To the Editor—The recent study “Evaluating a New Paradigm for Comparing Surface Disinfection in Clinical Practice”Reference Carling, Perkins, Ferguson and Thomasser ¹ by Carling et al has advanced the science of both cleaning and cleanliness with exploration of fluorescent markers and environmental screening. Undoubtedly, fluorescent gel applied to key surfaces leads to a more accurate assessment of cleaning, and the study design utilized this method to standardize the testing of 2 different disinfectants. The results equivocally demonstrate that one agent was better than the other for removing bioburden.Reference Carling, Perkins, Ferguson and Thomasser ¹ However, the authors then examined their quantitative data against microbiological standards proposed a decade ago.Reference Dancer ^{2 ,} Reference Griffith, Obee, Cooper, Burton and Lewis ³ They found that precleaning soil was uniformly low, which, according to these standards, represented a hygiene-level pass for ~85% surfaces. This finding is clearly unhelpful for both housekeeping and infection control staff because it negates further monitoring, research, and enthusiasm toward improvement.

It is possible that this hospital sustains exemplary cleaning practices as routine, or certainly did during the course of this study. Housekeepers and domestic staff always react to environmental monitoring,Reference Bogusz, Stewart, Hunter, Yip, Reid, Robertson and Dancer ^{4 ,} Reference Dancer ⁵ and this reaction could explain the low level of soil found on surfaces before cleaning. However, the interpretation of bioburden against previously proposed microbiological standards is subject to methodological confounders that were not detailed in the study. First, the original standards for surface level cleanliness were based on routine cleaning with detergent, not disinfectant, and were aimed at UK hospitals.Reference Dancer ^{2 –} Reference Bogusz, Stewart, Hunter, Yip, Reid, Robertson and Dancer ⁴ Routine use of disinfectant has a measurable and long-lasting effect on hospital surfaces.Reference Griffith, Obee, Cooper, Burton and Lewis ^{3 ,} Reference Dancer ^{5 ,} Reference Stewart, Bogusz and Hunter ⁶ It is not surprising, therefore, that the precleaning bioburden measured in this study was low; any proposed cleaning standards would require adjustment to reflect habitual exposure to biocidal disinfectants.

Second, while dip slides are relatively malleable, they could not be expected to pick up the full complement of bioburden on items such as a call button, a light switch, and some types of handles, rails, and bars. The preferred method for accurately screening irregular and/or small surfaces is to swipe a moistened swab over a specified area and then inoculate the slide or plate with the swab.Reference Griffith, Obee, Cooper, Burton and Lewis ³

Third, the article does not mention the pressure used to apply the dip slides to the selected surfaces. This is important, because if too much pressure is applied on the surface, the agar breaks up and renders quantitative assessment of counts invalid. If too little pressure is applied or pressure is not applied for an adequate length of time (10 seconds is advised), the dip slide will fail to pick up all superficial (newly shed/planktonic) microbes on sampled surfaces.Reference Griffith, Obee, Cooper, Burton and Lewis ^{3 ,} Reference Griffith ^{7 ,} Reference Vickery, Deva, Jacombs, Allan, Valente and Gosbell ⁸ The correct pressure for dip-slide sampling has been quoted as 25 g/cm² (without lateral movement) by food industry microbiologists and should have been predetermined within an appropriate training process before the study began.Reference Griffith, Obee, Cooper, Burton and Lewis ^{3 ,} Reference Griffith ⁷

Fourth, the dip slides were incubated for only 24 hours; this time period is insufficient to permit retrieval of environmental organisms, and particularly so when the study surfaces have been habitually exposed to disinfectants.Reference Griffith ⁷ In our experience, both agar plates and dip slides should be incubated for at least 48 hours at 30°–35°C to recover the greatest possible yield of cultivable aerobic organisms.Reference Dancer, White and Robertson ⁹ Additionally, the agar(s) used on the dip slides and incubation conditions are not mentioned in the Methods.

Finally, 2 standards were originally proposed: 1 quantitative (<5 cfu aerobic flora/cm²) and 1 qualitative (<1 cfu specific pathogen/cm²).Reference Dancer ² These standards were designed to be used together and, indeed, have been shown to be linked (for coagulase-positive staphylococci) when screening hand-touch sites.Reference Dancer, White and Robertson ⁹ The second standard was not used in the present study. The choice between 2.5 cfu/cm² (as in this study) vs 5 cfu/cm² (as originally suggested) does not necessarily represent a significant problem; several studies have examined both and little difference overall was found.Reference Griffith, Obee, Cooper, Burton and Lewis ^{3 ,} Reference Dancer ⁵ Future work will demonstrate which density adequately predicts risk in a range of healthcare environments. However, quantitative aerobic colony counts performed in isolation only provide a general level of contamination and not necessarily an infection risk for patients.Reference Griffith, Obee, Cooper, Burton and Lewis ³

Considering these concerns together, it is possible that the low level of bioburden reported in this study did not reflect true contamination of hospital surfaces and should not have been interpreted in accordance with previously proposed microbiological standards. Surface sampling is fraught with potential pitfalls and has always complicated reliable assessment of cleanliness. Recent work on surface biofilm in the healthcare environment has introduced yet another hurdle for healthcare monitoring.Reference Vickery, Deva, Jacombs, Allan, Valente and Gosbell ⁸ Despite these new findings and the concerns listed above, it is very gratifying to see increasing interest and support of basic cleaning in our hospitals. It has been a long time coming.Reference Dancer ¹⁰