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Sterile Assurance Level (SAL) vs. Log Reduction

Sterile Assurance level (SAL) assured the effectiveness of the sterilization method. It Express, the one product item in a lot, is non-sterile after being subject to a Sterilization Method.

Sterile Assurance Level (SAL)

SAL is defined as “the probability of a single viable microorganism found in or on a product item in a lot after sterilization. If a sterilization method has a Sterility assurance level (SAL) of 10-1, which means there is a 1 in 10 chance that micro-organisms are able to survive during the sterilization process. SAL of 10–2 specifies a 1 in 100 possibilities of microorganisms surviving to the end of the sterilization process.

Sterile assurance level vs. Log reduction

SAL is used in the sterilization industry because the ultimate goal is to have the lowest probability. Generally, the Sterile assurance level (SAL) 106 is applied to achieve the sterility level.

It is important to know that SAL is not the same measurement as log reduction.

SAL measures the chances of microorganisms surviving the sterilization process.

Log Reduction

Log reduction measurements show the amount or percentage of live microbes eliminated after sterilization.

One log reduction shows a decrease in microbial levels to 10 times. It means if a product surface has 1000 microbes (CFU), after 1 log reduction, the number of microbes is reduced to 100 microbes (CFU). After 2 log reduction, it reduced to 10 microbes. If we apply 1 log reduction, It is a 90% reduction of the specific microbial population.

Remember: log reduction only indicates the percentage of microbial removal.

Percentage of microbial reduction with SAL value

1 log-90% meansSAL -10–1
2 log-99% meansSAL -10–2
3 log- 99.9% meansSAL -10–3 
4 logs- 99.99% meansSAL -10–4

Generally, SAL 106 is required to achieve the sterilization value, and this is assured by a 6-log reduction.

Calculation of D- value

The most important characteristic of biological indicators is the level of resistance. it is defined by the Decimal reduction value (D-value).

D- value is the time required to reduce the population of known microorganisms by 1 log (at a certain temperature).

Let’s understand it with an example:

For example, A organism (hypothetical) is reduced by 1 log after exposure to the temperature at 121°C for 1.5 minutes. thus the D-Value would be written as D121°C=1.5 minutes.

Related: Aseptic and Sterile in Pharmaceutical

Calculation for the time required for log reduction

To achieve sterilization value up to 6 log reduction.

1 log consumption time is 2.5×1.0= 2.5 minutes at 121°C, so for 6 log reduction 2.5×6.0 = 15 minutes. In pharmaceutical sterilization with 6 log SAL where 12 log reduction is required, sterilization time should be 2.5×12 = 30 minutes.

In Pharmaceutical companies, 106 or 6 logs are required, whereas 12 logs are also required with 2.5×12=30 minutes sterilization time.

A 10–6 SAL or a SAL providing greater assurance of sterility (i.e., 10–6, 10-7, etc.) is used for:

  • Products intended to come into contact with breached skin or compromised tissue
  • Invasive products that enter normally sterile tissue
  • Products with claims of sterile fluid pathways
  • Surgically implanted devices

A 10–3 SAL or a SAL providing greater assurance of sterility (i.e., 10–4, 10–5, etc.) is used for:

  • Products not intended to come into contact with breached skin or compromised tissue.
  • Topical products that come in contact with intact skin or mucous membranes.

When a product requires a 10–6, but it is incapable of withstanding the sterilization process, the selection of a SAL other than 10–6 may be necessary.

A different SAL can be applied when all of the following apply:

  • The product cannot be designed to allow a sterilization process that achieves a sterile assurance level (SAL) of 10–6 without adversely affecting its function and safety.
  • The product offers unique or superior benefits for patient diagnosis, treatment, or care.
  • No alternative product is available that can be sterilized with a process that achieves a 10–6 SAL.
  1. ANSI/AAMI ST67:2003. Sterilization of health care products – Requirements for products labeled “STERIE”
  2. Wikipedia
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