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Low specificity of GenoType® MRSA Direct test when result interpretation is restricted to hybridisation of PCR products

Abstract number: 1733_403

Borgmann S., Kick A., Schuierer M., Reber A., Gruber H., Beyser K., Abel R., Lindauer A.

Objectives: Identification of MRSA bearing patients might be accelerated by performing PCR from primary materials (swabs, etc.) circumventing bacterial culture. Recently, Robert-Koch-Institute recommended GenoType® MRSA Direct test (Hain, Nehren, Germany) to reveal MRSA colonisation. In our lab this test has been performed since January 2006 servicing more than 40 hospitals in Northern Bavaria with microbiological analyses. Here we present data covering all PCR examinations collected over a time period between April and October 2006.

Methods: MRSA PCR and hybridisation of PCR products against specific probes linked to nitrocellulose strips were performed according to the manufacturers' recommendations. In addition, all PCR products were examined by agarose gel electrophoresis. All results were documented and compared to those of bacterial cultures performed within the following 7 days.

Results: A total of 861 samples was examined by PCR out of which 457 samples were grown and characterised through microbiological routine within the next 7 days (Table 1).

Table 1. Summary of MRSA direct PCR and bacterial culture resultsa

CultureGel- Hyb-Gel- Hyb+Gel+ Hyb+
n.d.376523
+7260
-3511225
aGel: results of agarose gel electrophoresis. Hyb: results of hybridisation reaction; n.d., not done.

Among the cohort of PCR negative MRSA samples only 2% showed bacterial growth (column 2). Hybridisation positive MRSA without detectable PCR products on agarose gels (column 3) could be grown in bacterial cultures in only 14% of the samples, suggesting a high percentage of false positive PCR results. Even with a detectable PCR product on an agarose gel combined with a positive hybridisation signal (column 4) growth of MRSA was observed in only 71% of the samples. Since this result also indicates a low specificity these 25 cases were backtracked in more detail. Eventually there were only 5 cases left for a fair comparison of positive PCR results versus negative bacterial growth results suggesting specificity higher than 90%.

Conclusion: GenoType® MRSA Direct test is a sensitive tool for identifying MRSA bearing patients. However, interpretation of PCR results needs to be based on hybridisation combined with agarose gel electrophoresis.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Subject:
Location: ICC, Munich, Germany
Presentation type:
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