Antimicrobial use in Primary Care

Susceptibility testing for Vancomycin in S. aureus : What do we do now?

14th March 2013

Robin A Howe

Susceptibility testing for Vancomycin in S. aureus : What do we do now?• History

• What are we aiming to identify?

• Testing issues

• Current recommendations

• 1997: VISA– Mu50 Vanc MIC 8mg/L

• 2001: SARV (S. aureus with Reduced susceptibility to Vancomycin)– MIC 4mg/L– Associated with mortality in Case-control study

• 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

Hiramatsu et al. (1997) JAC 40: 135Fridkin et al. 41st ICAAC (2001)

• 1997: hetero-VISA– Mu3 Vanc MIC 2-4mg/L but population able to

grow in higher vanc concentrations

Hiramatsu et al (1997) Lancet 350: 1670

0 1 2 3 4 5 6 7 810 2

10 3

10 4

10 5

10 6

10 7

10 8

10 9

10 10

Mu 3

Mu50

MSSA

MRSA

Vancomycin (mg/L)

Col

ony

Cou

nt (c

fu/m

L)– Numerous case reports associating hVISA with treatment failure (lack of systematic studies)

Is there an association between high MICs below BP and poor outcome?

• Meta-analysis• high-VMIC

– VMIC >1 mg/L but ≤2 mg/L)

• 33 studies identified

Mavros et al. (2012) IJAA. 40: 496Mortality

Is there an association between high MICs below BP and poor outcome?

• Meta-analysis• high-VMIC

– ≥1 mg/l by BMD– ≥1.5 mg/l by Etest

• 20 studies identified

Jacob et al. (2013) IJID. 17: e93

Treatment failure

What should we aim to identify?

• Vanc MIC >2mg/L (ie BP)• ?Vanc MIC ≥1.5mg/L

– Caution about method• ?hVISA

– PAP analysis can help to define

resistance

Wootton et al (2005) AAC 49(9): 3982

What should we aim to identify?

• Vanc MIC >2mg/L (ie BP)• ?Vanc MIC ≥1.5mg/L

– Caution about method• ?hVISA

– PAP analysis can help to define

resistance

Wootton et al (2005) AAC 49(9): 3982

Pennsylvania VRSA(vanA +ve)MIC 32 mg/L

Tenover et al. AAC (2004) 48: 275.

Disc testing

14 VSSA 49 hVISA20 VISA tested by disc

Vancomycin 5 MHA

0

10

20

30

40

50

60

70

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Zone Diameter (mm)

Freq

uenc

y

V5 (VISA)

V5 (hVISA)

V5 (VSSA)

Vancomycin 5 ISO

0

10

20

30

40

50

60

70

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Zone Diameter (mm)

Freq

uenc

y V5 (VISA)

V5 (hVISA)

V5 (VSSA)

Vancomycin 30 MHA

0

10

20

30

40

50

60

70

17 18 19 20 21 22 23 24 25

Zone Diameter (mm)

Freq

uenc

y

V30 (VISA)

V30 (hVISA)

V30 (VSSA)

Vancomycin 30 ISO

0

10

20

30

40

50

60

70

18 19 20 21 22 23 24 25 26 27

Zone Diameter (mm)

Freq

uenc

y

V30 (VISA)

V30 (hVISA)

V30 (VSSA)

Davies LE et al. ICAAC (2009). D803.

Disc testing

MIC testing• “Gold Standard”:

– Microbroth dilution MIC– ISO 20776– MH broth

• Ease of use standard:– Gradient strips– (Etest, MICE, MICtest)

0.5

0.750

0000

0000

0001 1 1.5 2

02468

1012

Vancomycin vs. ATCC 29213

V Etest MHAV MICE MHAV Etest ISOV MICE ISO

Replicate testing (x15) of QC strains by MICE & Etest gradient strips

CLSI QC Range: 0.5 – 2 mg/L

Richards J et al. ECCMID (2012). P1652.

• 8 VISA, 59 VSSA(MRSA)

• MIC determination by– Etest (BioMerieux)– MICE (Oxoid)– MIC test

(Liofilchem/Launch diagnostics)

– ISO MBD.

Richards J et al. ECCMID (2012). P1652.

• 182 pts with SAB

• Related vancomycin AUC/MIC to outcome(target >400)

• Median AUC/MIC– MBD – 436.1– Etest - 271.5

• CART analysis– AUC/MIC (MBD)

>373 assoc with ↓mortality

0.25 0.38 0.5 0.75 1 1.5 2 30

20406080

100120140160

Etest BMD

MIC (mg/L)

No

of is

olat

es

Holmes et al AAC (2013) epub.doi:10.1128/AAC.01485-12

“Etest tends to give slightly higher MICs”

Commercial systems• Vitek2 undercalls

resistance• Phoenix overcalls

resistance

Swenson et al. (2009) JCM 47: 2013

Method SN SP NPV PPV hGISA GISA Total GI hGISA GISA Total GI MET 58.8 92.3 71.1 100 70.4 96.2 69.4 100 GRD (24) 58.8 100 72.7 100 70.4 100 70.4 100 GRD (48) 80.4 100 87 100 83.3 100 83.3 100

Screening tests

BHIA6V MHA5T MET0

25

50

75

100

SDBHIA6V - 14.59MHA5T - 9.93

MET - 8.44

Method type

%ag

e st

rain

s co

rrec

tly

CT

'd p

er la

b

% Sensitivity

% Specificity

BHI - 4V 71 88 Macro Etest 96 97 BHI - 6V 22 97 MHA - 5V 20 99

• Screening agars– BHI/MHA– V/T – 4/5/6– MHA5T best

– Macro Etest (2 McF)– Good performance

– GRD Etest (0.5 McF)– Good performance

Walsh et al (2001) JCM 39: 2439Wootton et al (2007) JCM 45: 329Wootton et al (2008) ICAAC D2214

Conclusions• Aim to identify MIC >2mg/L• Closeness of BP to MICs of wild population

challenges all methods• Disc testing not reliable• MIC methods require close attention to QC/QA• Automated systems struggle• Population analysis can confirm reduced

susceptibility• Screening methods may have a role but lack

sensitivity

Current recommendations• Do not use disc testing• Use MIC method

– BMD gold standard– Control all tests with QC strains

(monitor QC results for trends even within appropriate QC range)

• Consider targeted testing

AcknowledgementsDr Mandy WoottonLeanne DaviesJennifer RichardsProf Tim Walsh

Jenny AndrewsBSAC AST Working Group