8.3 Sterilization Modalities, Chemical/Biological Indicators & Storage

8.3 Sterilization Modalities, Chemical/Biological Indicators & Storage

Sterilization represents the highest level of microbicidal assurance, achieving the complete destruction of all viable microorganisms, including resistant bacterial endospores. Infection preventionists must thoroughly understand the operational parameters of primary sterilization modalities—such as steam under pressure, Ethylene Oxide (EtO), and Vaporized Hydrogen Peroxide (VHP)/Gas Plasma—along with the comprehensive monitoring triad (physical, chemical, and biological indicators) and event-related sterile storage parameters.

---

Sterilization Modalities & Cycle Parameters

1. Steam Sterilization (Autoclaving)

Steam sterilization is the most widely used, reliable, non-toxic, and cost-effective sterilization method. It utilizes moist heat under pressure to coagulate and denature essential microbial proteins. There are two primary types of steam sterilizer cycles:

A. Gravity Displacement Sterilizers

Steam is injected into the top of the chamber, pushing cold, dense air downward and out through a temperature-sensitive discharge trap at the bottom. Air removal is passive.

• Standard Operating Parameters:
  • 250°F (121°C) for 30 minutes exposure time at 15 pounds per square inch (psi) gauge pressure.
  • 270°F (132°C) for 15 minutes exposure time at 27 psi.

B. Dynamic Air Removal (Prevacuum) Sterilizers

A mechanical vacuum pump actively removes air from the chamber prior to steam admission. This creates a rapid vacuum, allowing steam to instantly penetrate dense textile packs and complex lumens.

• Standard Operating Parameters:
  • 270°F (132°C) for 4 minutes exposure time at 28–30 psi.
  • 275°F (135°C) for 3 minutes exposure time at 30 psi.

2. Ethylene Oxide (EtO) Gas Sterilization

Ethylene Oxide is a low-temperature chemical gaseous sterilization process used for heat- and moisture-sensitive medical equipment. EtO kills microorganisms through alkylation of cellular proteins, DNA, and RNA.

• Cycle Parameters: Temperatures between 99°F–145°F (37°C–63°C), relative humidity of 40%–80%, exposure times ranging from 1 to 6 hours.
• Mandatory Aeration: EtO is toxic, mutagenic, carcinogenic, and flammable. Processed items require extended mechanical aeration in dedicated cabinets (typically 8–12 hours at 50°C–60°C or 36 hours at room temperature) to desorb toxic EtO residues before patient contact.

3. Vaporized Hydrogen Peroxide (VHP) & Gas Plasma

Vaporized Hydrogen Peroxide (VHP) and Hydrogen Peroxide Gas Plasma are low-temperature, dry sterilization processes. Hydrogen peroxide vapor is injected into a vacuum chamber; in gas plasma systems, radiofrequency energy excites the vapor into a plasma state, generating free radicals (hydroxyl and hydroperoxyl radicals) that disrupt microbial lipids, proteins, and nucleic acids.

• Cycle Parameters: Temperatures between 113°F–131°F (45°C–55°C); short cycle times (28 to 55 minutes).
• Key Limitations: Incompatible with cellulose materials (paper, cotton, linen wrappers), liquids, powders, and long, narrow copper or brass lumens, which absorb or degrade hydrogen peroxide.

---

Sterilization Monitoring Systems: The Triad

Sterilization quality assurance relies on three interdependent monitoring systems: Physical, Chemical, and Biological indicators.

1. Physical Indicators

Physical monitoring involves continuous real-time verification of chamber parameters using digital printouts, gauges, and sensors recording cycle time, temperature, and pressure. Operators must verify physical parameters after every cycle before releasing the load.

2. Chemical Indicators (CIs)

Chemical indicators utilize chemical dyes that change color when exposed to specific physical sterilization parameters. ANSI/AAMI/ISO 11140-1 categorizes CIs into six distinct classes:

• Class 1 (Process Indicators): External tape or labels applied to packages to distinguish between processed and unprocessed items (e.g., steam indicator tape).
• Class 2 (Specific Test Indicators): Designed for specific test procedures, most notably the Bowie-Dick Air Removal Test. Run daily in an empty chamber of prevacuum steam sterilizers to detect air leaks, residual air pockets, or inadequate vacuum function.
• Class 3 (Single-Variable Indicators): Responds to a single critical parameter (e.g., temperature tube).
• Class 4 (Multi-Variable Indicators): Responds to two or more critical parameters (e.g., temperature and time).
• Class 5 (Integrating Indicators): Reacts to ALL critical cycle parameters (time, temperature, steam quality). Performance correlates directly with the kill rate of biological indicators. Can be used to monitor loads not containing implantables.
• Class 6 (Emulating / Cycle-Verification Indicators): Reacts to all critical variables for a specific, defined sterilization cycle.

3. Biological Indicators (BIs)

Biological indicators are the gold standard for sterilization assurance, consisting of standardized carrier strips or vials containing high concentrations ($10^6$) of bacterial endospores highly resistant to the specific sterilization process.

Critical Implantable Load Standard

Every sterilization load containing implantable medical devices (e.g., orthopedic screws, prosthetic joints, pacemakers) MUST be monitored with a Biological Indicator along with a Class 5 integrating indicator. The implantable items MUST be quarantined and held until the BI incubation yields a verified negative result. In emergency clinical situations where an implant must be released prior to BI incubation completion, an emergency release form must be signed by the infection preventionist or sterile processing manager and documented in the recall log.

---

Event-Related Sterile Storage Principles

Healthcare facilities have transitioned from time-related shelf life (arbitrary expiration dates) to Event-Related Sterility Maintenance. Under event-related principles, a sterile item remains sterile indefinitely unless an event occurs that compromises package integrity (e.g., tearing, punctures, moisture contamination, broken seals).

Storage Environmental Parameters

• Temperature: Maintained between 68°F to 73°F (20°C to 23°C).
• Relative Humidity: Maintained between 30% to 60%.
• Airflow & Pressure: Positive air pressure relative to surrounding hallways; minimum of 10 Total Air Changes per Hour (ACH).

Physical Storage Clearances

• Floor Distance: Lowest shelf must be at least 8 to 10 inches off the floor to prevent contamination during floor cleaning. Solid bottom shelves are required.
• Wall Distance: Items must be stored at least 2 inches from exterior walls to prevent moisture condensation.
• Ceiling / Sprinkler Clearance: Items must be stored at least 18 inches below fire sprinkler heads (or 24 inches below the ceiling in rooms without sprinkler systems) to ensure unobstructed water distribution during a fire emergency.

Product Recalls & Reprocessing Failure Response

When sterilization or high-level disinfection monitors fail—or a manufacturer/internal review identifies potentially contaminated equipment, instruments, medications, or supplies—infection prevention collaborates on recall and lookback actions: quarantine affected inventory, stop use of implicated devices, notify clinical leaders, determine patient notification needs with risk management and public health when required, and document corrective actions. Domain 8 items increasingly emphasize investigating suspected or confirmed reprocessing failures, not only routine monitoring parameters.