Screening for MRSA

Abstract

Rapid screening is no more effective at reducing acquisition than conventional screening

Controversy about the effectiveness of screening for meticillin resistant Staphylococcus aureus (MRSA) stems from the scarcity of robust data from controlled studies. Typically, studies supporting screening have used multiple control measures to curtail hospital outbreaks of MRSA and have lacked control groups.¹ The effectiveness of screening depends on key factors including compliance with and sensitivity of screening, capacity to isolate or form cohorts out of identified MRSA carriers, efficacy of decolonisation regimens, and compliance with standard infection control precautions (such as hand hygiene, aseptic procedures when handling vulnerable sites or devices, and prophylaxis).² In the accompanying study, Jeyaratnam and colleagues report a randomised controlled trial of the effect of rapid screening for MRSA on acquisition of MRSA on hospital general wards in the United Kingdom.³

In some healthcare systems—for example, in the Netherlands—MRSA screening has helped maintain low MRSA colonisation and infection rates. However, in MRSA endemic settings in the UK, the demand for isolation facilities (either to segregate known MRSA carriers or those at high risk) often exceeds availability.⁴ This compromises one of the main principles of controlling the spread of healthcare associated pathogens.

Furthermore, the efficacy of MRSA decolonisation is suboptimal and may be exacerbated by resistance to mupirocin, the main agent used for nasal clearance, particularly if used repeatedly or for long periods.^{2 5 6} A systematic review of randomised controlled trials of the efficacy of nasal mupirocin to reduce MRSA colonisation and prevent infection concluded that insufficient evidence exists to support the widespread use of topical or systemic antimicrobials for eradicating MRSA.⁵ Short course mupirocin for selected patient groups, such as those about to undergo major surgery, may be helpful. UK guidelines advocate that MRSA screening should be targeted at patients at high risk of MRSA colonisation (for example, patients being transferred or re-admitted and previously known carriers) or infection (people being admitted to intensive care, cardiothoracic, orthopaedic, trauma, and vascular surgery wards).² Such categories will inevitably vary from hospital to hospital, which compounds the lack of standardisation of approach.

Screening on discharge can measure the prevalence of MRSA colonisation or acquisition, but it increases costs and does not guide the targeted use of measures to reduce the risk of infection. MRSA carriage is not a valid reason to delay discharge of people to their own homes or for exclusion from residential or nursing homes. Also, labelling a patient as MRSA positive at discharge may cause undue anxiety and have medicolegal implications. Despite the lack of supportive evidence for universal screening,² the Department of Health in England has recently stipulated that MRSA screening of all elective admissions should be in place by March 2009, and provision for screening of emergency admissions as soon as is practical.⁷

A valid criticism of most MRSA screening studies has been that the delay in obtaining a result means that transmission of MRSA from colonised patients may occur before carriage has been detected. There are two approaches to laboratory screening for MRSA—conventional culture based testing, which can give a provisional result in 24 (but more usually 48) hours, and the newer “rapid” nucleic acid amplification methods. Although these newer techniques can produce results within two to four hours, in reality sampling, transport, receipt, recording, and reporting increase this figure substantially and may make even same day results difficult to achieve for some patients. Two important recent studies are notable for their size (over 4000 and 20 000 patients) and rigour of evidence concerning the effectiveness of MRSA screening.^{3 8} Both were crossover studies and used a rapid test to detect MRSA. The most recent of these is reported by Jeyaratnam and colleagues, who found no benefit of rapid versus conventional screening in terms of rates of MRSA acquisition by contacts of positive patients on 10 wards of a London teaching hospital.³ Inappropriate isolation or cohorting of patients awaiting a screening result decreased significantly, but the authors conclude that the additional expense associated with rapid screening is unlikely to be justifiable. Laboratory costs associated with rapid screening are about three or four times higher than for conventional testing, although this difference is likely to narrow.

Jeyaratnamand colleagues’ study was well designed and importantly controlled for the effects of key confounding factors, such as other MRSA control measures and antibiotic use.³ The findings do not preclude a benefit of rapid screening for admissions to specialised units (such as intensive care), which were not studied. However, these units probably have robust infection control measures already in place, which would limit any added value of rapid screening. Four other reports on the effectiveness of rapid MRSA screening were subject to confounding as they used historical controls, so their results are unreliable.^{9 10 11 12}

A Swiss study examined the effectiveness of MRSA screening of all surgical admissions compared with standard control measures.⁸ While the rate of MRSA infection was low, the prevalence of MRSA colonisation was similar to that measured by Jeyaratnam and colleagues (5% v 7% of those screened).^{3 8} Neither the rate of MRSA surgical site infection nor MRSA acquisition changed significantly when all admissions were screened.⁸ More than half of the patients who developed MRSA infection on intervention wards were MRSA negative when screened on admission, and therefore probably had hospital acquired infection, despite generally good adherence to infection control precautions. Notably, the median time for “rapid” screening results to be reported was around 22 hours in both studies.^{3 8} Acting on a positive result takes even longer.

MRSA infection in hospitals can be reduced by rigorous application of standard control principles, as shown by recent precipitous decreases in the incidence of bloodstream infections in England.¹³ Such approaches have the added benefit of reducing risk for other healthcare associated infections. Targeted MRSA screening has a potential role as an adjunct to other infection control measures. Further large studies should investigate which patient groups benefit most from screening at admission in MRSA endemic settings, crucially when compliance with standard control measures can be demonstrated. Current evidence does not support the effectiveness of rapid as opposed to conventional MRSA screening. The cost effectiveness of MRSA screening should be compared with rigorously enforced standard control measures.