What is Sterility Assurance?

What is the difference between 10-5 and 10-6?

SAL, or the Sterility Assurance Level, is important in any production operation where there is a threat of contamination that could affect the product (in the case of pharmaceutical manufacturing), health of a client, customer, or any subsequent user.  The SAL addresses the possibility of bacteria, viruses, spores, or other microorganisms surviving the sterilization process.

In the U.S. National Library of Medicine/National Institutes of Health (NIH) journal PubMed, the de facto acceptable levels are as follows: SAL of 10-3 is a designation which applies to low-level sterilization often employed in industry; 10-4 is considered high-level sterilization suitable for (heat-resistant) medical instruments.  The standard for injectables, or anything that enters the body anywhere other than through the digestive tract, is 10-6. This standard is also recommended for sterile cleanrooms that involve pharma compounding, consumer drug manufacturing, or highly sensitive aerospace work.

Just what do these numbers actually mean?  We’ll discover that below, and show why this applies across the Health Care industry, but also as a fundamental part of the Food and Beverage industry as well as conventional household cleaning products and generic categories.

Microbiology 101

Let’s consider an example order for 100,000 single-use units.  Presented with a SAL specification of 10-5, and a cost of 13¢ per unit, versus a 10-6 specification, costing 18¢ per unit, the difference between orders will be $500.  Why would an administrator choose the more expensive option?

Looking at the Numbers

Let’s say an unprocessed sample has been deliberately hyper-contaminated and has one million bacteria on its surface before being treated.  Assuming each step of sterilization destroys 90% of the bacteria, after one treatment, there is one order of magnitude less, so only 100,000 remain; after two steps, 10,000 remain; step three leaves 1,000; followed by 100, 10, and finally just 1, for steps 4, 5, and 6, respectively. 

Each step is called a log-reduction, and refers to a decrease by an order of magnitude, which is why each step is referred to by negative exponents; these take the form of 10-1, 10-2, 10-3, 10-4, 10-5, and 10-6.

Achieving a SAL of 10-1 does not mean the object is sterile.  It almost certainly is still contaminated because only 90% of the bacteria have been killed.  Kill 90% more of the remainder (10-2) and the likelihood is still that there is significant contamination.  It is not until 10-3 that we can begin to think of something as fairly clean for conventional (non-medical) use.

What SAL is addressing is the likelihood of survival of a viable microorganism.  A value of 10-5 suggests that only 1 in 100,000 will survive sterilization.  10-6, on the other hand, suggests that the survival rate will only be 1 in 1,000,000. 

Such a distinction becomes extremely important when you’re making sterile dressings by the millions for hospitals.  The same is true it you’re making hundreds of frozen TV dinners per minute, or bottling 100,000 units of soda pop per hour. 

SAL was created because nothing is absolutely certain to be sterile; it is impossible to guarantee 100% sterility.  By processing repeatedly, you can be confident that your odds are very, very high against it being compromised by any surviving bacteria, viruses, spores or other microorganisms.

Depending on the application, say vials for accepting blood samples, compared to squirt bottles of dish detergent, you would need 10-6 rather than 10-5 just to assure testing integrity in the vials.  For the squirt bottles you might require only 10-3, since the detergent itself is toxic to most household microorganisms.

Medical Devices

Anything designed to exist within a human body, such as a pacemaker or insulin pump, cannot accept a mere SAL 10-5 reduction.  It simply wouldn’t be safe, and such a device might even specify a 10-7 level rather than a 10-6.  An in-dwelling catheter, or cannula, should be sterilized to 10-6, just like a syringe.  Even the vials in which drugs are brought into an operating theater must be completely sterile on the outside of the container, too.

International Enviroguard’s GammGuard® CE is sterilized to 10-6.  Many competitive products are satisfied to achieve 10-5.  When it comes to the health and safety of your products, customers, or their customers, do you really want to rely on odds of 100,000:1 instead of 1,000,000:1?  Of course not!

The Takeaway

Whether sterilization takes place using thermal effects (heat), photo effects (UV energy), or nuclear effects (irradiation), each level of increasing efficacy costs energy, and well as time.  The current standard for pharmacopoeial sterilization procedures requires a probability of one viable microorganism (or less) in one million sterilized items of the final product and that means a SAL of 10-6.

Don’t settle for anything less.  It simply is not worth the risk to your clients, your customers, or your reputation!  Contact us with any questions, we're here to help.