ObjectiveTo investigate clinical characteristics and influencing factors of lower respiratory tract infection of Acinetobacter baumannii (AB-LRTI) in respiratory intensive care unit (RICU).MethodsClinical data were collected from 204 RICU patients who were isolated Acinetobacter baumannii (AB). The bacteriological specimens were derived from sputum, bronchoscopic endotracheal aspiration, bronchoalveolar lavage fluid, pleural effusion and blood. The definition of bacterial colonization was based on the responsible criteria from Centers for Disease Control and Prevention/National Medical Safety Network (CDC/NHSN). The patients were divided into three groups as follows, AB colonization group (only AB was isolated, n=40); simple AB-LRTI group (only AB was isolated and defined as infection, n=63), AB with another bacteria LRTI group (AB and another pathogen were isolated simultaneously, n=101). The epidemiology, clinical characteristics and influencing factors of each group were analyzed and compared. ResultsCompared with the AB colonization group, the AB with another bacteria LRTI group had higher proportion of patients with immunosuppression, specimens from sputum and bronchoalveolar lavage fluid, more than 4 invasive procedures, 90-day mortality, white blood cell count >10×109/L (or <4×109/L), neutrophil percent >75% (or <40%), lymphocyte count <1.1×109/L, platelet count <100×109/L, albumin <30 g/L, high sensitivity C-reactive protein >10 mg/L, and neutrophil-to-lymphocyte ratio (NLR). The frequency of bronchoscopy and days of infusing carbapenem within 90 days before isolating AB, the Acute Physiology and Chronic Health Evaluation Ⅱ score, the proportion of patients with invasive mechanical ventilation and the duration of invasive mechanical ventilation in the AB with another pathogen LRTI group were higher than those in the AB colonization group (all P<0.05). Days of infusing carbapenem and β-lactams/β-lactamase inhibitors within 90 days before isolating AB, proportion of septic shock, NLR and 90-day mortality of the patients from the AB with another pathogen LRTI group were more than those in the simple AB-LRTI group (all P<0.05). After regression analysis, more than 4 invasive procedures, or immunosuppression, or with more days of infusing carbapenem within 90 days before isolating AB were all the independent risk factors for AB-LRTI.ConclusionsThere are significant differences in epidemiology, clinical symptoms and laboratory indicators between simple AB-LRTI, AB with another pathogen LRTI and AB colonization in RICU patients. For RICU patients, who suffered more than 4 invasive procedures, immunosuppression, or with more days of infusing carbapenem within 90 days before isolating AB, are more susceptible to AB-LRTI.