Direct application of Etest on positive blood cultures more than fast susceptibility testing only?

Abstract number: P1871

Bjørnholt J.V., Brastad T., Kløvfjell L., Johnsen B.O., Andersen C.T., Leegaard T.M.

Objective: To validate and implement direct Etest on positive blood cultures in a routine laboratory.

Methods: Samples taken directly from consecutive Bactec blood culture bottles flagging positive and with Gram stain showing bacteria. They were plated onto 140 mm Mueller-Hinton plates with 5% sheep blood and Etest MIC's were determined for: 1) Gram-negative rods: cefotaxime, meropenem, ciprofloxacin, co-trimoxazole, gentamicin and one optional antibiotic. 2) Gram-positive cocci in clusters: vancomycin, gentamicin, ampicillin, clindamycin with adjacent erythromycin disk for MLS-B, cefoxitin, one optional antibiotic. In addition they were plated onto an oxacillin screening plate. 3) Gram-positive cocci in chains: Penicillin G (low), vancomycin, gentamicin (high), ampicillin, clindamycin with adjacent Erythromycin disk for MLS-B and one optional antibiotic. The plates were read as early as possible, after 3–8 hours dependent on growth (Etest I), and reincubated for reading next work day (Etest II). All isolates were identified and susceptibility testing was performed according to routine methods (VITEK 2 or Etest depending on species and additional methods if needed). Time to Etest I and discrepancies between Etest I and II and routine susceptibility were recorded.

Results: 155 positive blood cultures were included, 120 analysed per protocol. (19 mixed cultures and 16 not indicated when Etest was read were excluded). 5 positive cultures did not show any growth on resistance plates (3 anaerobic isolates, 1 Haemophilus influenzae, 1 Actinomyces spp.). Median time to Etest I 6 hours (range 3–24). There were no major inconsistencies between Etest I and II or between Etest I and routine susceptibility, except from difficulties in interpretation of co-trimoxazole for Gram-negative rods and clindamycin for staphylococcus and enterococcus species, see table 1.

Conclusion: Direct Etest on positive blood cultures gives rapid and reliable results on antibiotic susceptibility although bacteriostatic antibiotics should be interpreted carefully. Maybe even more important than the fast susceptibility testing was the continuous monitoring of growth which resulted in a much faster identification of the isolates. An automated ID or manual ID could be initiated at the same time as the reading of Etest I, meaning that in many cases identification and susceptibility testing was on the clinicians desk 24 hours earlier than previously.

Table 1. Proportion of major groups of isolates with inconsistencies between Etest I and II, and affected antibiotics. Proportion of isolates where Etest I could not be read due to insufficient growth

 Inconsistencies*No growthDrugs affected
Staphylococcus aureus3/162/16clindamycin/ampicillin
Coagulase neg. staphylococci10/295/29clindamycin/ampicillin
Enterococcus species1/103/10clindamycin
Other Gram-positive cocci2/90clindamycin/penicillin G
Corynebacterium speciesNA6/6All
*defined as geqslant R: gt-or-equal, slanted2 dilutions difference between readings.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
Back to top