Medical devices: Hygiene monitoring through protein detection

Selective sampling for residual proteins is under the gaze of international standards for its effectiveness on areas hidden from the human eye in cracks and internal spaces. Dr Juan Díaz of infection control specialist Terragene presents an alternative

Lack of senstivity with detection methods can mistakenly be blamed on the swab technique

Cleaning processes of instruments and materials for medical use must be thoroughly controlled since their results determine the subsequent success of the disinfection and or sterilisation processes.

A common practice is to rely on visual inspection to detect residual dirt levels. However, this method has low sensitivity, is subjective and can be influenced by a number of factors that include intensity and nature of lighting in the inspection area. In addition, this methodology becomes irrelevant in cases of instruments with cracks, joints, lumens, etc., where the human eye can’t spot the dirt.

Ideally, the verification of cleanliness by users should include three basic methodologies. Firstly, visual inspection combined with other verification methods that allow evaluation of both external surfaces, as well as internal spaces and channels of medical devices. Next, checking the cleaning efficiency of the equipment. And lastly, monitoring the key cleaning parameters (e.g. temperature, time).

Today, the detection of proteins derived from blood and other fluids on the surfaces of surgical instruments has gained great importance due to the continuous risks of transmission of infectious protein agents, such as prions (causes of transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease).

Additionally, proteins are one of the components of residual dirt that presents the greatest challenge to cleaning, due to its high adhesion, especially when they undergo a denaturation process due to high temperatures (for example in thermodisinfection).

The detection of proteins derived from blood and other fluids on the surfaces of surgical instruments has gained great importance due to the continuous risks of transmission of infectious protein agents

Currently, the most used methodology for the detection of residual proteins is based on the selective sampling of the most challenging places in the instruments (cracks, joints, lumens, internal spaces, etc.) through systems using the swab technique.

Controversial standards

The different international regulations refer to the possible methodologies to use in order to quantify protein residues in surgical instruments after the washing stage. ISO 15883-1 describes the general requirements, terms, definitions and test to be applied in the procedures that use disinfector washers. The international reference standard recommends three test methods for detection and evaluation of residual protein contamination: the methods are based on the reactions of Ninhydrin, OPA (orthophthaldehyde) and Biuret.

This regulation recommends the use of the swab methodology for the two methods in which it is possible to use it, Ninhydrin and Biuret, and then establishes the limits of acceptance for them.

Different studies, from Lombardo et al. in 1995 to Holmes et al. in 2017, highlight that the use of the swab technique is recommended for the detection of residual proteins —and even the most widely recognised regulations around the world—recommend the use of this technique. These include ANSI/AAMI, FDA, SHTM 01-01, Health Canada, and HTM 01-06.

The swab method is recommended, as it is able to detect proteins in places of difficult access, such as in instruments with porous parts, cannulated elements, or the channels of an endoscope

A particular case represents the Health Technical Memorandum (HTM), which is implemented in the UK as regulations for the management and decontamination of surgical instruments (medical devices). In sections 1–6, the use of the swab method is recommended, as it is able to detect proteins in places of difficult access, such as in instruments with porous parts, cannulated elements, or the channels of an endoscope.

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The information used to advise against the swab technique in this memorandum is based on a paper entitled “Critical evaluation of ninhydrin for monitoring surgical instrument decontamination” by Nayuni et al. in 2013. This work correctly points out that the Ninhydrin method does not reach the level of sensitivity required by the standard (≤ 5 µg per side of the instrument, HTM 01-01: 2016), but then mistakenly associates this lack of sensitivity with the swab technique. In fact, it does not specify the technique used to perform the swabbing, a determining point to take into account due to the large dispersion in the protein recovery rates obtained by the different ways of using the swab and its supplements.

The Chemdye PRO1 MICRO system is made of a combination of a highly-abrasive swab and a formulated moisturiser

One of the biggest controversies about this memorandum focuses on the inability of the in situ detection method to be used in a wide range of elements that must be tested according to the standard. These included aspects such as the walls of the washing machine chambers, the freight cars, porous materials, hinges, cannulated elements and large volume instruments (to give but a few examples). For this reason, this methodology becomes unfeasible for a large number of the instruments used in a sterilisation plant.

False negatives

The use of this method for the quantification of residual protein has been widely criticised, mainly due to the low penetrability of the OPA/NAC (NAC: N-acetylcysteine) reagent in areas where dirt tends to accumulate more easily, the false negative results obtained in porous material testing, and the variable affinity of different proteins for this reagent.

The penetrability of the OPA reagent in the dirt layer may not be complete due to its thickness and composition, so it is not possible to determine a priori what percentage of the proteins are being taken into account at the time of quantification.

A porous material absorbs a greater amount of dirt that lodges in the pores, and to which the fluorescent solution is not able to reach, leading to an underestimation of the amount of protein detected

The porosity of the material to be tested is a critical limitation in this system. A porous material absorbs a greater amount of dirt that lodges in the pores, and to which the fluorescent solution is not able to reach, leading to an underestimation of the amount of protein detected. This leads to erroneous results and causes false negative results, with all of the serious repercussions that this entails for patients.

The aforementioned limitations make the detection of proteins in situ a methodology that is not very applicable for Central Sterile Services Department when it comes to testing most surgical instruments and their complements, in order to guarantee a correct cleaning test.

The ideal method is one that employs the technique of surface swab (suitable for any instrument size and complexity), that uses an alternative to the OPA and Nihindrina reactions

For everything listed above, the ideal method is one that employs the technique of surface swab (suitable for any instrument size and complexity), that uses an alternative to the OPA and Nihindrina reactions, such as the Biuret reaction, and that is capable of quantifying with a sensitivity ≤ 5 µg of proteins per instrument side.

Terragene SA proposes the use of its Chemdye PRO1 MICRO system for protein detection and quantification on instrument surfaces, complying with the characteristics mentioned above and with international regulations.

Moreover, it is made of a combination of a highly-abrasive swab and a specially formulated moisturiser solution that ensure a protein recovery of more than 85%. It represents an easy, fast and accurate option for monitoring the cleanliness of reusable medical devices in the context of medical, dental and industrial practices based on the detection and quantification of surface proteins, allergens and reducing agents.

N.B. This article is featured in the November 2019 issue of Cleanroom Technology. Subscribe today and get your print copy!

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