Singapore first isolated MRSA in the early 1980s. The incidence and prevalence of MRSA infections have remained steady during the last two decades in major academic medical centers, although methicillin resistance appeared in 35% of all S. aureus isolates in one study. The incidence of HIV infection in Singapore has increased rapidly over the last decade.
A previous study at CDC showed that the prevalence of MRSA colonization in HIV-positive outpatients was higher than the prevalence of MRSA colonization in other populations. A study by Villacian et al. using only nasal swab reported a lower point prevalence of MRSA colonization (3% vs. 5.1% in our study) and 20% colonization with methicillin-sensitive Staphylococcus aureus (MSSA). Most studies and screening programs evaluating the prevalence of MRSA colonization have utilized swabs from the nares, axilla and groin[22, 23]. Mertz et al. have shown that the inclusion of a throat swab increased the sensitivity of detection among S. aureus carriers by 25.7%.
In this study, we showed that inclusion of throat and/or peri-anal swabs increased the sensitivity of NAG screening by 20%. The inclusion of additional swabs could be especially important for HIV positive patients, as unrecognized carriers can serve as reservoirs for transmission during frequent hospital visits. Chow et al. found that the inclusion of additional perianal and throat swabs increased MRSA detection by 12.5% in HIV positive patients.
Our finding that recent diagnosis of lymphoma was associated with MRSA colonization could be explained by the increased likelihood for lymphoma in advanced HIV infection when there is a greater extent of immunosuppression[25–27]. Some studies have shown an association between MRSA colonization and cutaneous T-cell lymphoma but not among HIV patients[28, 29]. Similarly, we identified low CD4 count as a risk factor for MRSA colonization; this has been described in other studies among HIV-positive patients[13, 30, 31].
Although univariate analysis showed an association between MRSA colonization and hospitalization within the past year, this effect was negated in multivariate analysis. Recent hospitalization has been frequently described as a predictor for MRSA colonization among HIV-positive patients in the published literature[14, 15]. Interestingly, in this study, we found a strong association between MRSA colonization and previous hospitalization history of household members. Studies have shown that family members of colonized patients are at risk for MRSA colonization and subsequent MRSA infection for many months[7, 32, 33].
The association between MRSA colonization and the presence of a percutaneous device within the past year in this study was significant. Onorato and colleagues found a seven-fold increased risk between the insertion of a central venous catheter and MRSA colonization among HIV-positive patients. Both findings highlight the importance of aseptic insertion and appropriate care of such devices and may reflect that patients with indwelling devices could have more contacts with healthcare workers, making them more susceptible to MRSA.
Our observation that receipt of ARV within the past year was not associated with decreased risk of MRSA colonization is inconsistent with previous studies. We found strong association between age and MRSA colonization. Studies have shown that elderly age (>65 years) is a risk factor for MRSA colonization despite patient group[24, 34]. AUC value in our study showed that predictors in the final model comprising six variables can differentiate well between MRSA colonizers and non-colonizers.
Our study had some limitations. Our population was predominantly male, and the sample size was small; more independent risk factors may have been identified with a larger study population. As our data were collected at a single public hospital for only outpatients, it may not reflect all HIV-infected patients. We may have underestimated prior hospitalizations, as data collection for this study did not account for admission to other hospitals. Data collection was also limited for some risk factors previously associated with MRSA colonization in HIV infected patients such as prior incarceration and high risk sexual behaviors[35, 36]. Many healthcare related factors assessed by the questionnaire in this study may be subjected to recall and reporting bias. However, this study highlights that a proportion of MRSA carriers would have been undetected without multiple-site screening cultures. HIV-positive patients are at increased risk of MRSA colonization due to immunosuppression and also their exposure to healthcare facilities.