Quality controlQC of antibiotic susceptibility testing

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Quality control(QC) of antibiotic susceptibility
testing

• The routine laboratory

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A culture of QC is essential
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Other Essential ingredients

• The Commitment
• Value of antibiotic QC
• For the patient
• The laboratory
• The Hands and Heads
• Skilled personnel
• The media
• The incubator
• The bug
• The antibiotics
• The standard operating procedures (SOP)

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Commitment recognising the value of QC

• To monitor and ensure the accuracy and precision
  of our antibiotic susceptibility testing
• Allows us to rely on the reproducibility and
  accuracy of our daily results
• Allows communication of accurate and meaningful
  results to the clinician

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The hands and heads - personnel

• Essential that everyone is aware of the
  importance of QC
• Training of personnel in correct technique
• Storage of discs
• Preparation of a standard inoculum
• Swabbing of plates
• Choice and storage of media
• Timing and methods of incubation of plates
• Measurement of zone sizes
• Recording of results

Preparing a standard inoculum
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The Media essentials

• Thickness
• pH
• Cations
• Magnesium
• Calcium
• Storage
• Dry before use
• At room temperature before use

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Incubator

• Knowledge of requirements for incubation
• Ambient air at 350C for non fastidious aerobic
  and facultative anaerobic bacteria
• Timing generally 16-18 hours
• 24 hours for S.aureus and enterococci
• 5 CO2 for Haemophilus
• Stacking of plates
• No more than 5 plates high

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The bug

• ATCC strains
• Correctly stored
• Sub-cultured regularly
• Pure culture
• Correct inoculum preparation
• Timing of swabbing
• within 15 minutes of preparing inoculum

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Antibiotic discs

• Stored and handled correctly
• Refrigeration taken out 1 hour before use
• Expiry dates noted
• Discs at room temperature before use
• Avoid condensation
• Placing of discs within 15 minutes of swabbing

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Decide on your procedure

• Who will do it?
• How often?
• What does the NCCLS say?
• Daily
• Weekly
• Who will assist with troubleshooting?
• Designate an individual
• How will problems be communicated to clinicians
• When?

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When things go wrong

• Questions to ask?
• Is the procedure correct?
• Check test materials including test strains
• Check equipment
• Fridges
• Incubators
• Freezers
• Review technique of personnel

Culture of general QC in lab essential
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Troubleshooting
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Its Tuesday - the ENT bench reads their AST
QC..

• The ATCC strain of E.coli has been used to QC the
  gram negative antibiotic panel
• The zone size for Ampicillin is 10
• Accuracy limit from NCCLS is 16-22
• Zone too small
• What now?

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Possible errors?
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Possible errors

• Inoculum too heavy?
• Antibiotic deteriorated?
• Mueller Hinton agar too deep?
• Incorrect reading of result?
• Wrong QC strain?

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Investigation ATCC strain E.coli 35218 was used
this is a beta lactamase producing strain
Corrective action Test repeated with correct
ATCC strain 25922 Plates clearly labelled for
future use Personnel to check labels carefully
before use
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Blood culture bench QC performed weekly

• Monday
• New sub-culture of ATCC strain of Pseudomonas
  aeruginosa
• Tuesday
• AST QC performed for antibiotic panel for
  Pseudomonas
• Wednesday
• Plates read
• Zone sizes universally too large

Investigate
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Possible errors?
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Possible errors

• Inoculum too light
• Antibiotic too potent
• QC strain
• Agar too thin
• Incorrect recording of results

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Investigation Mueller Hinton agar depth 2.5mm.

• Corrective action new batch of agar used with
  correct depth of 4mm

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Any additional action required?
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Additional action

• If possible, results should be held back until QC
  error is corrected
• Significant blood culture results for patients
  tested on this batch of media should be reviewed
  and discussed. Repeated if required.
• Highlights some of the disadvantages of weekly
  testing

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Pus bench AST QC

• This bench performs a weekly QC or whenever a new
  batch of discs is opened
• ATCC Enterococcus faecalis strain used to QC
  trimethoprim sulphamethoxazole (TMP-SMX)
• Zone too small
• ATCC S.aureus used for gram positive panel
• Zone for erythromycin too small

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Investigation

• Quality of media should be investigated
• pH will affect macrolides, tertracyclines and
  aminoglycoside
• In this case the pH was too low acidic
• Ideal pH 7.2-7.4
• TMP-SMX is affected by the amount of thymidine in
  media
• This QC may indicate the presence of excess
  thymidine in the agar which will allow the
  bacteria to bypass the inhibitory effects of
  TMP-SMX
• Corrective action should be taken in house or
  with the manufacturer and QC repeated with a new
  batch

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Your experiences
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Some final thoughts

• AST QC needs a culture of general QC in the
  laboratory
• Systems for performing, recording and
  troubleshooting should be documented in writing
• Any errors should be investigated timeously and
  systematically
• Results can then be reported with confidence and
  permit appropriate and safe antimicrobial use