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Evaluation of the Efficiency of Sterilization Methods

Phenol Effects, Filter Paper Method, Use Dilution Test, Thermocouples, Brown Tubes, Bacillus Stearothermophillus Spores, Autoclave Tape

Sterilization Effectiveness Evaluation by Chemical Agents

The Phenol Effects

Phenol is used as a standard disinfectant to measure and compare the performance of other disinfectants. If a disinfectant has a phenol coefficient equal to 1, it has the same activity as phenol, but if the phenol coefficient is less than 1.0, its activity is reduced compared to phenol. In order to determine the phenol coefficient, two organisms are used, the first one being Staphylococcus aureus, and the second one being Salmonella typhi. After a certain period of time, the activity of the phenol and the disinfectant can be measured in two separate culture samples. For different microorganisms, phenol coefficients can be the same or different. Lysol is a good example of how it works. A dilution of Lysol at 1:150 does not produce growth after 10 minutes, while a dilution of phenol at 1:90 does not produce growth after 10 minutes. Lysol therefore has a 5 times higher phenol coefficient than phenol, meaning that it is 5 times more effective than phenol.

Filter Paper Method

This method involves soaking a small disk of filter paper in the disinfectant and then putting it on an agar plate containing microorganisms for testing. An inhibition zone will appear after a while. Disinfectants with a greater zone of inhibition are said to be more active for the test sample. Disinfectant activity varies with the organism being tested.

Use Dilution Test

It involves coating the stainless-steel cylinder with a broth culture of the test microorganism and allowing it to dry. Once these cylinders have been dipped in broth and the disinfectant diluted, they are then placed in test tubes. It is possible to determine the effectiveness of a disinfectant by measuring the smallest concentration at which the growth of microorganisms is inhibited.

Sterilization Control

The following methods can be used to evaluate the effectiveness of sterilization by moist heat:


An autoclave's temperature can be measured using thermocouples. A particular autoclave temperature is deemed to be the appropriate temperature for sterilization.

Brown Tubes

Along with the articles, these tubes go into the autoclave. When the temperature inside the autoclave reaches 121°, these tubes turn green, which helps determine whether the articles are properly sterilized.

Bacillus Stearothermophillus Spores

These spores must be exposed to 121°C for 12 minutes in order to be killed. Within the envelope are placed paper strips containing 106 spores before being placed in the autoclave. Inoculated with culture media, these strips are autoclaved and then inoculated. It is determined if these spores are not growing in the culture media if sterilization has been properly performed.

Autoclave Tape

In the autoclave, the product changes color when heated to 121°C due to lead carbonate.

To check whether the sterilization process has the ability to kill bacteria (microorganisms), and then to ensure that all microorganisms have been killed.

This can be measured by the following three factors:

D – value (decimal reduction time)

The D-Value indicates the time in minutes it takes to destroy 90% of viable microorganisms at a constant (defined) temperature. The less the value of D decreases (less), then the more efficient sterilization will be. This is because D value is the time required to destroy microorganisms, so less time (D value) results in killing more bacteria, so sterilization becomes very efficient.

Z – value

This represents the amount of temp. change required to decrease one point in D – value

Change in temperature is the Z-value.
  • As the temperature rises, more bacteria are killed and the D - value decreases and the Z - value increases.
  • A Z - value is proportional to the D - value, and a lower D - value will result in a more efficient sterilization.

F – value

Efficacy of a sterilization method is determined by the number of minutes it takes to kill bacterial spores through heating. It is possible to calculate the probability of survivors remaining from a load using the F value as follows:

[ F = D (log No – log N)]

No = initial population volume/original population number
N = number of final residents/volume of units
D = D - the organism's temperature at 121°C.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, 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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