Describe the methods of disinfection and sterilisation.

Disinfection and Sterilisation

• Sterilisation is the complete destruction or removal of all forms of microbial life including bacterial spores.
• disinfection is the destruction of most pathogenic organisms but not necessarily spores.
• The presence or absence of spores is what separates the two terms, and it is the distinction on which every choice of method rests.
• The related terms — antisepsis is disinfection applied to living tissue; decontamination renders an article safe to handle; sanitation reduces microbial numbers to a safe public health level; and asepsis is the practice of preventing contamination altogether
• Spaulding’S classification determines what level of treatment an article requires, and reasoning from it is what an examiner wants:
• Critical items — those entering sterile tissue or the vascular system (surgical instruments, implants, needles, catheters) — require sterilisation
• Semi-critical items — those contacting mucous membranes or non-intact skin (endoscopes, respiratory equipment, laryngoscopes) — require high-level disinfection
• Non-critical items — those contacting intact skin only (stethoscopes, blood pressure cuffs, bedpans, furniture) — require low or intermediate-level disinfection
• The methods of sterilisation:
• Moist heat (autoclave) — 121°C at 15 lb per square inch for 15 to 20 minutes, or 134°C for 3 minutes. It is the method of choice for most items, being rapid, reliable, cheap and non-toxic
• Moist heat kills by coagulating and denaturing proteins, whereas dry heat kills by oxidation — which is why moist heat is effective at a lower temperature and in a shorter time
• The critical requirement is that steam must make direct contact with every surface, so air must be fully displaced, the load must not be packed tightly, and articles must be cleaned first — organic matter protects organisms and no sterilant penetrates it
• Dry heat (hot air oven) — 160°C for 2 hours; used for glassware, oils, powders, and sharp instruments that steam would damage or fail to penetrate
• Chemical sterilisation — ethylene oxide for heat-sensitive items, which is effective but toxic, mutagenic and requires prolonged aeration; hydrogen peroxide plasma; and glutaraldehyde 2% for high-level disinfection of endoscopes
• Radiation — gamma irradiation for single-use disposables at industrial scale
• Filtration — for heat-labile fluids, and HEPA filtration of air
• Monitoring of sterilisation is essential and is examined — physical (time, temperature and pressure recordings); chemical indicators such as autoclave tape and BOWIE-DICK tape, which confirm only that the item was processed, not that it is sterile; and biological indicators using Geobacillus stearothermophilus spores for the autoclave and Bacillus atrophaeus for dry heat and ethylene oxide, which are the only true proof of sterility
• Disinfectants in common use — alcohol 60 to 90%, which needs water to act and is therefore less effective at 100%, and is not sporicidal; hypochlorite (bleach), cheap and broadly effective including against viruses, but inactivated by organic matter and corrosive to metals; iodophors and chlorhexidine as skin antiseptics; phenolics for surfaces; and quaternary ammonium compounds for low-level use
• The principles that govern practice — clean before disinfecting or sterilising, since organic matter defeats every agent; match the method to the risk using Spaulding; observe the required contact time and concentration; prepare solutions fresh; and store sterile items so that sterility is maintained
• The commonest failures — omitting cleaning; overloading the autoclave; insufficient contact time; using diluted or old solutions; and relying on chemical tape as evidence of sterility
• Clean before sterilising, since no agent penetrates organic matter and blood physically shields organisms
• Autoclave tape proves processing, not sterility; only a biological indicator proves the latter
• Spores are what separate the two terms, and that distinction governs every choice of method
• Use Spaulding to match method to risk, sorting items into critical, semi-critical and non-critical
• Moist heat coagulates and dry heat oxidises, which is why the autoclave works at a lower temperature and shorter time
• Steam must reach every surface, so air must be displaced and the load must not be packed tightly
• Alcohol needs water to act, and is therefore less effective at full strength than at sixty to ninety per cent
• Alcohol is not sporicidal, which limits it to disinfection and rules it out for critical items
• Hypochlorite is inactivated by organic matter, so surfaces must be cleaned before it is applied
• Ethylene oxide suits heat-sensitive items but is toxic and mutagenic and needs prolonged aeration
• Glutaraldehyde gives high-level disinfection of endoscopes, which cannot withstand autoclaving
• Prepare disinfectant solutions fresh, since diluted or old solutions are a common cause of failure
• Observe the stated contact time, which is as important as the concentration and is routinely cut short