# Presentation: 528807

## Quality control(QC) of antibiotic susceptibility testing 

- Quality control(QC) of antibiotic susceptibility testing

- The routine laboratory

## A culture of QC is essential  

- A culture of QC is essential

## 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)

## 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

## 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**

## The Media essentials  

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

## 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

## The bug

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

## 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

## 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?

## 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**

## Troubleshooting

## It’s 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?

## Possible errors?

## Possible errors

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

## 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

## 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**

## Possible errors?

## Possible errors

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

## Investigation – Mueller Hinton agar depth 2.5mm. 

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

## Any additional action required?

## 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

## 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

## 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

## Your experiences

## 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