Rapid microbiological diagnosis of infections in sterile body fluids including blood

Abstract

Infections of normally sterile body fluids (SBF) including blood, cerebrospinal-, pleural-, peritoneal-, pericardial and synovial fluid are critical and require early diagnosis and rapid initiation of appropriate antimicrobial therapy. With the advanced and invasive care of modern medicine, infections of SBF are increasing worldwide. The clinical microbiology laboratory is crucial in facilitating targeted therapy through early microorganism detection, identification and antibiotic susceptibility testing. This actionable information will allow correct antimicrobial therapy early in the infectious process. Blood culture systems as well as blood culture bottles are the standard diagnostic method in blood cultures and are utilized for SBF cultures. The overall aim of this thesis was to evaluate and improve the microbiological diagnosis of SBF including blood from positive blood culture bottles.

The first study focused on BacT/Alert Virtuo blood culture system introduced into the market to replace the BacT/Alert 3D blood culture system by the same company. The study presented in this thesis showed a significantly shorter time to detection for bottles incubated in the Virtuo system than those incubated in the BacT/Alert 3D system (median 12 h and 15 h, respectively; p < 0.0001) in simulated blood cultures. In Study 2, the performance a unique method of short-term culture followed by MALDI-TOF MS was analyzed prospectively. The study showed a successful identification of 424/515 (82.3%) bacteria with MALDI-TOF MS after 5.5 h subculture on solid media. In Study 3 and 4, the FilmArray BCID panel was prospectively evaluated for accurate identification and limited antibiotic susceptibility testing. In Study 3, the FilmArray BCID showed accurate identification of microorganisms in 153/167 (91.6%) and 17/24 (71%) culture positive blood culture bottles with monomicrobial- and polymicrobial-growth. In Study 4, the performance of FilmArray BCID was analyzed for SBF other than blood. The assay could identify all microorganisms in 84/84 (100%) and 18/24 (75%) SBF samples with mono- and polymicrobial growth respectively. These studies showed that the FilmArray BCID panel is a reliable and fast method that may be utilized in blood cultures as well as other SBF that are cultured in blood culture bottles. In Study 5, the performance of four S. pneumoniae antigen tests including ImmuLex™, SlideX® pneumo-Kit, Wellcogen™, and BinaxNOW® were evaluated. The study revealed similar sensitivity of 99.6%-100% for all tests when excluding invalid results in the calculation. However, the study showed that the BinaxNow® had the lowest specificity (64.1% 9, p<0.01) among the four methods. The ImmuLex™ test had lower specificity than the Slidex® test (82.6% vs 97.6%, p<0.01) and performed similar to the Wellcogen™ test (84.5%, p=ns).

In conclusion, the present thesis showed that the new Virtuo blood culture system allows faster detection of microorganisms in blood culture bottles. The thesis further presented several methods for rapid identification of microorganisms after detection of growth in blood culture bottles. These tests included FilmArray BCID panel and the latex agglutination tests directly from blood culture bottles while MALDI-TOF MS was performed after sub-culture on solid media. These rapid tests are important complements to the rigorous and extensive conventional culture methods and provide early critical actionable information for appropriate antimicrobial stewardship.