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P ROTE IN DE TECT ION


additional reprocessing cycles.2


As this was


concerning, a significant amount of research ensued and the transmissible agent of CJD was identified as being a truncated form of a single protein found on the surface of neurons. In one in a million humans, this protein is made into a pathological, infectious form (called prion) capable of aggregating unstoppably in the central nervous system (CNS) of affected patients.3 Prions adsorbed on neurosurgical instruments were found to be particularly resistant to existing decontamination treatments due to their hydrophobic nature and appeared unaffected by standard sterilisation, explaining the iatrogenic case above. This problem was initially thought to be an issue for neurosurgical sets only, as far as SSD procedures were concerned. This was until the ‘mad cow’ crisis of the mid- nineties and the subsequent cases of (then new) variant CJD which peaked in the UK during 2000.4


Other research had then established that transmissible spongiform encephalopathies (TSEs) like CJD affect many vertebrate species and all have in common that prion protein.5,6


In the case of vCJD the bovine protein consumed by humans ‘jumped the species barrier’ and infected an estimated 1 in 2,000 of the British population at the time.7


Only a small fraction of patients rapidly developed symptoms and died, but this event has led to a significant reconsideration of the importance of residual proteins on reusable surgical instruments.8


Targeting prions and residual proteins on surgical instruments Laboratory studies confirmed that hydrophobic proteins such as prions can resist water-based chemistries and be retained on surfaces while most other proteins are desorbed and washed away.9-12 This suggests that the proteins which remain on instruments after the manual cleaning and washer disinfector cycle are potentially those more harmful to patients. As an alternative to enzymatic chemistries, alkaline solutions have been tried as high pH can alter the structure of prions (if not removing them from the surfaces), rendering them less infectious. However, dilutions during preparation or application reduce the pH and efficacy of these solutions against prion infectivity. Solutions which would remain sufficiently alkaline to affect prion infectivity would also damage equipment. Therefore, procedures have focused on total protein removal relying on equipment-compatible chemistries and quality control, notably through closer inspection of instruments in the ‘clean room’ of SSDs. Washer disinfectors do not always produce a perfect result, and all SSDs have a certain return rate which they try to keep


as low as possible, since sending instrument trays for a second washing cycle implies delays. Failures in decontamination occur rarely, although instruments returned often harbour macroscopically visible stains or deposits, generally located in crevasses, hinges or similar areas that are particularly difficult to reach when attempting decontamination. The water source for AWDs is usually the common water supply and therefore out of control from the end users, although additional water purification and/or filtration steps may be included at different stages of the reprocessing cycle. There is evidence that the quality of the water employed might impact on the overall cleaning result.13


All other parameters are pre-set by the manufacturer and occasionally adjusted by engineers. SSD staff rely on the recommendations from chemistries and equipment manufacturers, and AWD self- generated reports after each cycle. Just like the decontamination market, the protein detection market for SSDs has expanded in the wake of CJD and vCJD in the UK. One key factor to any additional procedure introduced into busy SSDs is the associated cost and time requirement. For many years, swabbing has been used to try to desorb proteinaceous residues from the surface of instruments. Tests such as Biuret and Ninhydrin became very popular, being rapid and easy. One problem with such tests is that microscopic, surface-bound proteinaceous residues, which resisted a manual cleaning followed by an AWD cycle, are unlikely to be desorbed by a wet cotton bud.14


limitations, the Department of Health and Social Care (DHSC) introduced in the Health Technical Memorandum (HTM) 01-01, Management and decontamination of surgical instruments (medical devices) used in acute care,16


recommendations for direct


(in situ) detection of residual proteins on reprocessed surgical instruments. There is a range of highly sensitive technologies used in research laboratories to detect minute quantities of biomolecules. Obviously, technologies applicable to the needs of SSDs must remain sufficiently simple, rapid, cheap, and any required equipment should easily fit in the clean room. Episcopic differential interference contrast microscopy combined with epi-fluorescence (EDIC/EF) was developed nearly 20 years ago for laboratory studies of biofilms on surfaces (Best Scientific, UK).17


It combines


long working distance lenses allowing the examination of complex shapes and a Nomarski prism which provide a pseudo- 3D image. A CCD camera mounted on the microscope is linked to a computer to record images which can be analysed for various purposes, stored, and re-examined at will. In laboratory studies this direct in situ


observation technology proved far more sensitive for the detection of proteinaceous residues on surfaces than biochemical tests relying on swabbing.14


We could


demonstrate the differential removal of amyloid (particularly prions) and other proteins.11


We also evaluated the impact


On the other hand, handling the previously decontaminated instruments with bare hands may contribute to inadvertently increasing the protein load.


Since swabbing does not guarantee 100% recovery, indirect biochemical colorimetric tests can also be ambiguous to interpret as they are at best a semi- quantitative measurement of an unknown proportion of proteins recovered from residues that are effectively present on the surface swabbed (only some of the ‘tip of the iceberg’).15


Taking into account these 70 l WWW.CLINICALSERVICESJOURNAL.COM


of delays in reprocessing and the potential influence of transport conditions during the transfer of instruments between theatres and centralised SSDs. We could measure the negative impact drying has on the efficacy of instruments reprocessing.18,19


Thanks


to NIHR-funded collaborative research, an EDIC/EF system was installed in our clinical collaborators’ SSD in 2011 (Figure 1). Using EDIC/EF located within an SSD, we were able to carry out various studies to identify existing limitations in protocols and try to find applicable solutions. Our most recent work demonstrated that while achieving compliancy, a number of issues remain which are linked to high demand,


APRIL 2021


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