8 th July 2014 Why some tests are no longer recommended Robin A Howe, Cardiff, UK Antimicrobial use in Primary Care
Tests that are no longer recommended Burkholderia cepacia complex testing Stenotrophomonas maltophillia testing (except co-trimoxazole) Glycopeptide disc testing of staphylococci Colistin disc testing
Testing aims to predict efficacy Clinically Susceptible (S) a micro-organism is defined as susceptible by a level of antimicrobial activity associated with a high likelihood of therapeutic success Requirements for susceptibility testing Reliable/reproducible test Relationship between test and clinical outcome
B. cepacia complex: a reliable/reproducible test? Gold standard MIC performed to ISO 20776-1 (2006) Microbroth dilution ID Mueller-Hinton Broth Total B.ambifaria 6 B.anthina 4 B.cenocepacia 42 B.cepacia group K 4 B.cepacia gv I 10 B.dolosa 3 B.multivorans 17 B.pyrrocinia gv IX 4 B.stabilis 4 B.vietnamensis 6 Grand Total 100
B. cepacia complex: ISO MBD vs Etest MIC breakpoint S R > BSAC/ PKPD 4 8 EUCAST Pseudomonas 8 8 CLSI B. Cepacia 8 16 Ceftazidime ISO Microbroth Dilution MIC (Mueller-Hinton) 0.5 1 2 4 8 16 32 64 >128 0.5 2 0.75 1 2 1 1 1 5 3 1.5 3 14 5 1 2 7 10 3 2 3 5 6 2 1 4 1 4 5 6 2 1 8 12 1 1 16 1 24 1 32 1 48 1 64 96 1 >128 2 1 2 Etest
B. cepacia complex: BSAC AD vs Etest S R > MIC breakpoint BSAC/ PKPD 4 8 EUCAST Pseudomonas 8 8 CLSI B. Cepacia 8 16 Ceftazidime BSAC Agar Dilution MIC (Isosensitest) <0.008 0.25 0.5 1 2 4 8 16 32 64 128 >128 0.25 0.5 1 1 0.75 1 1 1 1 1 3 4 1 1 1.5 3 3 7 9 1 2 2 2 2 7 6 2 1 3 2 4 6 2 4 1 2 2 3 2 6 1 1 1 8 12 2 16 1 24 1 32 1 48 1 64 96 1 128 >128 1 3 1 Etest
B. cepacia complex: ISO MBD vs BSAC AD Ceftazidime BSAC Agar Dilution MIC (Isosensitest) MIC breakpoint S R > BSAC/ PKPD 4 8 EUCAST Pseudomonas 8 8 CLSI B. Cepacia 8 16 ISO Microbroth Dilution MIC (Mueller-Hinton) 0.5 1 2 4 8 16 32 64 128 >128 <0.008 1 1 0.25 2 3 1 0.5 2 2 1 4 5 4 2 1 2 11 9 2 2 4 1 9 8 6 1 3 8 4 3 2 16 1 32 2 2 1 64 128 2 1 >128 1 1
B. cepacia complex: a relationship between MIC and clinical outcome? Relationship not defined in vivo vs in vitro expression of resistance Biofilm infection Intracellular infection Often mixed Often treated with combinations MIC breakpoint S R > BSAC/ PKPD 4 8 EUCAST Pseudomonas 8 8 CLSI B. Cepacia 8 16
B. cepacia complex: Ceftazidime MIC distribution 25 20 Number 15 10 5 B. Cenocepacia B. Cepacia BCC Other Non BCC 0 0.5 1 2 4 8 16 32 64 128 >128 MIC
B. cepacia complex Multiple resistance mechanisms (?variable expression) No reliable gold-standard test No defined relationship between MIC & clinical outcome Wide MIC distribution across PKPD BP No AST method
Stenotrophomonas maltophilia Multiple resistance mechanisms (?variable expression) No gold standard test Results markedly affected by incubation temperature, culture medium and technique V little data to relate test to clinical outcome only test Co-trimoxazole
EUCAST guidance
2010 BSAC v9 2011 - BSAC v10 disc criteria removed MIC breakpoint (mg/l) Disc content (mg) Interpretation of zone diameters (mm) R > S R S Enterobacteriaceae 2 2 25 19 20 Pseudomonas 2 2 25 13 14 1981 NCCLS disc criteria removed ~2002 CLSI re-instate disc criteria (amended criteria)
