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Sterility Indicators

Dry heat, Filtration methods, Gaseous methods, Moist heat, Witness tubes, Royce sachet, Heat sensitive tape, Chemical dosimeter, Brown's tube
A sterilized surface, item, or product is one that is totally devoid of microorganisms. Sterilization processes must be strictly controlled to ensure that sterile products do not contain viable organisms.
Controls can be divided into two types,
  • Sterility Indicators or Sterility Monitors are used to control sterilization.
  • The products are tested for sterility.
You can monitor sterilization using either of these methods:
  • Biological indicator
  • Physical indicator
  • Chemical indicator

Biological indicators

  • As the name suggests, it involves depositing a suitable microorganism on a carrier and placing it into a load.
  • After sterilization, the items are collected and examined for viable microorganism's indicative of sterilization effectiveness.
  • 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 the viability of organisms, storage conditions, and incubation and storage temperatures.
  • Spores are commonly used because they are more durable life forms.
  • There are several common processes to identify biological indicators, such as,

Sterilization process

Species used

Autoclaving at 121 degrees

Clostridium sporogenes

Bacillus stearothermophilus

Membrane filter (0.22 micrometer)

Pseudomonas diminuta.

Ionizing radiations

Bacillus pumilus

Dry heat at 160°C

Bacillus subtilis var.niger

Membrane filter (0.45 micrometer)

Serratia marcescens

Ethylene Oxide

Bacillus subtilis var.niger

Physical indicators

Various chemical monitors are used to monitor the sterilization process. These include the following chemical monitors, which vary in properties depending on the sterilization process.

Dry heat

It is possible for a Master temperature record to be prepared for a machine to serve as a reference for each batch temperature setting.

Filtration methods

In bubble point pressure testing, integrity is confirmed and pore size is determined after the process is completed.

Test parameters include:
  • After soaking the filter in an appropriate fluid, it is pressed.
  • First bubbles of air breaks away when pressure difference equals maximum pore size of the filter.
  • Air bubbles erupt on the filter surface as air pressure increases, the pressure equal to the mean pore size.

Gaseous methods

The following steps are monitored during the gaseous sterilization process:
  • During a sterilization cycle, "Temperature Probes" record elevated temperatures.
  • Seals are tested for gas leaks to ensure they are “Gas-tight”.
  • Tests are conducted to measure gas concentrations, weight under pressure, and humidity.

Moist heat

  • For each autoclave and any specific product or load configuration, the "Master Process Record (MPR)" is prepared.
  • The batch process record is then used as a reference.
  • In modern sterilization cycles, microprocessors control the cycles and transducers measure pressure.

Chemical indicators

By sterilizing the chemical monitors, the physical or chemical properties of the chemical monitors' change.
These are the chemical monitors most commonly used:

Witness tubes

  • There is a substance within these tubes that has a known melting point. Benzoic acid (121°C) or sulfur (115°C) are examples.
  • Substances melted at a certain temperature. The temperature is reached through heat.
  • To indicate the melting process visually, a dye like methylene blue is sometimes added.

Royce sachet

  • "Ethylene Oxide Sterilization" uses this chemical indicator.
  • Ink and magnesium chloride are contained in a polyethylene sachet.
  • Polyethylene bags react with ethylene oxide when penetrated by that substance.
  • By exposing the mixture to a particular concentration and time, it changes color from yellow to purple due to the formation of "Ethylene chlorohydrin".

Heat sensitive tape

  • The tape is used to identify a specific temperature during autoclaving.
  • As autoclave tape is exposed to steam autoclaves that use 121 degrees Celsius for sterilization, it changes color afterward.
  • Before the items are placed in the autoclave, small strips of the tape are applied.
  • It is similar to masking tape, but with more adhesive, so that it will adhere to the material under the hot and humid conditions of an autoclave.
  • Inks are used in such tapes that change color when heated (usually from beige to black).

Chemical dosimeter

  • For measuring radiation dose absorbed during radiation sterilization, it is the best technique available.
  • On exposure to radiation, the radiosensitive materials turn yellow to red in a plastic container.

Brown's tube

  • Heat processes commonly use this material.
  • Reaction mixtures are contained in these small tubes with indicators.
  • When a tube is exposed to a high temperature, it turns red, yellow, brown, then green, representing its specific temperature.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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