What will happen if a medical device is not having the desired sterility level and it is going to place in the human body as an implant? It will lead to an infection or in a worst-case scenario, causes death to the patient.
So, how we will avoid this and how will we know that the device is sterile? How do the manufacturers assure us that their devices are sterile as per the requirements? This is something looks interesting to know, right? Yeah, Sterility is considered to be a critical parameter and every manufacturer should take the necessary steps to achieve this.
- ~ First of all, what is this sterility?
It is defined as free from viable microorganisms.
- ~ Good, then what about these viable microorganisms?
It is considered the ability of microorganisms to survive and replicate under given conditions.
- ~ Is it possible to remove all the microorganisms from the product during the sterilization process?
No, and it is not possible also.
So, there are some permissible limits are defined based on the product’s site of action in the humans. To achieve these limits, we prefer suitable sterilization methods, based on the product characteristics. Thermal, Chemical, and Irradiation methods are generally considered for the sterilization of medical devices.
Once we choose a sterilization method for a particular product, then the next question is, what are the other parameters we should consider for an effective sterilization process to achieve the desired bioburden levels?
Oh ..! bioburden, what is this?
It is the number of microorganisms contaminating an object.
The sterilization process should give assurance that the product meets the desired sterility level. The effectiveness of the assessment of any sterilization process is expressed by the Sterility Assurance level.
Sterilization Assurance Level (SAL)
What is this SAL?
A sterility assurance level (SAL) is defined as the probability of a single viable microorganism occurring on an item after completion of the validated sterilization process.
The nature of microbial death in any sterilization process is described by an exponential function. And the presence of viable microorganisms on any individual product can be expressed in terms of probability. As the process will not assure zero microorganisms on the product completely then this probability can never be reduced to zero.
SAL is considered as a target for any of the following sterilization processes using heat, chemicals, radiation, or a combination of these agents. It is generally expressed as the negative logarithm to base 10, i.e., 10-x.
For example SAL of 10-6 means, the product is 99.999999 % sterile.
Sterility Assurance Level (SAL) for Medical Devices
Which SAL value should we consider for the devices to obtain CE mark or US FDA approval?
European Union harmonized standard EN 556-1 specifies that a sterility assurance level (SAL) of 10−6 or less (e.g. 10−7) should be achieved in order to designate a terminally sterilized medical device as sterile. In the USA, the American National Standard ANSI/AAMI ST67 specifies that a maximal sterility assurance level of 10−6 is required for the majority of terminally sterilized health care products.
What if the medical device is unable to withstand a terminal sterilization process to achieve a SAL of 10−6? Can we choose another SAL as per the regulatory requirements?
Another Sterility assurance level may be applicable if the device offers superior benefits or there is no alternative product available in the market. In such cases, a sterility assurance level of greater than 10−6 (e.g. 10−5) is considered through the risk assessment undertaken by the manufacturer of the medical device.
Based on the intended use there is a choice to select the appropriate sterility assurance level for medical devices.
A 10-6 SAL is used for:
- Surgically Implanted devices: Sutures, Intraocular lenses
- Devices intended to contact with breached skin or compromised tissue: Hypodermic Needles, Incise Drapes
- Sterile Fluid Pathway: Fluid Pathways of IV sets
A 10-3 SAL, is used for:
- Devices not intended to contact with breached skin or compromised tissue
- Topical Devices: Surgical drapes and gowns
SAL of 10-6, how can we achieve this level? It is based on the D value.
Oh..! what is this D value then?
D-value refers to decimal reduction time (or decimal reduction dose) and It is the time (in minutes) or radiation dose required to reduce the microbial population by 90% or 1 log cycle (i.e., to a surviving fraction of 1/10), under stated conditions (ex. Temperature).
Considering this D value and bioburden of the manufactured product, the sterilization process is programmed in such a way as to reach the desired sterility assurance level.
Significance of Sterility Assurance Level
As mentioned above, if the device is intended to be designated as sterile then it should reach a SAL of 10-6. Hence, while preparing the sterilization process of a particular medical device, the manufacturer should consider the microbial burden during the manufacturing process, material characteristics, and packaging materials, as these have an influence on the sterilization process.
When it comes to the safety of a device, we should never compromise. Right?
Can someone help me to understand the relation between the sterility assurance level and shelf-life of medical devices!!
About the Author
Mr. Manoj Meghavaram is our senior consultant in medical devices Quality Assurance & Regulatory Affairs. He is having expertise in QMS, biocompatibility, sterilization validation, and technical file preparation. He is interested to share his experience with his colleagues and well-wishers across the Globe.