Nasal site MRSA surveillance may miss colonization

Abstract & Commentary

By Joseph F. John Jr. MD, FACP, FIDSA, FSHEA

Associate Chief of Staff for Education, Ralph H. Johnson Veterans Administration Medical Center; Professor of Medicine, Medical University of South Carolina, Charleston.

Dr. John reports no financial relationships relevant to this field of study.

This article originally appeared in the September 2012 issue of Infectious Disease Alert. It was edited by Stan Deresinski, MD, FACP, FIDSA, and peer reviewed by Timothy Jenkins, MD. Dr. Deresinski is Clinical Professor of Medicine, Stanford University, and Dr. Jenkins is Assistant Professor of Medicine, University of Colorado, Denver Health Medical Center. Dr. Deresinski does research for the National Institutes of Health, and is an advisory board member and consultant for Merck, and Dr. Jenkins reports no financial relationships relevant to this field of study.

Synopsis: Nasal swabs identified only two-thirds of MRSA carriers.

Source: Matheson A, Christie P, Stari T, et al. Nasal swab screening for methicillin-resistant Staphylococcus aureus—How well does it perform? A cross sectional study. Infect Control Hosp Epidemiol 2012;33:803-8.

David MZ, Medvedev S, Hohmann SF, et al. Increasing burden of methicillin-resistant Staphylococcus aureus hospitalizations at US Academic Medical Centers 2003-2008. Infect Control Hosp Epidemiol 2012;33:782-9.

The classic teaching is that if a human carries Staphylococcus aureus, it is most likely residing in the anterior nares. This concept held generally true for methicillin-susceptible S. aureus (MSSA) and for nosocomial methicillin-resistant S. aureus (MRSA) for many years. With the advent of community-based MRSA — so-called USA300 — there often was a conspicuous absence of nasal carriage in persons who had single or even multiple infections with community-MRSA/USA300. Thus, there has been an evolving question of what anatomic sites, if any, give the most reliable index of colonization and a risk of subsequent infection.

Now comes a study from Scotland in two of its acute care hospitals to determine which of four sites were the most likely to show colonization of MRSA at the time of admission. Four sites were swabbed for culture: nostrils, perineum, axilla and throat. Also a pooled swab was cultured in selective mannitol nutrient broth before being plated onto selective agar.

Of 12,889 admissions 6,533 patients were studied from Aberdeen Royal Infirmary and 3,781 from Crosshouse. When a positive wound or device culture was factored into the total positives, there were 298 positive colonizations. The nose was the most likely positive, (72.5%), followed by perineum (39.1%), throat (37.7%) and axilla (8.4%). The “gold standard” was the presence of at least one confirmed agar or broth/agar culture from any pooled swab. Nasal swabs identified 66% of the MRSA-positive admissions. Throat and perineal cultures add nearly 16%. Axillary cultures alone add only 2.4%.

Commentary

Not all patients are Scots, but if they were, our current approach to pre-admission carriage of MRSA would have to change, or accept a recognition rate of just above two thirds. A rate of nearer to 50% may be true for a real world experience due to compliance, lack of standard training programs, etc. The Dutch routinely do nasal and throat swab looking for MRSA carriage and have reported throat carriage without nasal carriage previously. In the present study throat cultures plus nasal swabs would bring the screening accuracy to about 70%, not bad if a hospital wants to do something to recognize the MRSA carrier at admission and cohort these carriers. A positive culture of a preexisting infected site plus a nasal swab identified 100% of confirmed carriers.

The benefit of the study is to show that carriers may have colonization at one or more sites yet not have nasal colonization. The study also suggests that the nose is becoming less of a true focus of staphylococcal carriage, at least in terms of MRSA-colonized patients at the time of admission. The authors did a valiant job in organizing, implementing and analyzing the study and are to be congratulated for adding to this literature and to the Pathfinder study which is illuminating the role of MRSA in nosocomial infections. Of course the overall rate of MRSA carriage in these two Scottish hospitals at admission was only 3%. So, hospital administrators would have to be convinced that isolation of that small a MRSA-colonized group would actually prevent significant spread and morbidity in their hospitals.

Additionally, in an article accompanying the Scottish report, David and co-investigators from the University of Chicago found that there was a doubling of MRSA-associated hospitalizations from 20.9 per 1000 discharges to 41.7 per 1000 discharges. This sharp increase was likely due in part to infection with community MRSA, the very issue that the Scottish paper highlights by showing nasal swabs alone will not uncover those patients who are transporting community MRSA into the hospital.