Case Study: High Correction Factor for Bioburden. How to troubleshoot your bioburden testing?

Case:

Company X requested bioburden testing for their product (suture) which will be sterilized by EO. As is routine, the correction factor determination test using repetitive method was conducted first before bioburden testing can be performed. The results showed high correction factor (CF) value, >15.0 corresponding to a low bioburden recovery efficiency percentage. When this CF value was applied to their bioburden results, the bioburden level was higher than expected. This raised concern since this result will affect their intended EO sterilization dose.

Discussion:

According to ISO 11737-1:2018, due to the variability in product manufacturing, design, materials and configuration, it is not required that a specific bioburden recovery efficiency or bioburden correction factor be obtained.

A risk-based approach can be adopted when setting the limit for bioburden correction factor or recovery efficiency. Depending on the purpose and criticality of your bioburden data, be it for setting sterilization dose or bioburden component screening, a high bioburden correction factor may be acceptable.

However, in Company X’s situation, since the high bioburden CF value will affect their intended sterilization dose, lowering the CF value is paramount.

The first step to this problem is to identify whether the material of the product is a contributing factor. The product’s surface characteristics may influence the adherence or retention of microorganisms to the product. Extraction of microorganisms will be more difficult for high porosity products or products with rough surfaces because the microorganisms may reside in the crevices of the products.

Following this, the method of extraction may need to be altered, either by using a completely different extraction technique or using a combination of extraction techniques. For example, if the initial method was extraction by mechanical shaking for 5 minutes, the method may be further improved by adding 2 minutes of sonication. If this doesn’t improve the bioburden recovery efficiency, an altogether different extraction method can be applied. In Company X’s case, the suture was indeed highly porous and bio-absorbable, so the initial extraction method using sonication for 2 minutes was not sufficient. The method was improved by adding 2 minutes of shaking.

Another approach to consider when dealing with a complex product with different components and materials is to group the similar materials together and determine their CF value separately. This may lower the overall bioburden estimate of the product since the CF values will be applied to each group respectively instead of one CF value, which will usually be high, being applied to the whole product.

An additional investigation can also be conducted to determine whether the product has antimicrobial properties which can affect bioburden recovery efficiency. Root cause analysis can be done to identify possible contributing factors for the presence of antimicrobial effects such as preservatives, antimicrobial materials or cleaning agents throughout the manufacturing process. 

Laboratory technician conducting membrane filtration to assess bioburden, utilizing a series of filtration units under sterile conditions.

The repetitive method for bioburden recovery efficiency test will typically apply a set number of the extraction step repetitively and the recovery efficiency will be calculated by dividing the bioburden count of the first extraction with the total bioburden from all the extractions. For products with antimicrobial properties, when using the repetitive method, the first extraction will yield a lower bioburden count due to the action of the antimicrobial agent. If the antimicrobial agent can be removed by dilution, we will typically see higher bioburden counts on the subsequent extractions since the diluent will “wash” off the agent and reduce their antimicrobial effects. To overcome this and improve your CF value, a neutralizing agent must be added either to the diluent or the agar to cancel out the effects of the antimicrobial agent.

Finally, when facing out-of-trend results related to microbiological testing, it is advisable to also review the laboratory procedures and the analyst’s competence in performing said tests. If found to be lacking or nonconforming, re-training should be considered. Microbiological tests heavily rely on the experience and skills of the laboratory analysts and whether proper aseptic techniques are constantly being adhered to. Contamination and improper training may lead to inaccurate results.

Laboratory technicians carefully swab blood agar plates as part of a microbiological analysis in a clinical setting.

In conclusion, Company X’s case showed the importance of product design and understanding how each component of your product may affect the bioburden recovery efficiency. In Company X’s, the CF value was able to be reduced by adding an extra 2 minutes of shaking in the extraction method as well as re-training their laboratory analysts on proper aseptic techniques and their new bioburden test method.

Reference:

1.      Clontz, L. (2009). Microbial limit and bioburden tests: Validation approaches and global requirements (2nd ed.). CRC Press.

2.      International Organization for Standardization. (2018). ISO 11737-1:2018 Sterilization of health care products — Microbiological methods — Part 1: Determination of a population of microorganisms on products.