Sterility indicators are tools, used to verify that a sterilization process has been effective. Sterility indicators are used to monitor and assess the effectiveness of sterilization processes.
They are crucial in ensuring that the equipment and materials are free from viable microorganisms after undergoing a sterilization procedure.
There are several types of sterility indicators, each serving different purposes and used in various sterilization methods. Here's an overview of the most commonly used sterility indicators:
There are several types of sterility indicators, each serving different purposes and used in various sterilization methods. Here's an overview of the most commonly used sterility indicators:
Biological Indicators
Chemical Indicators
Mechanical / Physical Indicators
Enzymatic Indicators
Biological Indicators
These are the live microorganisms to test the effectiveness of a sterilization process. They are the most reliable form of sterility testing because they directly measure the ability of the sterilization process to kill specific microorganisms.
Biological indicators are “standardised” preparations of specific microorganism with known characteristics (means definite population, absolute purity an resistance characteristics). The microorganism used to prepare biological indicators are those who capable to form endospores and the microorganism is used in the spore state.
The microorganism that will be used needs to have the following abilities:
• Non pathogenic
• Genetically stable
• Highly resistant to sterilizing agent that is supposed to be used.
Incubation and culture conditions must be standardized before use, including storage conditions, and incubation and storage temperatures. Spores are commonly used because they are more durable and resistant life forms.
Biological indicators are available in many different forms which includes spore strips, spore discs usually made of borosilicate paper or stainless steel, spore suspensions, test tubes made of expansion resistance glass etc.
Purpose: To validate that the sterilization process has effectively killed microorganisms.
Method for sterility indication: These indicators are usually prepared as spore strips or vials containing the test microorganism. After sterilization, the spores are cultured to check for any growth. If no growth occurs, the sterilization process is considered effective.
Using a biological sterility indicator is crucial for confirming the effectiveness of sterilization processes. Here’s a description of step-by-step process:
1. Select the Right Indicator
Choose an appropriate biological indicator (BI) based on the sterilization method (e.g., steam, ethylene oxide, dry heat).
2. Prepare the Sterilization Cycle
Place the BI in the area of the load that is hardest to sterilize, typically the center or a location with the most packaging.
3. Run the Sterilization Cycle
Complete the sterilization process according to standard operating procedures, ensuring proper conditions (temperature, pressure, time).
4. Post-Sterilization Processing
After the cycle, carefully remove the B.I. following safety protocols. Handle it aseptically to avoid contamination.
5. Incubate the Indicator
Place the BI in a suitable incubation medium. Follow the manufacturer's instructions for incubation time and temperature.
6. Observe Results
After the incubation period, check for growth. A color change or turbidity in the medium indicates growth of spores (biological indicator) as failure of the sterilization process.
No change (means no growth of Bio. Indicator) suggests that the sterilization was effective.
Chemical Indicators
Chemical indicators are the chemicals, which provided a visual confirmation when certain set conditions of the sterilization process have been met. They do not provide direct evidence of sterility but indicate that the process has reached the necessary conditions.
Types of Chemical Indicators:
There are multiple types of chemical indicators, ranging from Type 1 to Type 6.
Type 1: Process Indicators
Type 2: Specific-Use Indicators
Type 3: Single-Variable Indicators
Type 4: Multi-Variable indicators
Type 5: Integrating Indicators (Integrators)
Type 6: Emulating Indicators (Cycle Verification Indicators)
How Chemical Indicator works:
Chemical sterility indicators are used to monitor the effectiveness of sterilization processes. They contain chemical substances that change color or appearance when exposed to specific conditions, such as temperature, pressure, or humidity, during the sterilization cycle. Here's how they work:
1. Composition: These indicators typically contain dyes or chemicals that react to the specific sterilization parameters. For instance, steam sterilization indicators might contain a pH-sensitive dye that changes color in response to heat and moisture.
2. Exposure: The indicators are placed in the sterilization chamber alongside the items being sterilized. As the sterilization process occurs, the indicators are subjected to the same conditions as the items.
3. Reaction: If the conditions meet the necessary parameters for effective sterilization (e.g., proper temperature and exposure time), the indicator will undergo a chemical change, often resulting in a color shift.
4. Interpretation: After the sterilization cycle, the indicators are checked. A color change usually signifies that the sterilization conditions were adequate. If there is no change, it may indicate that the process was insufficient.
Type 1: Process Indicators: These are used to monitor whether the sterilization process conditions (such as temperature and time) have been met. They often change colour or display a pattern when exposed to the sterilization process.
For example, autoclave tape or sterilization pouches with indicators change colour when the correct temperature and pressure are reached.
