In this study we detected rectal colonization in 86% of patients colonized with ESBL-GNB and in 14% of ESBL-GNB carriers, no rectal colonization was detected. These 14% of ESBL-GNB carriers might potentially be missed if followed with rectal cultures only. This finding is in line with the scarce literature available on ESBL colonization. Papst et al. found that in 114 patients, 10% of those with positive sample sets had negative rectal swabs ; Tschudin-Sutter et al. found rectal colonization in only 69% of 133 patients .
Of course, our results are limited by the retrospective nature of the analysis. However, the standardized protocols for ESBL-GNB detection and follow-up at our centre and the relatively large sample size add to the validity of the retrospective analysis. Ideally, rectal samples would have been taken simultaneously with positive ESBL-GNB samples from non-rectal body sites. Therefore we have studied the number of negative rectal swab cultures and interval between an ESBL-GNB positive culture and negative rectal swab culture in the colonized patients without apparent rectal colonization. Thirty-nine percent of patients had negative rectal cultures taken within 8 days of an ESBL-GNB positive sample at another location: loss of colonization within this interval seems not likely. This finding might partly overcome the objection that the absence of rectal colonization in 14% of ESBL-GNB carriers might be caused by sampling bias. In patients with longer intervals between negative rectal samples and ESBL-positive samples from non-rectal sites, natural loss of colonization, the use of antibiotics and/or SDD might have led to negative rectal samples. However, we found similar rectal colonization rates in patients that presumably received SDD (e.g. were admitted to the ICU or haematology ward at some point in the study period), compared to patients that presumably did not receive SDD (e.g. were never admitted to the ICU or haematology ward during the study period). Although this study this has its limitations, we feel that our data can be of help to formulate infection control guidelines. Data on MDR-GNB colonization sites is scarce; more insight in colonization sites is urgently needed, and in the absence of large prospective studies, retrospective analysis with considerable sample sizes are welcomed. Future prospective trials should ideally use standardized sample protocols that will limit the time between rectal sampling and positive ESBL-GNB cultures at non-rectal sites. It is also advisable that such trials would focus on the cost-effectiveness of screening protocols.
It is of interest that we found that 28 of 506 patients with positive rectal cultures had a negative rectal sample obtained between two positive rectal samples. This could represent either loss of colonization or sampling error. The latter is the reason why in the Netherlands we require a patient to have two consecutive samples negative for MDR-GNB, before isolation precautions can be discontinued. However, when implementing MDR-GNB screening protocols the potential gain of extra cultures (higher sensitivity, less potential ESBL spreading) has to be balanced against the economic impact of performing more cultures. In addition, local and national screening protocols should be adapted to the local and national MDR-GNB epidemiology. In the Netherlands the prevalence of ESBL-producing Enterobacteriaceae is estimated to be around 10% in clinical isolates . In areas with a higher prevelance of ESBL-GNB it might be more cost-effective to direct MDR-GNB screening programs at carbapenemase producing bacteria. In addition, in more resource-limited countries the potential gain of extra cultures might be outweighed by the extra-costs of performing more cultures. Furthermore, we do not intend to recommend repeating invasive procedures for screening purposes.
In this study we questioned whether our current practice of MDR-GNB follow-up by rectal cultures plus a culture from the site where the MDR-GNB was initially found is more sensitive than follow-up by rectal cultures only. In conclusion, our data and the current available evidence indicate that 10 to 30% of colonized patients do not carry ESBL-GNB in the rectum [3, 4]. This finding should have implications for screening and follow-up programs of MDR-GNB colonization: ideally, screening and follow-up programs of MDR-GNB colonization should not solely rely on rectal cultures but should preferably also include the site where the MDR-GNB was initially found.