Abstract & Commentary

Does the nose still know when it comes to MRSA?

Many colonized patients may be undetected

By Joseph F. John, MD, FACP, FIDSA, FSHEA, Professor of Medicine, Medical University of South Carolina, Charleston

Dr. John reports no financial relationships in this field of study

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

Sources: 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 relatively susceptible 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 give the most reliable index of colonization and a risk of subsequent infection.

A study was done at two acute care hospitals in Scotland 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. Overall, 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 added nearly 16%. Axillary cultures alone added 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 put them in contact isolation. In this study a positive culture of a preexisting infected site plus a nasal swab identified 100% of confirmed carriers.

The real 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 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 22 per 1000 discharges to 42 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 all patients who are transporting MRSA into the hospital.