Evaluation of two swab types in the rapid detection of MRSA using a PCR analyser

With recent advances in automation in microbiology, such as automated plate spreading techniques, the use of liquid medium transport swabs has become popular. Such transport swabs are known to work well for the culture of specimens on solid media; however, polymerase chain reaction (PCR) analysis is now becoming routine, especially for urgent samples and to satisfy the need for quicker turnaround times.

Sigma Transwab with liquid Amies medium (Medical Wire & Equipment [MWE]) is used widely for the collection of specimens for methicillin-resistant Staphylococcus aureus (MRSA, Fig 1) screening using conventional methods, both manual and automated. However, automated tests are often only validated for the manufacturer’s dedicated collection device. Cumitech 31A (ASM) provides a method for validation of non-specified devices, and this was used as the basis of the present study to evaluate the performance of a liquid medium transport device (Sigma Transwab, Medical Wire & Equipment [MWE]) in the Cepheid GeneXpert PCR system for MRSA, in comparison to the dedicated Cepheid collection device (Copan Duo Swab).

Inoculation
The two devices on trial are used in different ways. The Cepheid/Copan collection device supplied for use with the GeneXpert is a double swab that is used to collect the specimen (Cepheid MRSA Specimen Collection Protocol). The entire swab bud is placed in the sample reagent vial, which is vortex-mixed and the contents dispensed into the specimen port of the GeneXpert test cartridge (Xpert MRSA /SA Nasal G3).