False resistance to amikacin in Pseudomonas aeruginosa with the MicroScan WalkAway 96 system

Abstract number: p444

Unda  F., Calvo  J., Armengol  M., Martínez-Martínez  L.


To determine the reliability of susceptibility testing results of P. aeruginosa strains reported as resistant to amikacin (AMK) with the MicroScan WalkAway 96 system (W/A).


We evaluated 131 consecutive non duplicated P. aeruginosa strains isolated during February 2004–January 2005, resistant to AMK according to the W/A system. Identification and susceptibility testing was performed according to manufacturer's instructions. Two type of panels with the following concentrations (mg/L) of aminoglycosides were used: Negative Combo 1S panel [gentamicin (GEN): 4 and 8; tobramycin (TOB): 4 and 8, AMK: 8 and 16] and Urine Combo 1S panel (GEN 4 and 8; TOB 4 and 8; AMK 16 and 32). The results were compared with those obtained by reference microdilution (CLSI guidelines). MICs were also obtained with Etest. For comparisons, MICs by Etest within two values of the 2-log dilution scale were rounded up to the next higher dilution. Percentages of agreements in clinical categories were calculated. Disagreements were defined as very major errors (VME: resistant by microdilution and susceptible by W/A or Etest), major errors (ME: susceptible by microdiluiton and resistant by W/A or Etest) and minor errors (mE: intermediate by one method and susceptible or resistant by the other one). Agreements of MICs by Etest within one and within two dilution steps of those obtained by microdilution were also calculated.


Percentages of agreement in clinical categories between W/A and microdilution for GEN, TOB and AMK were 81.7%, 92.4% and 3.0%, while for Etest and microdilution the corresponding values were 97.7%, 100% and 99.2%. No VME were obtained with W/A, but ME were observed for 18/131 (13.7%), 9/131 (6.9%) and 120/131 (91.6%) of the strains. mE with W/A were obtained in 6/131 (4.58%), 1/131 (0.7%) and 7/131 (5.3%) of the strains. Only mE were obtained with the Etest: 3/131 (2.3%) for GEN and 1/131 (0.7%) for AMK. Agreement within one and within two dilution step of MICs by Etest and by microdilution were 91.6% and 98.4% (AMK), 81.6% and 94.6% (TOB) and 87.0% and 97.7% (GEN).


In his study, the W/A system was not reliable for susceptibility testing of P. aeruginosa to AMK, as most strains reported as AMK-resistant actually were susceptible to this antibiotic. To a lesser extent, major errors were also noted for GEN and TOB. On the other hand the Etest agreed very well with reference microdilution for all three aminoglycosides.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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