Type 1 Process chemical indicators are used to show that the item has been sterilized and are used on the outside of packages.
Type 2: Specific-Use Indicators: Defined in relevant sterilization standards for specific test procedure. These indicators contain two components: a heat-sensitive indicator which changes color when exposed to the steam and a moisture-sensitive indicator which changes color when material exposed to moist heat or vapor.
Example: Bowie-Dick test packs, Test for air removal from sterilization device and to examine proper exposure to moist heat to complete the sterilization cycle.
Type 3: Single-Variable: A single variable indicator shall be designed to react to one of the critical variables and is intended to indicate exposure to a sterilization process at a stated value (SV) of the chosen variable.
Example: Chemical pellet, Melts at a specified temperature, it demonstrates that the steam sterilization process has achieved a specified temperature.
Type 4: Multi-Variable: Two or more critical variables in the sterilization process. Exposure to a sterilization cycle at stated values (SVs) of the variables, i.e. Time, temperature and sterilant concentration are examples of critical parameters.
A multi-variable indicator shall be designed to react to two or more of the critical variables and is intended to indicate exposure to a sterilization process at the stated values (SVs) of the chosen variables.
Example: Chemical indicator tubes: Change color only when exposed to a given temperature for a specified time in a steam sterilization application. Benzoic acid (121°C) or sulfur (115°C) are examples.
Type 5: Integrators: These are more complex than process indicators and provide a measure of the cumulative exposure to sterilization conditions. They are designed to react to multiple variables (temperature, time, etc.) and are often used alongside biological indicators.
These internal Chemical Indicators are usually paper strips printed with a Chemical Indicator.
Type 6 Chemical Indicators: Emulators, or Cycle Specific Indicators
React to all critical parameters for specified sterilization cycles.
Strips placed in packs that respond to and monitor specific cycles (i.e. Prevacuum 270°F/132°C, 10-minute steam sterilization cycles).
Mechanical or Physical Indicators
Physical or Mechanical indicators are instruments used to monitor and record the physical parameters of the sterilization process, such as temperature, pressure, and time.
Purpose: To ensure that the sterilizer is operating within the specified parameters.
Examples:
o Physical Devices for temperature measurement: Monitor the temperature and pressure within the sterilizer. Eg. Thermometer, Pressure gauge, temperature probes etc.
o Data Loggers: Record and provide detailed information about the conditions during the sterilization cycle.
o Dosimeters Sterilization by radiation can be monitored by use of plastic dosimeters which can detect amount of radiation dose absorbed during the process. The radiosensitive material turns yellow to red on exposure to radiation.
o Bubble Point Pressure: used for validating the integrity of sterilization grade filter (used for filtration sterilization method).
Enzymatic Indicators
Enzymatic indicators are the enzymes that are used to determine the level of sterility. Enzyme Indicator technology can be used for any bio-decontamination cycle, especially those using gaseous hydrogen peroxide.
Enzymatic sterility indicators are tools used to assess whether a sterilization process has effectively eliminated viable microorganisms.
Enzyme Indicators (EIs) offer a range of advantages over traditional Biological Indicators (BIs). One of the major benefits is EIs’ ability to provide clear, quantifiable results in seconds rather than days.
Key Components are the enzymes: Adenylate Kinase (TK), thermostable Adenylate Kinase (tAK)
Example and Mechanism of enzyme indicators:
Due to its thermophilic origin, tAK is an extremely stable enzyme with a high tolerance to high temperature and oxidising agents. The enzyme catalyses a reaction that produces light and the amount of light produced is directly comparable to the amount of remaining tAK activity on the indicator. The light output is measured using a luminometer and provides a quantifiable numerical value.
This tAK indicators placed with the material to be sterilized. Once the sterilization cycle completes, the enzyme activity can then be quantifiably measured using a bioluminescent reaction captured by a photometer.
The enzyme catalyses a reaction that produces light and the amount of light produced is directly comparable to the amount of remaining tAK activity on the indicator. The light output is measured using a luminometer and provides a quantifiable numerical value, which generates a Relative Light Unit (RLU) Value.
The higher the Relative Light Unit (RLU) value, the more enzyme is left on the strip, whereas a lower Relative Light Unit Value indicates less enzyme is left on the indicator. The lower the value, the more effective the sterilization.
Summary
Sterility indicators are essential tools for verifying that sterilization processes are effective. They include biological indicators, which provide direct evidence of microbial death; chemical indicators, which confirm that sterilization conditions have been met; and mechanical indicators, which monitor the physical parameters of the sterilization process. Each type plays a crucial role in maintaining the sterility of medical and laboratory equipment.