Sterilization Methods, Indicators and Pack Storage for Veterinary Technicians

Introduction

Sterilization is the process that destroys or removes all forms of microbial life, including bacterial spores.¹ In most veterinary hospitals the technician owns this job. You clean the instruments, wrap the packs, load and run the autoclave, read the indicators, keep the logs and decide whether a pack is safe to open.

This guide covers the Spaulding categories, steam sterilization (gravity displacement and prevacuum), ethylene oxide, hydrogen peroxide gas plasma, liquid chemical agents, the indicator classes, biological indicators, autoclave loading, event-related shelf life and basic autoclave checks. Each section focuses on what you do and what you check.

Key facts

How the VTNE tests this

• Choose the method for an item: steam for heat-stable metal, low-temperature gas for heat- or moisture-sensitive items (Domain 2 Task I; K18).⁶
• Interpret an indicator: what a color-changed tape, an internal strip, an integrator or a spore test result means (K18).
• Recall cycle numbers: 121°C for 30 minutes (gravity) versus 132°C for 4 minutes (prevacuum).
• Judge a stored pack: is it wet, torn or punctured, and what do you do next.
• Recognize hazards: ethylene oxide exposure, aldehyde irritation, burns from hot instruments (K19).⁶

Core concepts

Cleaning, disinfection and sterilization

Cleaning is the removal of visible soil, such as blood, tissue and debris, usually with water and a detergent or enzymatic product.¹ It does not kill everything, but it lowers the number of organisms the next step must kill. In one classic test, killing 10 B. atrophaeus spores took 30 minutes, while killing 100,000 spores under the same conditions took 3 hours.¹

Disinfection eliminates many or all pathogenic microorganisms on objects, except bacterial spores.¹ High-level disinfection kills all microorganisms except large numbers of bacterial spores. A few liquid disinfectants kill spores if the item stays immersed long enough (3 to 12 hours). Used that way they are called chemical sterilants. Used for a short time, such as 20 minutes for 2% glutaraldehyde, the same product only achieves high-level disinfection.¹

Sterilization destroys all microbial life. Steam under pressure, dry heat, ethylene oxide, hydrogen peroxide gas plasma and liquid chemicals are the main sterilizing agents in health care.¹

Spaulding categories

The Spaulding system sorts items by infection risk, and the risk sets the level of processing.

Source: CDC Guideline for Disinfection and Sterilization in Healthcare Facilities.¹

Steam sterilization

Moist heat kills by irreversible coagulation and denaturation of enzymes and structural proteins.¹ Steam sterilization has four parameters: steam, pressure, temperature and time. Pressure is not what kills. It raises the boiling point so the steam reaches 121°C or 132°C.¹ Steam is the preferred method for any instrument not damaged by heat, steam, pressure or moisture.¹ It is also the most common method in veterinary practice.² Oils and greases cannot be steam sterilized.²

The enemy of steam is trapped air. Air pockets keep steam from touching the surfaces inside a pack.

• Gravity displacement autoclaves let steam in at the top and push the cooler, heavier air out through a drain at the bottom. Air leaves slowly from porous loads, so cycles are long.¹ These are the most common type in private practice.²
• Prevacuum (high-speed prevacuum) autoclaves use a vacuum pump to remove air from the chamber and load before steam enters. Steam penetration, even into porous packs, is nearly instant.¹
• Steam-flush pressure-pulse units remove air by alternating steam flushes and pressure pulses.¹

Low-temperature methods

Ethylene oxide is a colorless gas that is flammable and explosive. It kills by alkylation of protein, DNA and RNA.¹ Its operating ranges are a temperature of 37 to 63°C, relative humidity of 40 to 80% and exposure of 1 to 6 hours. A basic cycle takes about 2.5 hours before aeration starts.¹ Absorbent materials hold toxic EO residue, so aeration is not optional. Contact with items that still hold EO can burn patients and handlers. Plastics and rubber need much longer aeration than stainless steel.²

Hydrogen peroxide gas plasma works under deep vacuum. Radio frequency or microwave energy turns hydrogen peroxide into a plasma full of free radicals, which attack cell components.¹ It runs at 37 to 44°C, has nontoxic by-products such as water vapor and oxygen, and needs no aeration. The cycle aborts if the load is wet. It cannot process paper, linens or liquids, so packs must use compatible wraps.¹

Liquid chemical agents ("cold sterilization") such as glutaraldehyde are used for items with lenses, such as endoscopes and arthroscopes.² Remember the time rule from the core concepts: a short soak is high-level disinfection, and only the long, label-specified soak reaches sterilization. Aldehydes irritate skin and mucous membranes, so items must be rinsed thoroughly with sterile saline before use.² A cold tray on the counter that items sit in all day is a disinfection tray, not a sterile one.

Sources: CDC Guideline for Disinfection and Sterilization;¹ University of Guelph Veterinary Surgery Online.²

Monitoring: mechanical, chemical and biological

Three kinds of monitor work together. Mechanical (physical) monitors are the time, temperature and pressure readouts or printouts for each cycle. Chemical indicators change color when exposed to conditions in the cycle. Biological indicators (BIs) carry living spores, and only they measure the actual killing power of the process.¹

The CDC guideline lists five classes of chemical indicator defined by the Association for the Advancement of Medical Instrumentation, in line with the ISO 11140-1 standard:¹⁵

• Class 1, process indicator: autoclave tape on the outside of the pack. It only separates processed from unprocessed packs.⁵
• Class 2, Bowie-Dick test: checks air removal in a prevacuum sterilizer.
• Class 3, single-parameter and Class 4, multi-parameter indicators.
• Class 5, integrating indicator: reacts to time, temperature and pressure together, so it tells you most about conditions at the center of the pack.⁵

