Background  Bloodstream infection (BSI) is a common infectious disease worldwidely. In recent years, the morbidity and mortality of BSI in elderly patients are high, resulting in a heavy disease burden.

  Objective  To investigate the clinical manifestation, species distribution and factors associated with bloodstream infection (BSI) in the oldest old (≥ 85 years old) patients, so as to provide evidence for clinical treatment and improve prognosis of elderly patients with BSI.

  Methods  Clinical data about all the oldest old patients diagnosed with BSI in Chinese PLA General Hospital from January 2017 to December 2019 were retrospectively analyzed. Logistic regression analysis was used to determine the independent associated factors of 28-day death.

  Results  A total of 318 oldest old patients with BSI were enrolled in this study, with an average age of 89.32 ± 3.02 years. Three hundred and forty five strains were isolated, including 168 strains of gram-negative bacteria (48.69%), 155 strains of gram-positive bacteria (44.93%) and 24 strains of fungi (6.38%). The most common pathogens were coagulase negative staphylococci (31.30%), Klebsiella pneumoniae (14.49%), Escherichia coli (13.62%), Acinetobacter baumannii (6.67%) and Enterococcus (5.51%). Multivariable logistic regression analysis showed that with ≥ 5 comorbidities (OR=1.641, 95%CI=1.376-2.219, P=0.013), invasive mechanical ventilation (OR=1.628, 95%CI=0.532-3.036, P=0.006) were associated with death within 28 days in BSI patients aged 85 years and older.

  Conclusion  Common pathogens of BSI in oldest old patients include coagulase-negative Staphylococcus, Klebsiella pneumoniae and Escherichia coli, etc., Combined with five or more comorbidities and invasive mechanical ventilation are associated with death in these patients.