Laboratory evaluation of selective mannitol broth for MRSA screening

Screening methods for meticillin-resistant Staphylococcus aureus

The UK National External Quality Assessment Service (UK NEQAS) distributes clinically relevant and educational specimens for external quality assessment (EQA). The aim of this report was to assess the suitability of using a freeze-dried specimen format for the EQA of conventional and rapid methods and to review the methods used by participants to screen for meticillin-resistant Staphylococcus aureus (MRSA). Of the 714 laboratories that returned a result, 678 reported the presence of MRSA, and results showed a mean of 73 c.f.u. per 25 μl and a median of 50 c.f.u. per 25 μl confirming that the specimen was homogeneous. Four different approaches to MRSA screening were used routinely, including: (i) liquid culture; (ii) direct plating onto conventional media; (iii) direct plating onto chromogenic media; and (iv) rapid methods. A wide variety of methods were used within each of these four categories to screen for MRSA, and many laboratories reported using more than one method. Attempts should be made to determine the most appropriate approach to MRSA screening and to standardize the protocols across the UK.

Throat and rectal swabs may have an important role in MRSA screening of critically ill patients

OBJECTIVE

Methicillin-resistant Staphylococcus aureus (MRSA) is a major problem in intensive care units (ICU). International guidelines recommend screening patients for MRSA on admission, although consensus on sites required for optimum detection has not been reached. Our aim was to determine whether throat and rectal swabs identified a significant number of additional MRSA-colonised patients not captured by swabbing at keratinized skin carriage sites (anterior nares, perineum and axillae).

DESIGN

Prospective cohort study.

SETTING

30-Bed medical and surgical ICU in a tertiary teaching hospital.

PATIENTS

One thousand four hundred and eighty adult patients consecutively admitted over 15 months.

MEASUREMENTS AND RESULTS

Swabs from carriage sites (anterior nares, perineum, axillae, throat and rectum), wounds and clinical samples taken within 48 h of ICU admission were analysed to identify patients admitted with MRSA. A complete set of carriage swabs were received from 1,470 patients. 105 (7%) patients were admitted with MRSA of which 63 (60%) were detected by a pooled keratinized skin swab (anterior nares, perineum, axillae). A further 36 (34%) patients were detected only by throat or rectal swabs. Indeed, throat and rectal swabs combined had a higher sensitivity than pooled keratinised skin swabs (76 vs. 60% P = 0.0247). Swabs from all carriage sites together detected 95% (100) of MRSA positive patients, with five patients being positive at wound sites only.

CONCLUSIONS

The throat and rectum are important and potentially hidden sites of MRSA carriage in critically ill patients. These findings prompt the need for larger studies to determine the most cost-effective screening strategy for MRSA detection.

DESCRIPTOR

Non-pulmonary nosocomial infections.

Validation of the IDI-MRSA system for use on pooled nose, axilla, and groin swabs and single swabs from other screening sites

A commercial rapid polymerase chain reaction methicillin-resistant Staphylococcus aureus (MRSA) screening method (IDI-MRSA) is validated for the use with nasal swabs transported in liquid Stuart's medium. We investigated the use of IDI-MRSA for screening for MRSA in pooled nose, axilla, and groin swabs and in single swabs from skin puncture sites, wounds, throat, rectum, and groin using swabs transported in Amies medium without charcoal. We performed the IDI-MRSA test on swabs that had been used for routine MRSA broth culture and which were selected to be about 50% MRSA positive. We compared the IDI-MRSA result with the MRSA culture result. With 201 pooled sets, the sensitivity of IDI-MRSA was 85% and the specificity 95%. With 32 single screening swabs, sensitivity was 94% and specificity 80%. The method is not compromised by swab transport in Amies medium if an additional heating step is used. We had a low rate of initial inhibition (1.3%), which may have been due to the extra heating step used to liquefy gel from the Amies medium. Thus, in this study IDI-MRSA gives similar results to culture with pooled or single swabs from multiple screening sites.

Prevalence and risk factors for meticillin-resistant Staphylococcus aureus in adult emergency admissions--a case for screening all patients?

In this study we describe the prevalence and frequency of risk factors for meticillin-resistant Staphylococcus aureus (MRSA) colonization in emergency admissions during a one-year MRSA screening programme. Overall, 7801/13 826 (56.4%) adult emergency admissions by 6469 patients were screened for MRSA. Of those screened, 670/7801 (8.6%) admissions by 433 patients (6.7%) were colonized with MRSA. Risk factors studied were previous hospital admission, previous MRSA colonization and residence in care homes. Patients with any risk factor (MRSA positive: 366/3952 (9.3%) vs MRSA negative: 67/2450 (2.7%), P<0.001), advanced age (mean ages for MRSA positive and negative patients were 74.4 and 56.2 years, respectively (P<0.001)), and increasing frequency of hospital admissions (P<0.001), patients from care homes (41/184, 22.2%) and previous MRSA colonization (232/1855, 12.5%) had a significantly higher MRSA colonization rate. The cost of the screening programme was nearly 24,500 pounds. It was noticed that there was a substantial reduction in hospital-acquired MRSA and MRSA bacteraemia during the study period. The study was not designed to establish whether this reduction was causally associated with the screening programme. We conclude that there is a high MRSA colonization rate among emergency admissions, especially those with risk factors. Using a selective risk factor based screening strategy more than 60% of the patients would have to be screened but still 3952/6469, 67 (15.5%) MRSA carriers would have been missed. Screening of all emergency admissions to detect MRSA colonization is preferable to selective screening, relatively inexpensive, and might reduce the MRSA colonization rate of hospital-acquired MRSA and MRSA bacteraemia among emergency admissions.