Learn — Sterilisation, disinfection & hospital infection control

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Methods of sterilization

Sterilisation is the process that destroys or removes all forms of microbial life, including bacterial spores, from an article; disinfection only reduces organisms to a level that is not harmful and does not reliably kill spores. The distinction decides patient safety at the bedside, because anything that enters sterile tissue or the vascular system must be sterilised, while an item that touches only intact skin needs no more than disinfection.

  • Spaulding's classification sets the standard: critical items that enter sterile tissue or the bloodstream (surgical instruments, implants, needles) must be sterilised, semi-critical items that touch mucous membranes (endoscopes, laryngoscope blades, respiratory circuits) need high-level disinfection, and non-critical items that touch intact skin (blood pressure cuffs, bedpans) need low- or intermediate-level disinfection.
  • The autoclave is the workhorse of every central sterile supply department: saturated steam at 121 degrees Celsius and 15 pounds per square inch above atmospheric pressure for a holding time of 15 minutes, or 134 degrees Celsius at 30 pounds per square inch for about 3 minutes in a high-vacuum machine. The holding time is counted only after the whole load has reached temperature, never from the moment the machine is switched on.
  • Moist heat kills by coagulating and denaturing protein and is far more efficient than dry heat; the hot air oven needs 160 degrees Celsius for 2 hours, or 170 degrees for 1 hour, or 180 degrees for 30 minutes, and is used only for glassware, sharp metal instruments, oils, powders and greases that steam cannot penetrate or would spoil.
  • Boiling at 100 degrees Celsius and pasteurisation are disinfection, not sterilisation, because spores survive them, so an item that has only been boiled must never be counted as sterile for an invasive procedure.
  • Chemical sterilisation covers the heat-sensitive articles: ethylene oxide gas for plastics, catheters, pacemakers and heart-lung tubing (it needs a long aeration period afterwards because the residue is toxic, mutagenic and inflammable), 2 percent glutaraldehyde for high-level disinfection of endoscopes in about 20 minutes and sterilisation only after many hours, and low-temperature hydrogen peroxide plasma.
  • The cold methods are radiation and filtration: gamma radiation from cobalt-60 is ionising and sterilises pre-packed single-use disposables such as syringes, gloves, catheters and sutures on an industrial scale, ultraviolet light is non-ionising with poor penetration and only disinfects air and exposed surfaces, and a 0.22 micrometre membrane filter sterilises heat-labile fluids such as serum, antibiotic solutions and vaccines while HEPA filters clean the air of a theatre or laminar flow hood.
  • Every load is monitored three ways: physically by the gauge and the cycle printout of time, temperature and pressure, chemically by autoclave tape and the Bowie-Dick test (run in an empty pre-vacuum machine at the start of each working day to prove that air has been removed), and biologically by spores of Geobacillus stearothermophilus for moist heat and Bacillus atrophaeus for dry heat and ethylene oxide. Only the biological indicator proves that spores were actually killed.
  • Under the Biomedical Waste Management Rules 2016 laboratory, microbiology and blood-bag waste is disinfected or autoclaved on site before it leaves the department, and red-bag recyclable waste is autoclaved or microwaved and then shredded, so sterilisation is a waste-handling duty and not only a theatre one.

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