Sterilisation and Disinfection — Methods, Parameters and Spaulding Classification

Sterilisation is the complete destruction of all microorganisms, including bacterial spores. Disinfection eliminates most pathogenic microorganisms from inanimate objects but does not necessarily kill spores. Antiseptics are germicides applied to living tissue and skin; disinfectants are used only on inanimate objects because they can injure tissue.

The CDC guideline defines three levels of disinfection by what they leave alive. High-level disinfection kills all microorganisms except large numbers of bacterial spores. Intermediate-level disinfectants may kill mycobacteria, vegetative bacteria, most viruses and most fungi but do not necessarily kill spores. Low-level disinfectants kill most vegetative bacteria, some fungi and some viruses within about 10 minutes.

Spaulding classification assigns the required level of processing to a medical item according to the infection risk of its intended use. It is a favourite exam table because each category pairs an example item with a minimum method.

The logic follows the body barrier. Intact mucous membranes of the lung and gut resist common bacterial spores but are susceptible to other bacteria, mycobacteria and viruses, so semicritical items must be free of all organisms though a few spores are acceptable. Intact skin is an effective barrier, so the sterility of items touching only intact skin is "not critical".

Moist heat under pressure is the most widely used and most dependable method for heat- and moisture-stable items such as surgical instruments and linen packs. Pressure is only a means of reaching the high steam temperatures needed; the lethal agent is saturated steam (dryness fraction at least 97 per cent), which condenses on the item and releases latent heat that destroys cell walls, enzymes and structural proteins.

Two sterilizer designs appear in exams. In a gravity displacement autoclave, steam enters at the top or sides and, being lighter than air, pushes air downwards and out through a drain; penetration of porous loads is slower. A prevacuum (high-speed) sterilizer has a vacuum pump that removes air before steam enters, giving almost instant steam penetration even into porous loads and permitting a shorter, hotter cycle.

The Bowie-Dick test checks the air-removal system of a prevacuum sterilizer. A test pack of folded clean cotton towels with a chemical indicator sheet is placed in the front, bottom part of an otherwise empty chamber, near the door and over the drain, and run at 134 °C for 3.5 minutes, each day before the first load. A uniform colour change means adequate air removal; a spot indicates trapped air.

Dry heat kills mainly by oxidation of cell constituents. It penetrates materials and does not corrode metal or sharp instruments, which makes it suitable for metal and sharp instruments. Its disadvantages are slow heat penetration, a time-consuming cycle, and high temperatures that are unsuitable for most materials.

There are two types. The static-air (oven-type) sterilizer heats by coils at the bottom, with hot air rising by gravity convection; it heats slowly and is less uniform. The forced-air (mechanical convection) sterilizer uses a motor-driven blower to circulate hot air at high speed, giving faster, more uniform energy transfer.

Ethylene oxide (ETO) is a colourless, flammable and explosive gas used for heat- or moisture-sensitive medical equipment. Four parameters govern its efficacy: gas concentration, temperature, relative humidity and exposure time. Its drawbacks are the lengthy cycle and aeration time, cost, and hazards to staff and patients; it is infrequently used for flexible endoscopes for this reason.

Liquid chemical sterilants include 2.4 per cent or higher glutaraldehyde, 7.5 per cent stabilised hydrogen peroxide and 0.2 per cent peracetic acid; indicated sterilisation contact times range from 3 to 12 hours. They work only if cleaning precedes treatment and concentration, contact time, temperature and pH are respected. The same agents at shorter contact times serve as high-level disinfectants: for example, ortho-phthalaldehyde (0.55 per cent) has a 12-minute high-level disinfection claim.

Microorganisms differ in innate resistance. The CDC orders them from most to least resistant to chemical germicides: prions, bacterial spores, coccidia such as Cryptosporidium, mycobacteria, non-lipid or small viruses, fungi, vegetative bacteria, and finally lipid or medium-size viruses such as herpes and HIV.

A sterilisation process is monitored with mechanical (physical) readings of time, temperature and pressure, with chemical indicators, and with biological indicators. Only biological indicators measure killing power directly, because they use the most resistant organisms, bacterial spores.

Chemical indicators are heat- or chemically sensitive inks that change colour when a parameter is reached. They are fixed on the outside of each pack to show it has been through a cycle, and preferably also inside to show sterilant penetration. They do not prove sterility, can mislead at marginal exposure times, and should be used alongside, not instead of, biological indicators. Chemical liquid sterilants generally cannot be monitored with a biological indicator.

• Pressure is not the killer. It raises steam temperature; saturated steam at 121 °C is what sterilises.
• Alcohol, 70 per cent included, does not sterilise because it is not sporicidal; it is a disinfectant and skin antiseptic.
• Laryngoscope blades and endoscopes are semicritical, so high-level disinfection is the minimum, not sterilisation.
• Dry heat needs longer and hotter than steam because heat penetrates slowly: 160 °C for 2 hours versus 121 °C for about 30 minutes in a gravity autoclave.
• The most resistant agents to germicides are prions, followed by bacterial spores.
• Match the spore to the method: steam = G. stearothermophilus; dry heat and ETO = B. atrophaeus.
• Chemical indicator colour change on a pack proves exposure, not sterility.