Inability to control selected drugs on commercially obtained microdilution MIC panels

Area of Science:

• Clinical microbiology
• Antimicrobial resistance
• Diagnostic testing

Background:

• The proliferation of broad-spectrum antimicrobics necessitates comprehensive susceptibility testing.
• Manufacturers often compromise the range of dilutions tested in microdilution panels to accommodate new drugs.
• Accurate minimal inhibitory concentration (MIC) determination is crucial for effective antimicrobial therapy.

Purpose of the Study:

• To evaluate the reliability of Micro Scan 96-well microdilution panels for antimicrobial susceptibility testing.
• To determine if on-scale MIC endpoints could be achieved for drugs in both gram-positive and gram-negative panels.
• To assess the control procedures for specific antimicrobics as per manufacturer instructions.

Main Methods:

• Testing of Micro Scan gram-positive and gram-negative 96-well microdilution panels.
• Use of suggested control organisms over a 60-day period.
• Evaluation of MIC endpoint achievement for individual drugs.

Main Results:

• Seven of 13 antimicrobics in the gram-positive panel showed inadequate control (clindamycin, tetracycline, cephalothin, gentamicin, nitrofurantoin, trimethoprim-sulfa, streptomycin).
• Six of 18 antimicrobics in the gram-negative panel also exhibited poor control (tobramycin, trimethoprim-sulfa, cefoperazone, mezlocillin, ticarcillin, kanamycin).
• The accuracy of patient isolate results for these problematic antimicrobics could not be validated by the provided control procedures.

Conclusions:

• Several key antimicrobics in Micro Scan panels do not meet control criteria for reliable MIC testing.
• The current control procedures may be insufficient to guarantee the accuracy of susceptibility results for certain drugs.
• Further investigation and potential adjustments to panel design or control methods are warranted for improved diagnostic accuracy.