Dear Editor,
We read with interest the uncertainties page entitled ‘Should selective digestive decontamination (SDD) be used in critically ill patients?’ by Walden et al [1]. However, we would like to raise several points that deserve attention.
Firstly, the authors acknowledge the efficacy in reducing serious infection of lower airways and mortality by SDD defined as an antimicrobial prophylaxis using parenteral and enteral antimicrobials applied in throat and gut. However, they express uncertainty about SDD due to the theoretical possibility of antimicrobial resistance. Although they discuss the two recent Dutch trials [2,3] with respect to their efficacy, they fail to mention the resistance data from these two trials. De Jonge et al conducted an RCT in 934 critically ill adult patients [2]. Carriage of aerobic Gram-negative bacilli (AGNB) resistant to polymyxin E, tobramycin, ceftazidime, ciprofloxacin and imipenem was significantly reduced in SDD patients compared with controls (16 vs 26%; p = 0.001). Similarly, de Smet et al [3] showed that there were fewer patients with AGNB in rectal swabs resistant to the marker antibiotics in the SDD group when compared to the group who received selective oral decontamination (SOD), a modified SDD protocol without the parenteral and enteral gut component. In addition, bloodstream infections due to highly resistant pathogens was significantly reduced by SDD compared with SOD (OR 0.37, 95% CI 0.16-0.85) [4].
Secondly, the authors state that they believe a new trial with the endpoint of resistance is required to address the aforementioned area of uncertainty. The R-GNOSIS group is planning a cluster randomised clinical trial across Europe in countries with higher levels of resistance. This study will compare SOD, oral decontamination with chlorhexidine without the parenteral component of the calssic SDD regimen with standard care. It is remarkable that the classic SDD regimen is not included in that study, since this should be considered the gold-standard in this type of approach. Nor is it clear whether surveillance samples of throat and/or gut (carriage of resistance) or diagnostic samples of lower airways (colonisation/infection with resistant bacteria) are obtained in this new trial. It is generally accepted that surveillance samples are the most sensitive samples to detect resistance [5]. Of course R-GNOSIS will generate interesting resistance data but this trial cannot answer the original question ‘Does SDD cause resistance?’

References
1. Walden AP, Bonten MJ, Wise MP. Should selective digestive decontamination be used in critically ill patients? BMJ 2012; doi: 10.1136/bmj.e6697.
2. de Jonge E, Schultz M, Spanjaard L et al. Effects of selective decontamination of the digestive tract on mortality and acquisition of resistant bacteria in intensive care: a randomised, controlled trial. Lancet 2003; 363: 1101-1016.
3. de Smet AMGA, Kluytmans JAJW, Cooper BS et al. Decontamination of the digestive tract and oropharynx in ICU-patients. N Engl J Med 2009; 360: 20-31.
4. de Smet AM, Kluytmans AJ, Blok HE et al. Selective digestive decontamination and selective oropharyngeal decontamination and antibiotic resistance in patients in intensive care units: an open label, clustered group-randomised, crossover study. Lancet Infect Dis 2011; 11: 372-380.
5. D'Agata EM, Venkataraman L, DeGirolami P et al. Colonization with broad-spectrum cephalosporin-resistant gram-negative bacilli in intensive care units during a nonoutbreak period: prevalence, risk factors, and rate of infection. Crit Care Med 1999; 27: 1090-1095.

Competing interests: None declared