Two evaluation methods for screening specimens to detect methicillin-resistant Staphylococcus aureus

Abstract number: 1134_03_152

Martín Y., Rezusta A., Castillo J., Revillo M.J.


Rapid assessment of clinical specimens for the presence of MRSA is an important part to control measures for infections taken to reduce the spread of MRSA and thus, to decrease hospitalization costs. The purpose of this study was to evaluate the sensitivity, specificity and the optimal incubation time from patient specimens by using ORSAB (Oxacillin Resistance Screening Agar Base) as a primary culture medium compared with those obtained by using a previous enrichment broth medium, Tryptone Soy Broth added with 75 mg/ml of aztreonam (TSB Azt).


Every MRSA screening swab was inoculated initially onto ORSAB and incubated aerobically for 48 h at 35°C; the plates were examined after 24 and 48 h. At the same, each specimen was introduced in TSB Azt. After 24 h of incubation, they were subcultured onto ORSAB, following the same process described previously. All mannitol-fermenting colonies were confirmed as S. aureus by using a latex agglutination system (Pastorex Staph Plus Bio Rad). Resistance to methicillin was determined by the disk diffusion method according to the NCCLS.


Out of 404 studied samples, 90.35% were nose swabs. MRSA was detected in 15% of these samples. The sensitivity and specificity obtained when ORSAB alone was used were 96.6 and 48.3% respectively. When we previously used TSB Azt, the results were 94.9 and 49.5%. While 93.0% of MRSA was detected after 24 h of incubation in the first case, it was 98.2% after using an enrichment broth medium.


ORSAB gave a positive result in 93.0% of the samples after 24 h of incubation. The three MRSA isolated only from direct culture onto ORSAB belonged to samples which had a low amount of microorganisms. Low specificity was especially due to Staphylococcus haemolyticus, very common in nose samples. With such a high number of nose samples, our study showed that the use of TSB Azt gave a minimum increase of sensitivity.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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