Etest® Reading

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Etest® Reading

© AB bioMérieux 2008

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Etest Reading

Endpoint criteria relates to: Technical issues Drug specific Organism specific Resistance mechanisms

Consult technical documents Etest Reading Guides (ERG 001, 002) Customer Information Sheets (CIS 006, CIS 007) Etest Technical Guide (ETG 006) – Mycobacteria

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Small molecule, larger ellipse Large molecule, smaller ellipse

Molecule Size

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Basic Etest Reading

Read MIC where growth intersects the strip

If between markings, read upper MIC value

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Between Markings

Read the upper value

First Reading Step – Sharp Endpoints

Is it a sharp endpoint?

YES: then read MIC where growth intersects the strip. VA, MIC 1.0 µg/mL

Second Reading Step – Not a Sharp Endpoint

Is it a sharp endpoint? NO: Is the drug bacteriostatic or

bactericidal?

Rule: CIDAL DRUGS: Read at complete

inhibition of ALL GROWTH (haze, micro- and macrocolonies), Read 100% inhibition. MIC 0.064 µg/mL

STATIC DRUGS: Read at the first significant inhibition of growth, Ignore trailing and microcolonies, Read so-called 80% inhibition. MIC 0.032 µg/mL

static

cidal

Macrocolonies

Always read macrocolonies for both static and cidal drugs.

MIC ≥ 256 µg/mL

Sharp or Hazy Endpoint?

Sharp or Hazy Endpoint?

Sharp or Hazy Endpoint?

Sharp or Hazy Endpoint?

Sharp or Hazy Endpoint?

Sharp or Hazy Endpoint?

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Avoid too heavy an inoculum

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Ignore tunnel of growth along strip

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If uneven intersection - read upper value if ≤ 1 dilution

If > 1 dilution, test must be repeated

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Proteus - don’t read the swarm!

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Don’t read haemolysis

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Bactericidal Drugs

• β-lactams

• Carbapenems

• Cephalosporins

• Ethambutol

• Glycopeptides

• Fosfomycin

• Isoniazide

• Aminoglycosides

• Quinolones

• Metronidazole

• Rifampicin

• Amphotericin B

Read at complete inhibition

• Look for hazes, micro- and macrocolonies

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Pneumo - Pen G

Obtain good confluent growth

Look for microcolonies and hazes

MIC: 4.0 µg/mL

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Look for colonies in the ”dip”

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Observe microcolonies from 8 to 32

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Paradoxical effect - beta-lactams

Induction of regrowth at higher concentrations of the antimicrobial

Read the highest value

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Bacteriostatic Drugs

• Tetracyclines

• Streptogramins

• Oxazolidinones

• Macrolides

• Azoles

• Lincomycins

• Chloramphenicol

• Sulfonamides

• Trimethoprim

• Echinocandins

Read at 80% inhibition i.e. first point of significant change as judged by the naked eye (CIS 007).

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80% for Bacteriostatic Drugs

Bacteriostatic drugs

Endpoint rule:80% inhibition

MIC: 0.032 µg/mL

Comment:Read at the first point of significant inhibition as judged by the naked eye

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Linezolid

Static endpoint at 90% inhibition MIC: 1 µg/mL

Trailing: 0.5 – 2 µg/mL

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MRSA - low level heteroresistance

Other Reading Rules

Pipercillin/tazobactam

Ticarcillin/clavulanic acid

Ignore the inhibition of growth caused by the activity of the inhibitor (tazobactam, clavulanic acid) and extrapolate the ellipse to give the MIC of pipercillin or ticarcillin.

Echinocandins

Ignore regrowth at higher concentrations (paradoxical effect) for Echinocandins

MIC: 0.125 µg/mL

Distorted ellipse

Wet surface, repeat test

Interpretation of POSITIVE Etest ESBL

Cephalosporin with Clavulanic acid shows 3 log dilution lower than cephalosporin without Clavulanic acid; ratio of CT /CTL ≥8

Deformation of the ellipse Phantom zone

→ All are Positive for ESBL See Etest ESBL Package Insert

ESBL POSITIVE CT ≥ 0.5 and CT/CTL ≥ 8or

TZ ≥ 1 and TZ/TZL ≥ 8or

PM ≥ 0.25 and PM/PML ≥ 8or

“Phantom” zone or deformation of the CT, TZ

or PM ellipse

Report:

ESBL producer and

resistant to all penicillins,

Cephalosporins

and aztreonam (CLSI

M100-S series).

ESBL

MIC: CT 1.5 & CTL 0.047

CT/CTL 1.5/0.047= 32 = ESBL+

ESBL +

Deformation of ellipse = ESBL +

Phantom zone = ESBL +

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Categorisation of S, I, R

Use MIC breakpoints from most recent CLSI M100--S or own regulatory body (eg. EUCAST,BSAC)

If not available, consult the literature Etest gives values in-between 2-fold dilutions

Round-up to next upper dilution before S, I, R

Report the exact MIC and S, I, R

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Interpreting 1/2 Dilution

S: ≤ 0.064

I: 0.125 - 1

R: ≥ 2

MIC 1.5 becomes 2 µg/mL

Category = R

R

I

S

Technical Documents

Consult technical documents Etest Reading Guides

• ERG 001 – antimicrobials

• ERG 002 - antifungals Customer Information Sheets

• CIS 006 – list of static/cidal drugs

• CIS 007 – reading of sharp and hazy endpoints Etest Technical Guide (ETG 006) – Mycobacteria

Etest Reading Practice

© AB bioMérieux 2008

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