We have shown that a data-augmentation approach allows us to estimate the extent of undetected MRSA carriage and the effectiveness of barrier precautions (gown and gloves) in preventing transmission using only routinely collected high-compliance admission and weekly MRSA screening cultures. Such estimates would have been difficult or impossible with more conventional approaches and would have required assuming rather than estimating the unobserved carriage data.
Our results have shown that admission and weekly nares screening with conventional culture methods are likely to have detected at least 80% of MRSA colonized patient days in the ICUs studied, despite 48-hour culture turn-around times, and sensitivities to detect MRSA carriage of only 85% for detecting nares carriage and 59% for carriage at any body site. The explanation for this high detection of MRSA patient days despite a low sensitivity for non-nares sites is that clinical cultures of other sites are routinely being performed for medical reasons. Thus, nares screening in conjunction with usual routine culturing performed for medical reasons captures a large majority of positive MRSA carriers. While generalizability to non-ICU wards remains unclear, this level of detection in ICUs is reassuring since risk of MRSA infection is highest in these settings.
We estimate that approximately 10% of potential contact precaution days are missed due to delays in obtaining test results. If contact precautions cause a 28.1% reduction in the transmission rate (i.e. 1 - (β
1), using the pooled estimate from all the wards), then an additional 5% reduction in total transmission rates would be gained with instantaneous swab results, suggesting a limit to the potential benefit in using rapid screening technologies. However, the potential benefit of rapid test results would increase in line with the effectiveness of isolation measures.
While our best estimate is that gown and glove barrier precautions produce a 28% reduction in MRSA transmission, we recognize that these estimates have considerable uncertainty despite including over 11 ward-years of data and there is a substantial chance that the effect is both much lower and much larger. Thus we estimate that there is 30% chance that the reduction in transmission is above 44% and a 30% chance that isolation actually increases transmission.
This uncertainty may indicate that many more years of data are required, and/or may be explained by the fact that data from different types of ICUs were pooled. However the Woolf's test for heterogeneity did not reach significance at a 5% level (p = 0.982), indicating that these differences are consistent with chance. Moreover since comparisons between ward-types were not planned they may be of value for generating, but not testing hypotheses.
Recognizing these caveats, it is plausible that barrier precautions have differential effectiveness in different types of ICUs. The estimates found in this study suggest that medical ICUs may derive the most benefit from barrier precautions. This is important since the MRSA prevalence in medical ICUs is often higher than in other ICU types. Interestingly, our results suggest that no benefit was derived in general surgery ICUs serving burn and trauma patients. It is possible that barrier precautions may be less effective when the burden of MRSA is profuse, such as with surgical wounds or burns. Such an interpretation is supported by the fact that the rate of background transmission (β
0) is substantially higher on these units. This is also supported by other work in these units suggesting that contamination with MRSA is higher in surgical versus medical ICUs . Another possible explanation for the lack of benefit seen in select wards is low compliance with contact precautions, although this was not assessed. Finally, if barrier precautions provide a false sense of security and lead to poorer hand hygiene, then transmission might effectively worsen with this practice.
It is also possible that the differing effectiveness of barrier precautions among ICUs could be due to unmeasured effects. Specifically, we did not measure or account for variations in host risk factors for acquisition (comorbidities, severity of illness, wounds, devices), nor did we measure the effect of other ongoing infection control interventions such as hand hygiene campaigns. We do note that infection control interventions were not differentially applied to certain ICUs during this time, but levels of compliance are not known. In the absence of reliable data about these factors, none were considered in our analysis.
Our results may underestimate the full effect of barrier precautions since all ICU rooms were single occupancy; thus, we could not assess the impact of this component. In addition, studies have suggested that compliance with barrier precautions is around 70% [9, 10]. Our estimates represent estimates of "effectiveness" (under routine operation) rather than a theoretical efficacy that would apply only under perfect compliance. Thus, while our estimates can be considered conservative, we believe they are the most practically relevant outcome measures. More precise estimates would require more prolonged surveillance data, data with more frequent swabbing intervals, and/or extensive typing information to help identify transmission routes.
The model-based approach that we have adopted for data analysis has several advantages over standard statistical methods, yielding more powerful analyses based on more realistic assumptions. In particular, it accounts for unobserved but important events (e.g. the time at which a patient becomes colonized), and attempts to estimate these events and their uncertainty. It also is based upon biologically meaningful assumptions regarding transmission, addressing both environmental sources and patient-to-patient transfer.
In summary, we have used a stochastic model to estimate the importance of undetected MRSA carriage and the impact of gown and glove precautions on MRSA transmission from 11-ICU years of active MRSA surveillance data. Such isolation precautions are the foundation of infection control guidelines for MRSA control. Nevertheless, there are concerns that isolation techniques may reduce the quality of patient care and incur risks related to inattention [11–13]. Thus, quantifying the beneficial effects and understanding the types of hospital wards in which such measures are likely to be insufficient is a critical part of weighing the need for additional or alternative measures. This work shows that even with conventional MRSA detection technology and weekly screening, a very large proportion of the total patient MRSA reservoir can be detected. It also suggests that barrier precautions may afford substantial benefit in medical and possibly specialty surgical ICUs, but that barriers to its effectiveness may need further study. The large uncertainty in effectiveness estimates, however, illustrate the limitations of routine surveillance data and highlight the need for well-designed prospective intervention studies to evaluate such interventions with greater precision.