Sterility refers to the complete absence of viable microorganisms, including bacteria, fungi, and spores, on a product or in a substance.
In the pharmaceutical industry, sterility is critical for products like injectables, ophthalmic solutions, and surgical instruments to prevent infections and ensure patient safety.
Sterility testing is a key process in quality control to confirm that a product meets the required sterility standards before it is released for use.
Common Sterilization Methods
To eliminate microbial life, various physical and chemical processes are utilized:
- Heat: The most common method. Autoclaving uses moist heat and pressure (typically 121°C for 15–20 minutes). Dry heat ovens are used for heat-stable powders or glassware.
- Filtration: Used for heat-sensitive liquids. Microorganisms are physically trapped using membrane filters with microscopic pore sizes (usually 0.22 μm).
- Irradiation: Uses gamma rays or electron beams to destroy microbial DNA without generating high heat.
- Chemical Sterilization:Uses gases like ethylene oxide or hydrogen peroxide for delicate medical instruments that could be damaged by heat or moisture.
Sterility Testing
Because it is impossible to test every single item in a batch without destroying it, laboratories rely on sample testing to ensure compliance. Common testing techniques include:
- Membrane Filtration: Passing a liquid sample through a sterile filter and placing the filter into a growth medium.
- Direct Inoculation: Placing small portions of the product directly into nutrient broths, such as Fluid Thioglycollate Medium and Soybean-Casein Digest Medium.
Samples are typically incubated for up to 14 days. If the broth remains clear, the batch passes; if it becomes cloudy (turbid), it indicates microbial growth and batch failure.
Sterility Assurance Level (SAL)
In practice, achieving absolute “zero” microorganisms is nearly impossible to prove mathematically and is often too harsh for active ingredients. Therefore, microbiology and pharmaceutical industries use the Sterility Assurance Level (SAL) to define sterility practically.
*An item is considered sterile if the SAL is ≤ 10⁻⁶.
*This means there is less than a 1-in-1-million probability of a single viable microorganism surviving in or on the product
