7.2 Measuring zones 7.2.1 Measure the diameters of of inhibition to the nearest millimetre (zone edge should be taken as the point of inhibition as judged by the naked eye) with a ruler, callipers or an automated zone reader. 7.2.2 Tiny colonies at the edge of the zone, films of growth as a result of the swarming of Proteus spp. and slight growth within sulphonamide or trimethoprim zones should be ignored. 7.2.3 Colonies growing within the zone of inhibition should be subcultured and identified and the test repeated if necessary. 7.2.4 When cefoxitin for the detection of methicillin/oxacillin/cefoxitin resistance in S. aureus, measure the obvious zone, taking care to examine zones carefully in good light to detect minute colonies that may be present within the zone of inhibition (see Figure 3) 7.2.5 Confirm that the zone of inhibition for the control strain falls within the acceptable ranges in Tables 20-23 before interpreting the test (see section on control of the disc diffusion method). 7.3 Use of templates for interpreting zone diameters A template may be used for interpreting zone diameters (see Figure 2). A program for preparing templates is available from the BSAC (). The test plate is placed over the template and the zones of inhibition are examined in relationship to the template zones. If the zone of inhibition of the test strain is within the area marked with an the organism is resistant. If the zone of inhibition is equal to or ‘R’, larger than the marked area, the organism is susceptible. Figure 2: Template for interpreting zone diameters 8. Oxacillin/cefoxitin testing of staphylococci Methicillin susceptibility testing is difficult with some strains. Expression of resistance is affected by test conditions and resistance is often heterogeneous, with only a proportion of
7.2 Measuring zones 7.2.1 Measure the diameters of of inhibition to the nearest millimetre (zone edge should be taken as the point of inhibition as judged by the naked eye) with a ruler, callipers or an automated zone reader. 7.2.2 Tiny colonies at the edge of the zone, films of growth as a result of the swarming of Proteus spp. and slight growth within sulphonamide or trimethoprim zones should be ignored. 7.2.3 Colonies growing within the zone of inhibition should be subcultured and identified and the test repeated if necessary. 7.2.4 When cefoxitin for the detection of methicillin/oxacillin/cefoxitin resistance in S. aureus, measure the obvious zone, taking care to examine zones carefully in good light to detect minute colonies that may be present within the zone of inhibition (see Figure 3) 7.2.5 Confirm that the zone of inhibition for the control strain falls within the acceptable ranges in Tables 20-23 before interpreting the test (see section on control of the disc diffusion method). 7.3 Use of templates for interpreting zone diameters A template may be used for interpreting zone diameters (see Figure 2). A program for preparing templates is available from the BSAC (). The test plate is placed over the template and the zones of inhibition are examined in relationship to the template zones. If the zone of inhibition of the test strain is within the area marked with an ‘R’, the organism is resistant. If the zone of inhibition is equal to or larger than the marked area, the organism is susceptible. Figure 2: Template for interpreting zone diameters 8. Oxacillin/cefoxitin testing of staphylococci Methicillin susceptibility testing is difficult with some strains. Expression of resistance is affected by test conditions and resistance is often heterogeneous, with only a proportion of
BSAC-disc-susceptibility-testing-method-Jan-2015.docx