Colistin: Reliable/reproducible test? 200 isolates P. aeruginosa, Acinetobacter, S. maltophilia, B. cepacia, Enterobacter spp., Klebsiella spp. etc. 5% VME (1981 NCCLS criteria) 3.5% VME (modified criteria) Gales A et al (2001) JCM 38: 183
Colistin: Reliable/reproducible test? 61 Acinetobacter spp., 56 P. aeruginosa, 26 E. coli, 42 Klebsiella spp., 43 Enterobacter spp. Prod Lit: 81% R reported S Prod Lit (Gales criteria): 44% R reported as S BSAC: 79% R reported as S SFM: 89% R reported as S Tan & Ng (2006) JAC 58: 864
Colistin: Reliable/reproducible test? 88 P.aeruginosa Previous BSAC Zone criteria 8.0% VME 77.8% of R reported S ISO MBD MIC (mg/l) Zone diameter (10 mcg disc) 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 0.25 2 1 0.25 0.5 1 3 1 1 1 2 5 8 1 1 2 1 1 2 11 15 10 3 4 1 6 3 8 1 1 1 1 1 16 32 1 64 1 1 >64 1
Colistin Disc testing unreliable perform MIC MIC issues?tween 80 Etest Hindler & Humphries (2013) JCM 51: 1678
S. aureus & vancomycin 1997: VISA Mu50 Vanc MIC 8mg/L 1998: hvisa Mu3 MIC 2-4mg/L but population 10 4 able to grow in higher vanc 10 3 concentrations 10 2 0 1 2 3 4 5 6 7 8 2003: VRSA High-level resistance (>32mg/L) due to acquisition of vana Remains v rare VANCOMYCIN MIC (mg/l) 1 2 4 8 16 32 CLSI (OLD) VSSA VISA VRSA BSAC/EUCAST (OLD) VSSA VRSA CLSI (NEW) VSSA VISA VRSA BSAC/EUCAST (NEW) VSSA VRSA Colony Count (cfu/ml) 10 10 10 9 Mu 3 10 8 10 7 10 6 10 5 Vancomycin (mg/l) Mu50 MSSA MRSA
Disc testing Pennsylvania VRSA (vana +ve) MIC 32 mg/l Tenover et al. AAC (2004) 48: 275.
Disc testing Vancomycin 5 MHA Vancomycin 5 ISO 70 70 60 60 Frequency 50 40 30 20 10 0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Zone Diameter (mm) V5 (VISA) V5 (hvisa) V5 (VSSA) 14 VSSA 49 hvisa 20 VISA Frequency 50 40 30 20 10 0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Zone Diameter (mm) V5 (VISA) V5 (hvisa) V5 (VSSA) 70 60 Vancomycin 30 MHA tested by disc 70 60 Vancomycin 30 ISO Frequency 50 40 30 20 V30 (VISA) V30 (hvisa) V30 (VSSA) Frequency 50 40 30 20 V30 (VISA) V30 (hvisa) V30 (VSSA) 10 10 0 17 18 19 20 21 22 23 24 25 Zone Diameter (mm) 0 18 19 20 21 22 23 24 25 26 27 Zone Diameter (mm) Davies LE et al. ICAAC (2009). D803.
% isolates S 14 S 16 R reported S 27.8 0 S reported R 6.4 23.4
MIC testing Gold Standard : Microbroth dilution MIC ISO 20776 MH broth Ease of use standard: Gradient strips (Etest, MICE, MICtest)
Replicate testing (x15) of QC strains by MICE & Etest gradient strips 12 10 8 6 4 Vancomycin vs. ATCC 29213 V Etest MHA V MICE MHA V Etest ISO V MICE ISO CLSI QC Range: 0.5 2 mg/l 2 0 0.5 0.75 1 1.5 2 Richards J et al. ECCMID (2012). P1652.
S. aureus & vancomycin Disc testing should NOT be used. An MICbased method should be used. Whichever method is used, a control strain (ATCC 25923 or ATCC 29213) should be tested in parallel with each run of test organisms Monitor performance of QC (even within acceptable range)
Conclusions Susceptibility testing requires Reliable/reproducible test Relationship between test and clinical outcome B. cepacia Testing poorly reliable/reproducible Poor definition of relationship with outcome S. maltophila Testing poorly reliable/reproducible Poor definition of relationship with outcome Co-trimoxazole AST remains challenging Colistin AST Disc testing unreliable MIC testing remains challenging (method development) Vancomycin AST for staphylococci Disc testing unreliable MIC testing remains challenging (BP close to wild-type popln)