BSAC rolling programme for devising acceptable limits for control strains

1. BSAC rolling programme for devising acceptable limits for control strains Jenny Andrews The BSAC Standardized Method Development Centre

2. Acceptable limits for control strains • Used by diagnostic laboratories to monitor the performance of testing. • There are gaps in the tables that need filling. • At present S. aureus is used to control testing of fastidious organisms and this may not be ideal because the growth requirements of fastidious organisms are different to that of staphylococci.

3. Findings from a previous BSAC study • Confirmed that agar poured `in-house’ to a depth between 3.5-4.5 mm, and pre-poured plates from Oxoid and bioMerieux gave acceptable results by BSAC methodology. • When combined data from these plates was used to calculate the acceptable limits for all of the antibiotic/strain combinations tested, the ranges were similar to those found in previous `field studies’. So it was proposed that this procedure could be used to fill the gaps in the tables.

4. Programme design • After discussion the Working Party decided that it would be prudent if acceptable limits were calculated using data from several laboratories rather than one centre. • However, SMDC acceptable ranges could be used as a guide for comparison.

5. Request for help • Needed the help of laboratories using the BSAC method, but unsure if laboratories would take part. • Letter sent to known users of the BSAC method • Asking if they would take part and select from a list of antibiotic/strain combinations those they were prepared to test • 20 laboratories agreed to take part

6. QC Study • SMDC calculated acceptable limits by testing discs 40 times each on ISA poured to a depth of 3.5, 4 & 4.5mm and pre-poured plates from Oxoid & bioMerieux. 200 observations • The participating laboratories were asked to test discs (supplied by SMDC) 5 times on 10 separate occasions. 250 observations.

7. How are the acceptable limits for the control strains calculated? • Obtain at least 200 zone diameters for the strain/antibiotic disc combination. • Determine the number of observations at each zone diameter. • Calculate the cumulative % at each zone diameter. • Construct a graph of cumulative % versus zone diameter. • Read off the zones equivalent to 2.5% & 97.5% of observations to give the acceptable range.

8. Plot of cumulative % v zone diameter

9. CT2 (Chester MS) CT6 (Oxoid) CT1 (Oxoid) CT25 (Oxoid) CT24 (In-house) Pip/tazo Piperacillin Nalidixic acid Levofloxacin Ampicillin E. coli ATCC 25922

10. CT4 (Taunton PHL) CT8 (In-house) CT14 (bioMerieux) CT12 (Chester PHL) CT 20 (Oxoid) Pip/tazo Piperacillin Chloramphenicol Colistin E. coli NCTC 10418

11. CT17 (Oxoid) CT15 (E&O) CT16 (Taunton MS) CT29 (bioMerieux) CT10 (Leeds PHL) Pip/tazo Meropenem Imipenem Linezolid Azithromycin E. faecalis ATCC 29212

12. E. coli ATCC 25922: Ampicillin 25 ug Participant zones compared with SMDC acceptable limits Observations

13. E. coli NCTC 10418 : Chloramphenicol 10 ug Participant zones compared with SMDC acceptable limits Observations

14. E. faecalis ATCC 29212: Azithromycin 15 ug Participant zones compared with SMDC acceptable limits Observations

15. E. faecalis ATCC 29212: Linezolid 10 ug Participant zones compared with SMDC acceptable limits CT10 (Leeds PHL) Observations

16. E. faecalis ATCC 29212: Pip/tazo 85ug discParticipant zones compared with SMDC acceptable limits Observations

17. E. faecalis ATCC 29212: Pip/tazo 85ugAcceptable limits using data from 5 centres(SMDC 27-32 mm) CT10 (Leeds PHL) Observations

18. E. coli NCTC 10418: Centres where zone diameters outside the SMDC acceptable ranges

19. Summary of acceptable ranges (mm) for E. coli NCTC 10418 * Excluding CT12

20. E. coli ATCC 25922: Centres where zone diameters outside the SMDC acceptable ranges NB. NCTC E. coli: CT12 (Chester MS) also large zones

21. Summary of acceptable ranges (mm) for E. coli ATCC 25922 * Excluding CT24 & CT6

22. E. Faecalis ATCC 29212: Centres where zone diameters outside the SMDC acceptable ranges

23. Summary of acceptable ranges (mm) for E. faecalis ATCC 29212 *Excluding CT15 & CT17

24. Conclusions • Reasonable to exclude data from the analysis that is outside the acceptable limits generated by SMDC or that is different to the other laboratories

25. Conclusions • Very worthwhile exercise and the rolling programme is to be continued including looking at fastidious controls. • In future studies control strains will be provided. • Laboratories asked to test `once daily’ as this reflects `real life’.

26. Future meeting • Intend to have a meeting with the participating laboratories • Need to look at data from centres where zones are generally different from SMDC & other participants

27. Acknowledgments • QE Hospital, Woolwich, London • Department of Clinical Medicine, Manchester • Truro PHLS • Rotherham DGH • Craigavon Area Hospital • Addenbrookes Hospital, Cambridge • Singleton PHL, Swansea • University Hospital, Birmingham • Bedford Hospital • Bristol Royal Infirmary • QE2, Hertfordshire • UCL Hospitals, London • Russells Hall Hospital, West Midlands • Manchester Royal Infirmary • New Cross Hospital, Wolverhampton