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Rapid detection of methicillin-sensitive and resistant Staphylococcus aureus direct from blood cultures

Abstract number: p1429

Montgomery  J., Bywater  J., Mayall  B.

Objectives: 

Rapid detection of S. aureus (STAU) bacteraemia including those resistant to methicillin (MRSA) significantly improves outcomes in these infections, and may reduce empiric vancomycin use. Blood cultures with a Gram stain resembling Staphylococci had modified latex agglutination (MLA) testing if direct tube coagulase test (DTC) was positive on the day of detection (Becton Dickinson, USA). The aim was to determine the accuracy of DTC and MLA test directly from blood culture broth (BCB).

Methods: 

For the DTC: 0.1 ml of mixed BCB was added to 0.5 ml rabbit plasma, incubated at 36°C and read at 2, 4 and 24 hours. Incubation of bottles was continued at 36oC until DTC became positive. If positive, a modification of the PBP2a-Latex Test (Oxoid Ltd., England) was performed. A 10 ml aliquot of the BCB was centrifuged using a gel separator, the deposit washed twice in saline, and re-suspended in extraction reagent. The test was then performed as instructed by the manufacturer. mecA gene PCR was used as the reference.

Results: 

DTC were performed on 571 bottles with Gram-positive cocci resembling Staphylococci. Of those positive, 87% (186) were detected within four hours. There were 124 STAU and 91 MRSA. One false positive and one false negative were detected. Sensitivity (SN) and Positive Predictive Value (PPV) were 99.5%; specificity (SP) and Negative predictive values (NPV) were 99.7%. MLA was performed on 212 DTC positive bottles from 176 patients. Of these, 124 were mecA negative from 107 patients, and 88 mecA positive from 69 patients tested. Five false positive and five false positive MLA tests occurred. SN and PPV were 95% and SP and NPV were 96%. All false negatives occurred early in the study and may have been due to insufficient growth so extended incubation is important. Adequate washing of the bacterial pellet and adherence to the method is important to avoid false positive results.

Conclusion: 

The DTC and MLA test, performed as part of the routine processing of any blood culture with a Gram stain resembling Staphylococci, gives accurate phenotypic detection of MRSA at least twelve hours earlier than routine subculture. It has significant impact on the laboratory turnaround time and clinical relevance of results.

Session Details

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