Many steam strips used in practice change from white to black once a set temperature is reached. They show that the temperature was reached at some point, not how long it lasted.³ Plasma indicators usually change from red to yellow.³

Spore tests: G. stearothermophilus (10⁵ spores) is incubated at 55 to 60°C. B. atrophaeus (10⁶ spores) is incubated at 35 to 37°C.¹ Steam and low-temperature sterilizers should be tested at least weekly. A busy sterilizer running several loads a day benefits from daily testing, which finds failures sooner.¹ Results take time, from 1 to 7 days with conventional culture.³

Procedure: running a steam cycle

1. Clean and dry. Remove all blood and debris. An ultrasonic cleaner helps clear box locks and serrations. Dry the instruments.²
2. Open and arrange. Open box locks so every surface is exposed to steam.²
3. Add an internal indicator. Place a chemical indicator inside each pack.¹
4. Wrap and label. Double wrap cloth or paper. Seal with indicator tape and write the date, contents and your initials on it.⁴³
5. Load loosely. Stand packs on edge with space between them. Place perforated trays parallel to the shelf, put basins and other solid containers on edge, place small items loosely in wire baskets, and stand peel pouches on edge in mesh racks.¹²
6. Add the BI when scheduled. Follow the BI manufacturer's directions for placement, and incubate a positive control: a BI from the same lot that was not run through the cycle.¹
7. Run and record. Select the cycle in the manufacturer's instructions. Record the date, load contents, cycle time, temperature and pressure.¹
8. Dry and cool. Let the drying phase finish. Cool steam-sterilized items before handling them, and use heat-protective gloves for hot trays.¹
9. Check before storage. Confirm the tape changed and the pack is dry and intact. If the mechanical or chemical indicators suggest a failed cycle, do not use the load.¹

Autoclave maintenance and daily checks

• A prevacuum unit runs a Bowie-Dick test each day it is used: a test pack in an empty chamber, at the front bottom shelf over the drain, run at 134°C for 3.5 minutes. A uniform color change passes. A spot of unchanged color means trapped air.¹
• If the Bowie-Dick test fails, do not use the sterilizer until it has been inspected and passes the test.¹
• Keep maintenance records with dates for preventive maintenance and repairs, alongside the BI log.¹
• Follow the manufacturer for chamber cleaning, water quality and gasket care.

Storage and event-related shelf life

Store sterile packs in a limited-access, well-ventilated area protected from dust, moisture, insects and temperature and humidity extremes. Relative humidity in sterile storage should not exceed 70%.¹ Do not crush, bend or puncture packs on the shelf.

Many facilities now use event-related shelf life: a pack remains sterile until an event contaminates it, such as a tear, wetting or a broken seal. Time-related practice puts an expiration date on each pack instead.¹ Under either system, inspect the pack before use. If it is wet, torn or punctured, repack and reprocess it.¹ Some practices still use time-based dates by wrap type. The Guelph text, for example, gives 4 weeks for double cloth and up to 1 year for paper-plastic and plastic pouches.⁴ Follow your hospital's written policy.

Flowchart: which method for this item?

Species differences

The method depends on the item, not the species. Dog and cat surgical packs, feline spay packs and equine or bovine packs all follow the same Spaulding logic, indicators and storage rules.

• Dentistry (dogs and cats): instruments that cut soft tissue or bone, such as extraction forceps, scalpel blades, periodontal scalers and surgical burs, are critical and should be sterilized after each use or discarded.¹
• Large animal and field surgery: packs for horses and cattle are often bigger and heavier. Large, dense packs slow steam penetration, so loading spacing and an internal indicator at the center matter even more. A cold tray carried in a truck gives disinfection, not sterility.
• Exotics and endoscopy: small rigid scopes and fine instruments used in birds, reptiles and small mammals are often heat sensitive. Check the device label for steam, EO or gas plasma compatibility before choosing.

Calculation: converting cycle temperatures

Safety, legal and client education

• Ethylene oxide should be treated as a known human carcinogen. The OSHA permissible exposure limit is 1.0 ppm as an 8-hour time-weighted average.¹ Never cut aeration short to get an item back faster.
• Burns: cool instruments before use. Hot instruments can burn staff and patients; air-cool them or immerse in sterile saline.¹
• Aldehydes irritate skin, eyes and airways and can cause hypersensitivity. Wear gloves, keep containers covered and rinse items fully.²
• Immediate-use (flash) steam sterilization is not for convenience or to avoid buying more instruments, and it is not recommended for implants. If an implant must be flashed, record the load, the patient and the BI result.¹
• Records: keep cycle logs, BI results and maintenance records. They show the practice can trace a failed load back to the patients who received items from it.¹
• Client education: clients may ask how instruments are kept safe. A simple answer is that every pack is cleaned, run through a monitored cycle and checked with indicators, and the sterilizer is spore tested on a schedule.

Common mistakes and exam traps

Quick review

1. Clean before any disinfection or sterilization.
2. Critical items must be sterile; semicritical need at least high-level disinfection.
3. Gravity displacement: 121°C (250°F) for 30 minutes, wrapped.
4. Prevacuum: 132°C (270°F) for 4 minutes, wrapped; Bowie-Dick test daily.
5. Moist heat kills by coagulating proteins; pressure only raises the temperature.
6. Ethylene oxide needs aeration, 8 to 12 hours at 50 to 60°C by machine.
7. Gas plasma: no paper, linen or liquid; no aeration needed.
8. Short glutaraldehyde soak is high-level disinfection, not sterilization.
9. Class 1 tape shows processing only; Class 5 integrator reacts to time, temperature and pressure.
10. Spore test at least weekly: G. stearothermophilus for steam, B. atrophaeus for EO.
11. Load loosely, on edge; cool before handling.
12. Event-related shelf life: wet, torn or punctured means reprocess.