Rapid presumptive identification of gram-negative rods directly from blood cultures by simple enzymatic tests

Shortening identification times: comparative observational study of three early blood culture testing protocols

Background

While early appropriate antibiotic therapy is a proven means of limiting the progression of infections, especially bacteremia, empirical antibiotic therapy in sepsis is ineffective up to 30%. The aim of this study was to compare early blood culture testing protocols in terms of their ability to shorten the delay between blood sampling and appropriate antibiotic therapy.

Methods

In this french observational study, we compared three blood culture testing protocols. Positive blood cultures were tested using either GenMark ePlex panels (multiplex PCR period), a combination of MRSA/SA PCR, β-Lacta and oxidase tests (multitest period), or conventional identification and susceptibility tests only (reference period). Conventional identification and susceptibility tests were performed in parallel for all samples, as the gold standard.

Results

Among the 270 patients with positive blood cultures included, early and conventional results were in good agreement, especially for the multitest period. The delay between a blood culture positivity and initial results was 3.8 (2.9-6.9) h in the multiplex PCR period, 2.6 (1.3-4.5) h in the multitest period and 3.7 (1.8-8.2) h in the reference period (p<0.01). Antibiotic therapy was initiated or adjusted in 68 patients based on early analysis results. The proportion of patients receiving appropriate antibiotic therapy within 48 h of blood sampling was higher in the multiplex PCR and multitest periods, (respectively 90% and 88%) than in the reference period (71%).

Conclusion

These results suggest rapid bacterial identification and antibiotic resistance tests are feasible, efficient and can expedite appropriate antibiotic therapy.

A rapid and inexpensive protocol to screen for third generation cephalosporin-resistant and non-fermenting Gram-negative rods directly in positive blood cultures

Third generation cephalosporins are frequently used in the first-line treatment of Gram-negative rod (GNR) bacteremia but are unsuitable in the case of extended-spectrum-beta-lactamase-producing Enterobacterales (ESBL-E) or non-fermenting GNR infections. The aim of this study was to develop and evaluate a simple and rapid two-test protocol involving oxidase and β-Lacta tests performed directly on positive blood culture broth as a preliminary screen for non-fermenting or third generation cephalosporins-resistant GNR. The diagnostic performance of this approach was evaluated on 294 bottles for the oxidase test and 267 bottles for the β-Lacta Test. The sensitivity and specificity of the oxidase test were respectively 93.1% and 100%, and the sensitivity of the β-Lacta Test for ESBL-E was 100% and the specificity 99.5%. This simple protocol, which can be implemented in all laboratories and performed in only 20 min, may be a valuable tool to optimize first-line antibiotic therapy for bacteremia.

Identification of Gram-negative bacilli directly from positive blood culture vials

The provision of rapid results from positive blood cultures is important for the clinical management of septicaemia. This study tested the accuracy of direct inoculation of biochemical tests from positive blood culture vials for the identification of members of the Enterobacteriaceae and Acinetobacter species. A hundred and eighty-one samples were included in the study, with 25 % subsequently excluded as a result of mixed colonial growth. The study method successfully identified 133 (98 %) isolates from 136 vials to genus level and was technically simple to perform, requiring an additional 3 min for the processing of each positive vial. The results of this study demonstrate that a direct inoculation method provides acceptable genus identification of Gram-negative bacilli in positive blood culture vials, with a potential saving of 24 h compared with traditional methods.