# Assessing the potential for specimen pooling to streamline nosocomial surveillance of methicillin-resistant Staphylococcus aureus (MRSA)

## TL;DR

Pooling nasal and groin swabs can reduce PCR costs for MRSA detection by 54% while maintaining accuracy and speed.

## Contribution

This study demonstrates that specimen pooling with Xpert MRSA PCR is a cost-effective alternative to traditional methods in low-prevalence settings.

## Key findings

- Specimen pooling reduced PCR costs by 53.5% at a 6.6% MRSA prevalence.
- PCR pooling provided equivalent clinical performance to traditional culture-based detection.
- 66% of pools were MRSA negative, and 34% were MRSA positive, with PCR results matching culture results.

## Abstract

In hospitals, identification of methicillin-resistant Staphylococcus aureus (MRSA) is important to reduce possible transmissions and serious outcomes. Traditional culture and susceptibility testing requires 48–72 h, whereas Xpert MRSA polymerase chain reaction (PCR) can provide accurate MRSA detection in <1 h. Unfortunately, the high cost of such commercial PCRs precludes their use in many laboratories. Using MRSA as a model, this study hypothesized that specimen pooling in a setting of low prevalence could reduce PCR costs and provide rapid results. A total of 424 sequential nasal/groin specimens submitted for MRSA detection were subjected to routine culture-based detection using chromogenic media, and suspect colonies were confirmed using mass spectrometry and cefoxitin disk diffusion testing. These specimens were also pooled 1:8 or processed individually by Xpert MRSA PCR. Analytical sensitivity of PCR with and without pooling was compared to culture using triplicate 10-fold serial dilutions of an MRSA reference strain. The analytical sensitivity and clinical performance of specimen pooling paired with Xpert MRSA PCR were equivalent to traditional culture-based detection. Of specimen pools, 66.0% (35/53) were MRSA negative, and 34.0% (18/53) were MRSA positive. Pool resolution by PCR showed similar results as culture, identifying 116 MRSA-negative and 28 MRSA-positive specimens. At a prevalence of 6.6% (28/424), 1:8 specimen pooling with Xpert PCR provided equivalent results to culture-based methods and reduced the overall number of PCR reactions by 53.5%. Compared to individual PCR testing, specimen pooling would lower overall PCR costs, but the feasibility of this approach and the extent of benefits afforded would depend on MRSA prevalence.

Identifying antibiotic-resistant bacteria like methicillin-resistant Staphylococcus aureus (MRSA) is important to prevent their spread and potentially life-threatening infections. MRSA can be detected using bacterial culture and antibiotic susceptibility testing but requires up to 3 days for results. Molecular detection methods like polymerase chain reaction (PCR) are more rapid (<1 h), but their high cost prevents implementation for many laboratories. To reduce PCR costs, specimen pooling was considered. With specimen pooling, swabs from multiple individuals are combined and tested together. If pools are negative, all their members are considered negative. If pools are positive, each swab is tested individually to identify the one(s) with MRSA. By reducing the number of PCRs required, pooling reduces PCR costs. In this study, 6.7% of samples were MRSA positive, and pooling reduced overall PCR costs by 54%, provided results in 1–2 h, and identified the same number of MRSA cases as the comparator (i.e., culture).