Method of sterilization with definition
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Disinfection and Sterilization in Healthcare Facilities
Page 9
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physical chemical sterilization techniques heat radiation gas
Disinfection and Sterilization in Healthcare Facilities
Page 59
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Methods of Sterilization
Definition
Sterilization is a process that destroys or eliminates all forms of microbial life, including bacterial spores (the most resistant form). It is carried out in healthcare facilities by physical or chemical methods. When chemicals are used to destroy all microbial life, they are called chemical sterilants.
Key distinction: Sterilization is an absolute process — an item is either sterile or it is not. Terms like "partially sterile" are incorrect. Sterilization differs from disinfection, which reduces (but may not eliminate) microbial load. (Disinfection and Sterilization in Healthcare Facilities, p. 9)
Classification of Sterilization Methods
A. Physical Methods
1. Heat Sterilization
Most medical and surgical devices are heat-stable and undergo heat-based sterilization.
| Type | Method | Temperature | Duration | Mechanism |
|---|---|---|---|---|
| Moist Heat (Autoclave) | Steam under pressure | 121°C (15 psi) or 132–134°C (30 psi) | 15–30 min | Denaturation of proteins by steam |
| Dry Heat | Hot air oven | 160–170°C | 1–2 hours | Oxidation of cellular components |
| Boiling | Boiling water | 100°C | 20 min | Kills vegetative forms (NOT spores — not true sterilization) |
| Incineration | Burning | >1000°C | — | Combustion of all microbial forms |
- Autoclave (Steam under pressure) is the gold standard for heat-stable items (surgical instruments, drapes, glassware).
- Dry heat is used for items damaged by moisture (powders, oils, glass syringes).
2. Radiation Sterilization
| Type | Application | Mechanism |
|---|---|---|
| Ionizing radiation (Gamma rays, Electron beam, X-rays) | Industrial sterilization of single-use medical devices (syringes, gloves, sutures) | DNA strand breakage |
| Non-ionizing radiation (UV light) | Surface/air sterilization in labs, OTs | Formation of thymine dimers in DNA |
- Gamma irradiation is widely used for factory sterilization of pre-packaged disposables.
- UV is limited to surface sterilization due to poor penetration.
3. Filtration
- Used to sterilize heat-sensitive liquids and gases (IV fluids, vaccines, air in laminar flow hoods).
- Common filters: Millipore filters (0.22 µm pore size) — removes bacteria and fungi but NOT viruses.
- HEPA filters are used for air sterilization in operation theaters and biosafety cabinets.
B. Chemical Methods
1. Gaseous / Low-Temperature Sterilization
Used for heat- and moisture-sensitive devices (endoscopes, plastics, electronic equipment).
| Agent | Method | Notes |
|---|---|---|
| Ethylene Oxide (EtO) | Gas exposure at 37–55°C | Used since the 1950s; alkylates DNA/proteins; items require aeration post-sterilization (toxic residues) |
| Hydrogen Peroxide Gas Plasma | Low-temperature plasma | Rapid, no toxic residue; used for heat-sensitive devices |
| Formaldehyde Gas | Low-pressure steam + formaldehyde | Less commonly used; toxic |
| Ozone | Gaseous ozone | Oxidative mechanism; emerging technology |
(Disinfection and Sterilization in Healthcare Facilities, p. 59)
2. Liquid Chemical Sterilants (High-Level Disinfection/Sterilization)
Used for critical items that cannot withstand heat or gas sterilization.
| Agent | Examples | Mechanism |
|---|---|---|
| Glutaraldehyde | Cidex (2%) | Alkylation of proteins and nucleic acids |
| Peracetic Acid | Steris system | Oxidative destruction of cellular components |
| Ortho-phthalaldehyde (OPA) | Cidex OPA | Reacts with amino acids; superior mycobactericidal activity |
| Hydrogen Peroxide (6–7.5%) | — | Oxidative damage to membranes, enzymes, DNA |
Summary Table
| Method | Mechanism | Best Used For | Limitations |
|---|---|---|---|
| Steam autoclave | Protein denaturation | Metal instruments, drapes | Cannot use for heat-sensitive items |
| Dry heat | Oxidation | Powders, oils, glassware | Long exposure time, high temp |
| EtO gas | Alkylation | Plastics, electronics, endoscopes | Toxic, requires aeration |
| H₂O₂ gas plasma | Oxidation | Heat/moisture-sensitive devices | Cannot be used for lumened devices >1m |
| Gamma radiation | DNA strand breaks | Pre-packaged disposables | Expensive, industrial use |
| Filtration | Physical removal | Liquids, gases | Does not remove viruses |
| Peracetic acid | Oxidation | Endoscopes | Single-use; no long-term storage |
| Glutaraldehyde | Alkylation | Endoscopes, lensed instruments | Toxic fumes, long exposure (10 hrs) |
Key principle: The choice of sterilization method depends on the nature of the item (heat-stable vs. heat-sensitive), the material composition, and the intended use (critical vs. semi-critical vs. non-critical device).
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