WHY COLISTIN SHOULD NOT BE TESTED WITH DISC DIFFUSION METHODS?
Dr.T.V.Rao MD
A question for post graduates in CLINICAL MICROBIOLOGY Colistin continues to be the final resource to treat the people with septic, immunosuppressed and blood stream infections. The raise of SUPERBUGS CONTINUES TO BE A CHALLENGE. The increasing occurrence of infections due to multidrug-resistant (MDR) Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae led to the revival of old and neglected antibiotics that may remain active, such as polymyxins (polymyxin B and colistin) Colistin is increasingly being used as a last-resort treatment option for infections caused by MDR organisms as matter of fact we are isolating many gram negative bacteria with Carbapenem resistance in people with long term catheterized patients, majority of the laboratories still continue to use disc diffusion method with limited resources in our laboratories, HOWEVER IT IS BEST WE CAN DO, Colistin has re-emerged as an important antimicrobial in recent times owing to limited therapeutic options against carbapenem-resistant Gram-negative bacteria. Current guidelines (BSAC, CLSI and EUCAST) recommend routine colistin susceptibility testing by estimation of MIC because the disc diffusion test does not reliably detect low-level resistance, Disk diffusion, commonly used in many clinical laboratories, yielded high error rates compared to MIC-based methods and is considered unreliable for the detection of colistin resistance if we talk the reality most of us are doing a disc diffusion method for detection of colistin resistance the zone size of 11 mm is difficult to measure with a naked eye and certainly it is advised to measure with a scale or caliper, it is all the best we can do in practice. Trends toward elevated colistin MICs have been noted worldwide, underlining the importance of accurate colistin susceptibility results. Also, suggestions for the optimal methods to be used for colistin ST have not been formulated by the CLSI Broth micro-dilution (BMD) is widely used as a method of MIC estimation in Europe and the USA. Therefore, it is critical for clinical laboratories to be aware of these discrepancies and consider applying a reference method for colistin ST, especially when colistin administration is deemed necessary.
Antimicrobial susceptibility testing of colistin has been fraught with difficulties. A joint EUCAST and CLSI subcommittee recently issued recommendations confirming that broth micro dilution (BMD) is so far the only valid method and that disk diffusion does not work because of the poor diffusion of the large colistin molecule. The report did not evaluate gradient tests of colistin, but reports in the literature have questioned the validity of MICs obtained with gradient tests.
Disk diffusion cannot be used at present for susceptibility testing of colistin. Currently available gradient tests underestimate colistin MIC values and under call resistance, and should be avoided. The broth micro dilution tests we have managed to evaluate all seem to give correct results both for susceptible and non-susceptible isolates (2)
Best conclusion is that all colistin susceptibility testing should be quality-controlled by the inclusion of a sensitive and a resistant QC strain. It just means we have not perfected methods to detect the precision in establishing the true resistance in colistin , however MIC continues to be only option which is possible with Vitec many times only resource available in better laboratories in developing countries
Current resources for practicing Clinical Microbiologists
Ref 1 Comparative Evaluation of Colistin Susceptibility Testing Methods among Carbapenem-Non susceptible Klebsiella pneumoniae and Acinetobacter baumannii Clinical Isolates Konstantina Dafopouloua, etal, Antimicrob. Agents Chemother. August 2015 vol. 59 no. 8 4625-4630
2 EUCAST warnings concerning antimicrobial susceptibility testing products or procedures. EUCAST 4 MAY 2017
Formulated for post graduate studies on World wide web resource on caring for better laboratory services
Dr.T.V.Rao MD Clinical Microbiologist
Dr.T.V.Rao MD
A question for post graduates in CLINICAL MICROBIOLOGY Colistin continues to be the final resource to treat the people with septic, immunosuppressed and blood stream infections. The raise of SUPERBUGS CONTINUES TO BE A CHALLENGE. The increasing occurrence of infections due to multidrug-resistant (MDR) Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae led to the revival of old and neglected antibiotics that may remain active, such as polymyxins (polymyxin B and colistin) Colistin is increasingly being used as a last-resort treatment option for infections caused by MDR organisms as matter of fact we are isolating many gram negative bacteria with Carbapenem resistance in people with long term catheterized patients, majority of the laboratories still continue to use disc diffusion method with limited resources in our laboratories, HOWEVER IT IS BEST WE CAN DO, Colistin has re-emerged as an important antimicrobial in recent times owing to limited therapeutic options against carbapenem-resistant Gram-negative bacteria. Current guidelines (BSAC, CLSI and EUCAST) recommend routine colistin susceptibility testing by estimation of MIC because the disc diffusion test does not reliably detect low-level resistance, Disk diffusion, commonly used in many clinical laboratories, yielded high error rates compared to MIC-based methods and is considered unreliable for the detection of colistin resistance if we talk the reality most of us are doing a disc diffusion method for detection of colistin resistance the zone size of 11 mm is difficult to measure with a naked eye and certainly it is advised to measure with a scale or caliper, it is all the best we can do in practice. Trends toward elevated colistin MICs have been noted worldwide, underlining the importance of accurate colistin susceptibility results. Also, suggestions for the optimal methods to be used for colistin ST have not been formulated by the CLSI Broth micro-dilution (BMD) is widely used as a method of MIC estimation in Europe and the USA. Therefore, it is critical for clinical laboratories to be aware of these discrepancies and consider applying a reference method for colistin ST, especially when colistin administration is deemed necessary.
Antimicrobial susceptibility testing of colistin has been fraught with difficulties. A joint EUCAST and CLSI subcommittee recently issued recommendations confirming that broth micro dilution (BMD) is so far the only valid method and that disk diffusion does not work because of the poor diffusion of the large colistin molecule. The report did not evaluate gradient tests of colistin, but reports in the literature have questioned the validity of MICs obtained with gradient tests.
Disk diffusion cannot be used at present for susceptibility testing of colistin. Currently available gradient tests underestimate colistin MIC values and under call resistance, and should be avoided. The broth micro dilution tests we have managed to evaluate all seem to give correct results both for susceptible and non-susceptible isolates (2)
Best conclusion is that all colistin susceptibility testing should be quality-controlled by the inclusion of a sensitive and a resistant QC strain. It just means we have not perfected methods to detect the precision in establishing the true resistance in colistin , however MIC continues to be only option which is possible with Vitec many times only resource available in better laboratories in developing countries
Current resources for practicing Clinical Microbiologists
Ref 1 Comparative Evaluation of Colistin Susceptibility Testing Methods among Carbapenem-Non susceptible Klebsiella pneumoniae and Acinetobacter baumannii Clinical Isolates Konstantina Dafopouloua, etal, Antimicrob. Agents Chemother. August 2015 vol. 59 no. 8 4625-4630
2 EUCAST warnings concerning antimicrobial susceptibility testing products or procedures. EUCAST 4 MAY 2017
Formulated for post graduate studies on World wide web resource on caring for better laboratory services
Dr.T.V.Rao MD Clinical Microbiologist
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