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 n n n 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 n n Confirmed that agar poured `in-house’ to a depth between 3. 5 -4. 5 mm, and pre-poured plates from Oxoid and bio. Merieux 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. n However, SMDC acceptable ranges could be used as a guide for comparison. n

Request for help n n 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. 5 mm and pre-poured plates from Oxoid & bio. Merieux. 200 observations n The participating laboratories were asked to test discs (supplied by SMDC) 5 times on 10 separate occasions. 250 observations. n

How are the acceptable limits for the control strains calculated? n n n 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.

Plot of cumulative % v zone diameter

E. coli ATCC 25922 n n n CT 2 (Chester MS) CT 6 (Oxoid) CT 1 (Oxoid) CT 25 (Oxoid) CT 24 (In-house) n n n Pip/tazo Piperacillin Nalidixic acid Levofloxacin Ampicillin

E. coli NCTC 10418 n n n CT 4 (Taunton PHL) CT 8 (In-house) CT 14 (bio. Merieux) CT 12 (Chester PHL) CT 20 (Oxoid) n n Pip/tazo Piperacillin Chloramphenicol Colistin

E. faecalis ATCC 29212 n n n CT 17 (Oxoid) CT 15 (E&O) CT 16 (Taunton MS) CT 29 (bio. Merieux) CT 10 (Leeds PHL) n n n Pip/tazo Meropenem Imipenem Linezolid Azithromycin

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

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

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

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

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

E. faecalis ATCC 29212: Pip/tazo 85 ug Acceptable limits using data from 5 centres (SMDC 27 -32 mm) CT 10 (Leeds PHL) Observations

E. coli NCTC 10418: Centres where zone diameters outside the SMDC acceptable ranges Antibiotic CT 4 (Taunton PHL) CT 8 (Oxoid) CT 20 (bio. Merieux) CT 14 (Oxoid) CT 12 (Chester PHL) Pip/tazo 85 ug X Too large Piperacillin 75 ug X Too large Chloramphenicol 10 ug X Too large Colistin 25 ug

Summary of acceptable ranges (mm) for E. coli NCTC 10418 Antibiotic Chloramphenicol 10 ug SMDC (mm) 23 -27 5 centres 4 centres* SMDC & (mm) 4 centres* 20 -28 20 -26 21 -27 Piperacillin 75 ug 31 -35 30 -36 29 -35 30 -35 Pip/tazo 85 ug 31 -34 30 -36 29 -34 30 -35 Colistin 25 ug 15 -19 16 -20 16 -19 15 -19 * Excluding CT 12

E. coli ATCC 25922: Centres where zone diameters outside the SMDC acceptable ranges Antibiotic CT 25 (Oxoid) CT 11 (Oxoid) CT 24 (Chester) CT 6 (Oxoid) CT 2 (Oxoid) Pip/tazo 85 ug X Too large X Too large Piperacillin 75 ug X Too large X Too large Nalidixic acid 30 ug Levofloxacin 1 ug Ampicillin 25 ug X Too large NB. NCTC E. coli: CT 12 (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 75 ug 26 -31 29 -34 28 -32 27 -32 Pip/tazo 85 ug 26 -30 28 -32 26 -31 Nalidixic acid 30 ug 26 -30 26 -32 26 -31 26 -32 Levofloxacin 1 ug 29 -33 28 -36 28 -34 Ampicillin 25 ug 20 -26 22 -28 21 -28 * Excluding CT 24 & CT 6

E. Faecalis ATCC 29212: Centres where zone diameters outside the SMDC acceptable ranges Antibiotic CT 17 (Oxoid) CT 15 CT 16 (E&O) (Taunton) Pip/tazo 85 ug X Too large X Too small Meropenem 10 ug X Too large X Too small CT 28 (bio. Merieux) CT 10 (Leeds) X Too small Imipenem 10 ug Linezolid 10 ug X Too large X Too small Azithromycin 15 ug 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 85 ug 27 -32 24 -32 26 -31 26 -32 Meropenem 10 ug 24 -28 19 -28 21 -27 22 -28 Linezolid 10 ug 25 -30 22 -31 24 -28 24 -29 Imipenem 10 ug 28 -33 25 -32 28 -32 Azithromycin 15 ug 15 -20 11 -21 14 -21 15 -21 *Excluding CT 15 & CT 17

Conclusions n 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. n In future studies control strains will be provided. n Laboratories asked to test `once daily’ as this reflects `real life’. n

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

Acknowledgments n n n n 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 QE 2, Hertfordshire UCL Hospitals, London Russells Hall Hospital, West Midlands Manchester Royal Infirmary New Cross Hospital, Wolverhampton