In a letter published today in the Journal of Hospital Infection, Professor Peter Wilson and colleagues report on their retesting of the Deprox (Hygiene Solutions Ltd) HPV decontamination system. The retesting was in response to concerns widely raised that the earlier tests of the system, which purported to give a log 6 efficacy, were the result of the manufacturer misrepresenting the concentration and/or constituents of the Deproxin solution.
In brief, the retest reveals the following:
- The efficacy of the system, when using the correct 5% H2O2 solution, is around log 4 – similar to other, much less expensive fogging systems on the market. Hygiene Solutions Ltd will now be taxed to explain why on the initial test by Prof. Wilson, a log 6 efficacy was found. This 100 fold drop in efficacy comes between the first test series, in which the solution concentration was not checked, and the second series in which the concentration was independently measured. It seems highly likely that in the initial test series, the Deprox was running on a 30% H2O2 solution, rather than a 5% H2O2 solution as claimed.
- The aerial H2O2 concentration on re-entering the room at the end of the cycle was 3.3ppm which is in excess of both short term and long term H2O2 exposure health limits. This is in spite of a hasty retrofit of catalysts to the whole Deprox fleet, and the manufacturer’s claim that their system monitored the H2O2 levels, and would only allow re-entry when the level was below the 1ppm safety standard.
- Deproxin is confirmed to contain 10-20ppm silver nitrate, and this is indeed deposited on surfaces in the room, and may well be a contributor to the efficacy achieved. Hygiene Solutions informed the HSE over a year ago that Deproxin does not contain silver nitrate. This has now been exposed as untrue – and raises the question as to whether it is currently legal to sell or use the Deprox system.
- Poor efficacy results for two more sheltered locations is attributed to inhomogeneous vapour distribution, due to inadequate circulation of the vapour. This raises serious doubts as to Hygiene Solutions’ claims that the Deprox will decontaminate “inside small crevices and complex equipment”
It is very much to the credit of Professor Wilson and colleagues that they have thoroughly retested the system in response to widespread concerns. It is sad that commercial interests would abuse the trust and confidence of highly qualified academics in this way by misrepresenting the basic test parameters.
For those without access to the Journal of Hospital Infection, I have reproduced the letter below, and a PDF is downloadable here.
In response to the letter from Dr Singh commenting on our paper., 
The objective of our study was to evaluate the reductions in environmental contamination during in-use operation of two commercially-available hydrogen peroxide whole-room disinfection systems.2 Both manufacturers agreed test parameters prior to the trial to ensure methodology followed manufacturer instructions. Our findings suggested similar efficacy of the two systems against both surface contamination and biological indicators of common pathogens. Inocula used on the indicators far exceeded the likely levels seen in the environment.
Additional studies were performed as part of the original work using the same methodology with four strains each of MRSA, Klebsiella pneumoniae, Clostridium difficile spores and Acinetobacter baumannii. Three HPV decontamination cycles were evaluated for each system. Of 305/320 samples, >4-log10 reduction was achieved.
Aerial concentrations of hydrogen peroxide and relative humidity were monitored continuously during a further 6 cycles of both systems using a sensor (C-16 Portasens II Gas Detector; Analytical Technology Inc., Collegeville, PA, USA). In addition, horizontal surfaces in the near-patient vicinity were swabbed and analysed to detect fallout of silver and nitrate at the end of HPV decontamination cycles (n=3). Surfaces were swabbed and analysed for silver by titration (Silver Test Kit, DTK Water, Wellingborough, UK) and nitrate using Quantofix semi-quantitative test strips (Macherey-Nagel, Düren, Germany).
For the Deprox (Hygiene Solutions, King’s Lynn, UK) system, peak aerial values of 29-46 ppm hydrogen peroxide were achieved with similar bacteriological efficacy as other cycles. The mean level at the end of the cycles was 3.3ppm for 41.8% (30.8-58.1%) mean relative humidity at start of cycles. Silver and nitrate were detected on surfaces at 1.5-2.5mg/m2 following cycles with the Deprox system.
For the Bioquell Q10 system with the R10 aeration unit (Bioquell, Andover, UK), the peak aerial levels of hydrogen peroxide were 450-640ppm. The mean level at the end of the cycles was 0.0ppm with starting mean relative humidity 42.5% (34.5-49.7%). No silver or nitrate was detected on surfaces following cycles with the Bioquell Q10 system.
The aqueous concentration of hydrogen peroxide in a Hygiene Solutions cartridge (Deprox) tested on one occasion at the point of insertion into the machine was 5%. Nitrate was detected in the aqueous solution at 10-25mg/L. The aqueous hydrogen peroxide concentration in the Bioquell Q10 cartridge (Bioquell HPV-AQ) was 35% and no silver or nitrate was detected.
Dr Singh suggests C. difficile spores (but not the other organisms) persisted underneath the bed and on the window frame after decontamination using the Deprox system. The persistence of spores may have been minimised during the Bioquell Q10 cycles by the inclusion of an oscillating fan to facilitate aerial distribution and aid breakdown of hydrogen peroxide vapour.
As Dr Singh suggests, settling of active silver onto biological indicator coupons during a cycle of aerial HPV decontamination may have contributed to the bactericidal/sporicidal activity of the Deprox system. However further studies would be required to elucidate its role.