D-2220. Influence of Technical Variables on Etest Daptomycin (DPC) MIC Testing of Recent Clinical MRSA
Session: Poster Session: Staphylococcus aureus Antimicrobial Susceptibility Testing
Monday, October 27, 2008: 12:00 AM
Room: Hall C
Background: DPC testing by Etest can be affected by technical variables e.g. Ca 2+ levels in Mueller Hinton agar (MHA), inoculum density, and endpoint selection. Since DPC non -susceptible (NS) clinical strains of MRSA is still rare, reports of NS results by Etest unconfirmed by reference broth microdilution (BMD) lead us to investigate the influence of different brands of commercial MHA plates, Etest lots and inoculum. Methods: Etest and BMD were compared using 38 clinical MRSA with varying DPC susceptibility. To improve precision, BMD was based on 0.25 dilution increments. MHA plates (BD and Remel) and home-made (HM) plates were compared. Inoculum (0.1 to100 fold variations) was studied with both methods. All tests were performed in triplicate. Quality control strains (QC) S. aureus ATCC 29213 and E. faecalis ATCC 29212 and reference strains S. aureus ATCC 43300 (MRSA), 700698 (hGISA) and 700699 (GISA) were included. Essential (EA) and categorical (CA) agreements were calculated using CLSI criteria. Results: MIC (µg/mL) for DPC by BMD and Etest (4 lots) using different MHA lots
GenotypeNumberBMDEtest mode (N 152) (range)
(25-30 Ca2+)
(23 Ca2+)
MHA -Remel
(40 Ca2+)
MRSA100.25-0.50.25 (0.19-1)0.5 (0.19-1.5)0.25 (0.125-1)
MRSA130.75-10.5 (0.38-1.5)1 (0.5-2)0.5 (0.25-1.5)
hGISA70.38-0.750.5 (0.38-1.5)1 (0.5-3)0.75 (0.38-1.5)
MRSA61.25-1.751 (1-3)2 (1.5-3)1 (1-2)
hGISA21.25- 2.51.5 (1-2)3 (1.5-3)1.5 (1-2)
EA (% ± 1 dil.)979297
CA (%)866088
Substantially equivalent DPC MIC results were seen with BMD and Etest used with different MHA brands. For MIC values close to the DPC breakpoint (S≤ 1 µg/mL), Etest results on MHA with Ca 2+ < 25µg/mL gave an unacceptable level of NS categorical discrepancies. Higher inoculum density gave elevated DPC results for both methods. Since QC results did not reflect Ca 2+ variation and could not be used to control this variable, and an additional QC strain (DPC mode 1 µg/mL) may be needed. To optimise the use of Etest for DPC, technical variables should be strictly standardised.
Anette Engelhardt1, Anne Yusof2, Phion Ho2 and  A. Engelhardt,
AB BIODISK Role(s): Employee., (1)AB BIODISK, Solna, Sweden, (2)AB BIODISK