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Evaluation of the mastascanelite image analysis system for measuring zones of inhibition in disc diffusion susceptibility tests

British Journal of Biomedical Science, 2003 by Clarke, R E H, Winstanley, T G, Ridgway, E J

ABSTRACT

In this evaluation a mastascanelite image analysis system is compared with manual measurement of disc diffusion inhibition zones. Data for 213 diverse organisms and a total of 1679 organism/antibiotic combinations gave an overall correlation coefficient of 0.988. The mean difference between readings was +0.425 mm, with 95% confidence limits of ±2.94 mm, and the majority of scanned zones (97.51%) fell within ±3 mm of the manual measurement. The mastascanelite system forms part of a laboratory suite and was found to be objective, accurate and rapid, reading and interpreting each plate in less than a second. Interfacing to the laboratory computer system facilitated data handling and performance control.

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KEY WORDS: Disc diffusion.

Image processing, computer-assisted.

Introduction

Comparative disc diffusion susceptibility tests1 are simple and inexpensive but have two main shortfalls. Firstly, they have undergone unsupported, ad-hoc changes to such an extent that no single method remains. This means that susceptibility data from multiple centres can no longer be combined reliably for surveillance purposes. Secondly, they may permit subtle changes in overall susceptibilities to pass undetected.

To overcome these criticisms, the British Society for Antimicrobial Chemotherapy (BSAC) has published a standardised method in which zone diameters are correlated with minimum inhibitory concentration (MIC) breakpoints.2 Zones can be interpreted with a template but it is preferred that they be measured using a ruler, callipers or an automated zone reader.

The aim of this study is to evaluate the mastascanelite (Mast Group, Bootle, UK) in comparison with manually measured zones. This is the first evaluation of this instrument, although other automated zone readers have been formally evaluated previously.14

Materials and methods

Disc testing was performed according to the recommendations of the BSAC.5

Iso-Sensitest agar (IST; CM471, Oxoid) was poured to a depth of 4 mm in 90 mm Petri dishes. For fastidious organisms, IST was supplemented with 5% defibrinated horse blood (E & O Laboratories, Burnhouse, Bonnybridge, Scotland) or with defibrinated horse blood and 20 mg/L nicotinamide adenine dinucleotide (Sigma Chemicals, Poole, UK) as required. Inocula giving semi-confluent growth after overnight incubation were used.

All plates were incubated at 35-37°C in air for 18-20 h, the exceptions being for streptococci, haemophili and neisseria, where the atmosphere was enriched with 4-6% CO2. Antibiotic discs were supplied by Mast and zone diameters were determined on the mastascanelite by one person, and then measured independently (to the nearest mm) with a ruler by another.

The 213 organisms studied comprised Escherichia coli (n=10), Citrobacter freundii (n=5), C. diversus (n=4), Morganella morganii (n=5), Proteus mirabilis (n=5), P. vulgaris (n=1), Providencia alcalifaciens (n=3), Klebsiella pneumoniae (n=5) K, oxytoca (n=5), Enterobacter cloacae (n=4), E. aerogenes (n=6), E. agglomerans (n=2), Serratia spp. (n=3), Shigella spp. (n=2), Salmonella spp. (n=5), Aeromonas hydrophila (n=2), Stenotrophomonas maltophilia (n=5), Acinetobacter spp. (n=4), Pseudomonas aeruginosa (n=26), Enterococcus spp. (n=17), Haemophilus influenzae (n=11), Streptococcus pneumoniae (n=5); [beta]-haemolytic streptococci (Lancefield group A [n=12], group B [n=10], group C [n=5], group G [n=4]), Neisseria gonorrhoeae (n=20), Staphylococcus aureus (n=21; five penicillin-sensitive and five methicillin-resistant) and coagulase-negative staphylococci (n=6; two methicillin-resistant). Antimicrobials tested against each group of organisms are shown in Table 1.

Results

Correlation of results from the mastascanelite and the manual method is shown in Figure 1. These data comprise 213 organisms and a total of 1679 organism/antibiotic combinations. The correlation for individual genera is shown in Table 2 and ranges from 0.975 to 0.992, with an overall coefficient of 0.988.

When differences in zone diameter measured by the mastascanelite were compared with manual readings, the difference was found to be +0.425 mm, with 95% confidence limits of ±2.94 mm (± 2SD for a normal distribution). The majority of scanned zones (97.51%) fell within ±3 mm of the manual measurement, 90.12% within ±2 mm and 68.76% within ±1 mm.

When BSAC interpretative criteria were used to compare the two data sets for the Enterobacteriaceae, 23/760 individual test results differed. Overall, results for ciprofloxacin, gentamicin and ceftazidime were unaltered, but results for trimethoprim (n=7, mainly intermediate to susceptible), ampicillin (n=7), cefuroxime (n=1), cephalexin (n=4), nalidixic acid (n=1) and nitrofurantoin (n=3) differed.

The mastascanelite was able to determine the zone diameter unaided for the majority of organisms. With some (notably Lancefield group B streptococci and Streptococcus pneumoniae), however, manual adjustment of the zone edge by the operator was necessary. The instrument coped well with pigmented pseudomonads and could differentiate between growth and haemolysis caused by streptococci.

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Evaluation of the mastascanelite image analysis system for measuring zones of inhibition in disc diffusion susceptibility tests

British Journal of Biomedical Science, 2003 by Clarke, R E H, Winstanley, T G, Ridgway, E J

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