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Inducible resistance to clindamycin in Staphylococcus aureus: validation of Vitek-2 against CLSI D-test
B J Gardiner1, M L Grayson, G M Wood
1Infectious Diseases Department, Austin Health, Victoria, Australia. bradgardiner@gmail.com
Aims:
Inducible resistance to clindamycin in Staphylococcus aureus is common but not easily identified by routine testing, and can result in treatment failure if not detected. The gold standard method is the D-test described by the Clinical and Laboratory Standards Institute (CLSI). The Vitek-2 AST-P612 card contains an 'inducible clindamycin resistance' (ICR) test. We aimed to determine the accuracy of the Vitek-2 ICR test compared to the D-test.
Methods:
Isolates of erythromycin non-susceptible, clindamycin susceptible Staphylococcus aureus were identified. Routine antimicrobial susceptibility testing was performed using the Vitek-2 AST-P612 card, including the ICR test, and compared against the D-test.
Results:
217 isolates were obtained. All of the 191 isolates that were ICR positive were D-test positive. Of the 27 ICR negative isolates, 10 (37%) were D-test positive [9 methicillin-sensitive S. aureus (MSSA), 1 methicillin-resistant S. aureus (MRSA)]. This correlates with a specificity of 100%, sensitivity of 95%, positive predictive value of 100%, and negative predictive value of 72%.
Conclusions:
The ICR test is reliable in the presence of a positive result; however there is a false negative rate of approximately one in four. This will lead to susceptibility reporting errors, with potentially serious clinical implications. A negative ICR should be confirmed by CLSI D-test before reporting clindamycin as susceptible where the organism is not susceptible to erythromycin.
Insights
The Vitek-2 inducible clindamycin resistance (ICR) test accurately identifies resistance in Staphylococcus aureus when positive. However, a significant false negative rate necessitates confirmatory D-tests for accurate clindamycin susceptibility reporting.
Area of Science:
- Clinical microbiology
- Antimicrobial resistance
- Infectious diseases
Background:
- Inducible clindamycin resistance (ICR) in Staphylococcus aureus is a growing concern, often missed by standard testing.
- Undetected ICR can lead to clindamycin treatment failure.
- The Clinical and Laboratory Standards Institute (CLSI) D-test is the gold standard for detecting ICR.
Purpose of the Study:
- To evaluate the accuracy of the Vitek-2 'inducible clindamycin resistance' (ICR) test.
- To compare the Vitek-2 ICR test performance against the CLSI D-test for Staphylococcus aureus.
Main Methods:
- Analyzed 217 Staphylococcus aureus isolates with erythromycin non-susceptibility and clindamycin susceptibility.
- Performed routine antimicrobial susceptibility testing using the Vitek-2 AST-P612 card with its ICR test.
- Compared Vitek-2 ICR test results against the CLSI D-test.
Main Results:
- The Vitek-2 ICR test demonstrated 100% specificity and 95% sensitivity.
- All 191 ICR-positive isolates were also D-test positive.
- 10 of 27 ICR-negative isolates (37%) were D-test positive, indicating a 72% negative predictive value.
Conclusions:
- The Vitek-2 ICR test is reliable for detecting inducible clindamycin resistance when the result is positive.
- A false negative rate of approximately 25% exists for the Vitek-2 ICR test.
- Negative ICR results in erythromycin-resistant isolates require confirmation with the CLSI D-test to prevent reporting errors and ensure appropriate clindamycin susceptibility reporting.
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