating with tigecycline  Exclusion Criteria:  severe liver dis- eases (e.g., Child-Pugh score C); patients allergic to tetracycline and tigecycline; those who have participated in other clinical trials, or those who are considered unsuitable by researchers; pregnant women and lactating women.