# BSAC rolling programme for devising acceptable limits for

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

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 prepoured 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. 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. Cumulative % Plot of cumulative % v zone diameter

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 25 26 27 28 29

30 Zone diameter (mm) 31 32 33 34 E. coli ATCC 25922 CT2 (Chester MS) CT6 (Oxoid) CT1 (Oxoid) CT25 (Oxoid) CT24 (In-house) Pip/tazo

Piperacillin Nalidixic acid Levofloxacin Ampicillin E. coli NCTC 10418 CT4 (Taunton PHL) CT8 (In-house) CT14 (bioMerieux) CT12 (Chester PHL) CT 20 (Oxoid) Pip/tazo Piperacillin Chloramphenicol Colistin E. faecalis ATCC 29212

CT17 (Oxoid) CT15 (E&O) CT16 (Taunton MS) CT29 (bioMerieux) CT10 (Leeds PHL) Pip/tazo Meropenem Imipenem Linezolid Azithromycin Zone diameter (mm) E. coli ATCC 25922: Ampicillin 25 ug Participant zones compared with SMDC acceptable limits 31 30 29 28 27 26

25 24 23 22 21 20 19 CT25 (Oxoid) CT1 (Oxoid) CT24 (In-house) Observations CT6 (Oxoid) CT2 (Chester MS) Zone diameter (mm) E. coli NCTC 10418 : Chloramphenicol 10 ug Participant zones compared with SMDC acceptable limits 36 35 34 33 32 31 30

29 28 27 26 25 24 23 22 21 20 19 18 CT4 (Taunton PHL) CT8 (In-house) CT20 (Oxoid) Observations CT14 (bioMerieux) CT12 (Chester PHL) Zone diameter (mm) E. faecalis ATCC 29212: Azithromycin 15 ug Participant zones compared with SMDC acceptable limits 23 22 21

20 19 18 17 16 15 14 13 12 11 10 CT17 (Oxoid) CT15 (E&O) CT16 (Taunton MS) Observations CT29 (bioMerieux) CT10 (Leeds PHL) Zone diameter (mm) E. faecalis ATCC 29212: Linezolid 10 ug Participant zones compared with SMDC acceptable limits 34 33 32 31

30 29 28 27 26 25 24 23 22 21 20 19 CT17 (Oxoid) CT15 (E&O) CT16 (Taunton MS) Observations CT29 (bioMerieux) CT10 (Leeds PHL) E. faecalis ATCC 29212: Pip/tazo 85ug disc 34 Participant zones compared with SMDC acceptable limits

33 Zone diameter (mm) 32 31 30 29 28 27 26 25 24 23 CT17 (Oxoid) CT15(E&O) CT16 (Taunton MS) CT29 (bioMerieux) CT10 (Leeds PHLS) Observations E. faecalis ATCC 29212: Pip/tazo 85ug Acceptable limits using data from 5 centres (SMDC 27-32 mm) 34 33 32 Zone diameter (mm)

31 30 29 28 27 26 25 24 23 CT17 (Oxoid) CT15(E&O) CT16 (Taunton ) Observations CT29 (bioMerieux) CT10 (Leeds PHL) E. coli NCTC 10418: Centres where zone diameters outside the SMDC acceptable ranges Antibiotic CT4 (Taunton PHL) CT8 (Oxoid)

CT20 (bioMerieux) CT14 (Oxoid) CT12 (Chester PHL) Pip/tazo 85ug X Too large Piperacillin 75ug X Too large Chloramphenicol 10ug X Too large Colistin 25ug

Summary of acceptable ranges (mm) for E. coli NCTC 10418 Antibiotic SMDC (mm) Chloramphenicol 10ug 23-27 20-28 20-26 21-27 Piperacillin 75ug 31-35 30-36 29-35 30-35 Pip/tazo 85ug

31-34 30-36 29-34 30-35 Colistin 25ug 15-19 16-20 16-19 15-19 * Excluding CT12 5 centres 4 centres* SMDC & (mm) (mm) 4 centres* E. coli ATCC 25922: Centres where zone diameters outside the SMDC acceptable ranges Antibiotic CT25 (Oxoid)

CT11 (Oxoid) CT24 (Chester) CT6 (Oxoid) CT2 (Oxoid) Pip/tazo 85ug X Too large X Too large X Too large X Too large X Too large Piperacillin

75ug X Too large X Too large X Too large X Too large X Too large X Too large X Too large X Too large Nalidixic acid 30ug Levofloxacin 1ug Ampicillin 25ug

X Too large NB. NCTC E. coli: CT12 (Chester MS) also large zones Summary of acceptable ranges (mm) for E. coli ATCC 25922 Antibiotic SMDC (mm) 5 centres (mm) 3 centres* (mm) SMDC & 3 centres* Piperacillin 75ug 26-31 29-34 28-32 27-32

Pip/tazo 85ug 26-30 28-32 28-32 26-31 Nalidixic acid 30ug 26-30 26-32 26-31 26-32 Levofloxacin 1ug 29-33 28-36 28-34

28-34 Ampicillin 25ug 20-26 22-28 22-28 21-28 * Excluding CT24 & CT6 E. Faecalis ATCC 29212: Centres where zone diameters outside the SMDC acceptable ranges Antibiotic CT17 (Oxoid) CT15 CT16 (E&O) (Taunton) Pip/tazo 85ug X Too large

X Too small Meropenem 10ug X Too large X Too small CT28 (bioMerieux) CT10 (Leeds) X Too small X Too small Imipenem 10ug Linezolid 10ug X Too large

X Too small Azithromycin 15ug X Too large X Too small X Too large Summary of acceptable ranges (mm) for E. faecalis ATCC 29212 Antibiotic SMDC (mm) 5 centres (mm) 3 centres* (mm) SMDC & 3 centres* Pip/tazo

85ug 27-32 24-32 26-31 26-32 Meropenem 10ug 24-28 19-28 21-27 22-28 Linezolid 10ug 25-30 22-31 24-28 24-29

Imipenem 10ug 28-33 25-32 28-32 28-32 Azithromycin 15ug 15-20 11-21 14-21 15-21 *Excluding CT15 & CT17 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

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. 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 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

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