Sterilisation means killing all microorganisms, including bacterial spores, on a surgical instrument. Cleaning removes soil. Sterilisation kills what remains. You cannot sterilise a dirty instrument. Clean first. Then sterilise.
Most common method. Saturated steam under pressure. Temperature: 121°C to 135°C. Time: 3 to 30 minutes depending on load and temperature. Higher temperature means shorter time. 134°C for 3 minutes kills everything. 121°C for 15 minutes also kills everything but takes longer.
How it works: Steam transfers heat to the instrument surface. The heat denatures microbial proteins and nucleic acids. Moist heat kills faster than dry heat because water conducts heat better.
Requirements: Instruments must tolerate heat and moisture. Stainless steel handles steam. Plastics, rubbers, and some lenses do not. The load must be dry before and after the cycle. Wet packs wick contaminants through the wrapper.
Cycle phases:
Conditioning: Air removed from chamber. Gravity displacement pushes air out the bottom. Pre-vacuum pumps pull air out. Pre-vacuum is faster and more reliable. Most hospital sterilisers use pre-vacuum.
Exposure: Chamber holds at target temperature and pressure for the required time.
Exhaust: Steam released. Pressure drops.
Drying: Vacuum pulls moisture off the load. Wrapped instruments need 20 to 30 minutes of drying.
Steam is the gold standard. Fast, non-toxic, inexpensive, and monitored easily. Use it unless the instrument cannot take it.
Low-temperature method for heat- and moisture-sensitive instruments. Brand name: Sterrad. Temperature: 45°C to 55°C. Cycle time: 35 to 75 minutes.
How it works: Hydrogen peroxide vaporises into a gas. Radio frequency energy creates plasma. Plasma breaks down the gas into free radicals. Free radicals attack microbial membranes, DNA, and proteins.
Requirements: Instruments must fit in the chamber. Lumens must be narrow enough for the gas to reach the end. Maximum lumen length: 50cm for single lumen, 40cm for double lumen. Minimum lumen diameter: 1mm for standard cycle, 1.5mm for rapid cycle. No cellulose materials (paper, cotton, linen). They absorb hydrogen peroxide and prevent sterilisation.
Advantages: Fast turnaround compared to other low-temperature methods. No toxic residues. No aeration needed. Disadvantages: Expensive. Limited lumen compatibility. Items must be completely dry.
Low-temperature method for heat- and moisture-sensitive items that cannot take hydrogen peroxide. Temperature: 37°C to 63°C. Cycle time: 2 to 24 hours plus aeration.
How it works: EtO gas alkylates microbial DNA and proteins. The microbe cannot reproduce or function. EtO penetrates plastics, rubbers, and long narrow lumens better than any other low-temperature method.
Requirements: Proper ventilation. EtO is toxic, flammable, and explosive. Staff must monitor exposure. Facilities need dedicated EtO rooms with air exchange systems. Aeration time: 12 hours at 50°C for most materials, 24 hours for some plastics.
Advantages: Compatible with almost everything. Penetrates packaging and lumens completely. Disadvantages: Long cycle plus aeration. Toxic. Expensive equipment. Regulations restrict where and how you can use it.
Low-temperature steam with formaldehyde. Temperature: 60°C to 80°C. Cycle time: 2 to 3 hours.
How it works: Formaldehyde gas kills by cross-linking microbial proteins and nucleic acids. Low-temperature steam carries the gas to instrument surfaces.
Use is limited. Formaldehyde is carcinogenic. Residues remain on instruments unless properly aerated. Most facilities that used it have switched to hydrogen peroxide or EtO. Some European countries still use it for specific items. The US uses it rarely.
Ozone gas (O3) oxidises microbial membranes. Temperature: 30°C to 35°C. Cycle time: 4 to 5 hours.
Limited adoption. Equipment is expensive. Cycle times are long. Ozone degrades rubber faster than other methods. Not common in hospitals. More common in dental and veterinary settings.
Liquid immersion sterilisation. Brand name: Steris. Temperature: 50°C to 56°C. Cycle time: 30 to 60 minutes.
How it works: Peracetic acid oxidises microbial proteins and enzymes. Items sit fully submerged in the solution. Rinse with sterile water after the cycle.
Requirements: Items must be submersible. Heat-tolerant plastics and metals work. No lumens longer than the manufacturer specification. Immediate use only after sterilization. Items cannot be wrapped and stored. They go directly from the steriliser to the sterile field.
Used for endoscopes and some surgical instruments. Not for implantables unless the manufacturer approves.
| Instrument type | Method |
|---|---|
| Stainless steel, heat-tolerant | Steam |
| Heat-sensitive, simple shapes | Hydrogen peroxide |
| Heat-sensitive, long narrow lumens | EtO |
| Endoscopes, immediate use | Peracetic acid |
| Almost everything but slow | EtO |
Three types of indicators. Use all three.
Physical indicators: Time, temperature, pressure readings from the steriliser itself. The cycle log. Shows the machine ran correctly. Does not prove items inside actually got sterile.
Chemical indicators: Tape, cards, or strips that change colour when exposed to sterilant. Placed inside each package. Shows the package went through a cycle. Does not prove kill. Type 6 chemical indicators respond to specific parameters (time and temperature matched to a specific cycle). Most hospitals use Type 4 or Type 5.
Biological indicators: Vials or strips containing bacterial spores (Geobacillus stearothermophilus for steam and hydrogen peroxide, Bacillus atrophaeus for EtO). Placed in the hardest-to-reach part of the load. After the cycle, incubate the vial. If spores grow, the load fails. If no growth, the load passes. Run a biological indicator in every steriliser at least once per day. Run one in every load containing implants. Implants cannot be released until the biological indicator shows no growth, which takes 24 hours. Some rapid biological indicators return results in 1 to 3 hours.
Wet packs. Moisture inside a wrapped package after the cycle. Causes: overloaded chamber, short drying time, wet instruments before packaging, steam quality issues. Rewrap and reprocess.
Instrument damage from steam. Tarnish, stain, pitting, corrosion. Causes: poor water quality, mixing dissimilar metals (copper with stainless steel), excessive detergent residue. Use demineralised water. Rinse instruments thoroughly before sterilisation.
Incomplete air removal in steam cycles. Air pockets prevent steam from contacting instrument surfaces. Causes: failed vacuum pump, overloading, improper loading. Pre-vacuum cycle with Bowie-Dick test every morning to check air removal.
Failed biological indicator. Spores grow after the cycle. Immediate action: recall all loads processed since the last passing biological indicator. Reprocess everything. Quarantine the steriliser until the problem is found and fixed.
Sterilisation with an s is British English. Sterilization with a z is American English. Both are correct. Pick one and be consistent. This article uses the s spelling except where quoting American sources or referencing American brand names.