How ‘sterile’ are your cleanroom consumables?

The importance of validating sterility

Author: Tim Sandle

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The expectations for the sterility assurance of cleanroom consumables and single-use products has increasingly become an area of regulatory concern. Purchasers of items intended to be sterile must have confidence that the manufacturer has undertaken the sterilisation process correctly, using an established method and that the sterilisation process has been validated. Purchasers should also ensure that the actual product they are obtaining has also been subject to validation (that is, the method selected is capable of eliminating any bioburden present). The most common method of sterilisation for cleanroom consumables is gamma radiation.

What is gamma radiation?

Gamma radiation is one of the three types of natural radioactivity, the other two being alpha and beta radiation. Gamma radiation is in the form of electromagnetic rays, like X-rays or ultra-violet light, of a short (less than one-tenth of a nanometre), and thus energetic, wavelength.  Gamma rays are a form of electromagnetic radiation, whereby gamma radiation kills microorganisms by destroying cellular nucleic acid(1). The main reason for why gamma radiation is selected as a sterilisation method is due to its relatively high penetrability and as there is only a small temperature rise (typically less than 5oC) associated with its use. This means that the technology is suitable for sterilising heat-liable and heat-sensitive products, which could not be processed by steam sterilisation.

 

How gamma radiation works

Gamma radiation is a physical means of sterilisation or decontamination as the rays pass through the product being sterilised (or ‘irradiated’). In doing so, gamma radiation kills bacteria, where there is sufficient energy, at the molecular level by breaking down bacterial DNA and inhibiting bacterial division(2).

The most common source of gamma rays for radiation processing comes from the radioactive isotope Cobalt 60, which decays at a specific rate and gives off energy in the form of gamma rays and other particles. The gamma process does not create residuals or impart radioactivity in processed products(3). The important variables for gamma radiation are the strength of the radiation dose and the exposure time. The measurement of radiation is expressed in units called kiloGrays (kGy)(4).

 

Dotch-Ds

Importance of validation

The gamma sterilisation process involves the product being placed into special containers either totes (constructed from aluminium) or by full pallet depending on the lay out of the sterilization facility. The amount of product that can go into a tote or on the pallet is established during validation.

 

For process validation, the following must be assessed:

  1. Material compatibility. Since ionising radiation generates free radicals in plastic polymers leading to degradation from chain scission (changes in molecular weight) or alterations to cross-linking, potential radiation effects on some materials include embrittlement (change to material hardness), discoloration (often yellowing caused by surface oxidation), unpleasant odour (from volatile material formed by reactions from within the polymers), or lack of functionality due to a compromised physical trait, such as tensile strength. An assessment must also be made with the outer packaging, ensuring that this does not become damaged through the irradiation process.
    The maximum radiation dose and time must be configured for each individual product. Then it must be demonstrated that the material meets the specifications after radiation both initially and across the shelf-life of the material.
  1. Sterilisation validation. The aim of this validation step is to determine the dose required to achieve a sterility assurance of 1 x 10-6. For this exercise it is necessary to understand the minimum dose that will achieve sterilisation and the maximum dose to avoid damaging the consumables that are to be sterilised.

    The international standard for radiation sterilisation is ISO 11137(5). According to the standard, to ensure consistent sterility assurance, lot-to-lot variability of bioburden must be known and controlled. As part of the validation, data needs to be collected and analysed for each batch of raw material, intermediate, or product to ensure process control. Post-sterilisation, a sterility test is required.

    This assessment involves the following steps(6):
    1. Determination of the bioburden of the product. This is normally undertaken using ten units per batch and from three different batches of product (10). There are different methods for bioburden determination. One of the most common methods is the Repetitive (Exhaustive) Recovery Method. This method involves washing the sample product repeatedly with sterile diluent and testing the eluent from the washing using a total viable count (TVC) test method.
      Variations to the bioburden method are permitted within the ISO 11137 standard, such as the VDmax25, which permits fewer units of product to be tested and for similar items to be grouped together for the validation (a matrix approach). This variation can only be used where it has been established that the bioburden level is relatively low (at less than 1,000 CFU).

    2. The calculation of the verification dose based on the resistance of an identified microbial population (this is based on the total number of bacteria and fungi and the types of species recovered, as characterised using microbiological identification techniques). The species recovered should be compared with species known to have some resistance to radiation. Some bacteria are relatively more resistant than others to gamma radiation (most notably Streptococcus faecium and Deinococcus radiodurans). The verification dose can be assigned using the table of standard resistance as set out in the ISO 11137 standard).

    3. The calculated dose is verified using an appropriate sample size (e.g, 10 units of the product in the VDmax25) to determine if the dose is efficacious via a sterility test.

    4. A further important aspect of the validation is dose mapping. For this, the product, in its final packaging configuration, is profiled in order to identify the high and low zones of absorbed dose in the product load in relation to the energy field it travels through. The mapping process also establishes the sterilisation cycle time. The level of radiation is assessed using dosimeters. It is important to assess the number of dosimeters required to assess the radiation dose.
      The key parameters for the assessment are: Product weight and volume, dimensions of packaging components and density, and the configuration of the packaging components. With the dimensions, it is important that the product is evenly distributed because the radiation dose is applied at the same level from both sides.

The object of the validation is to set processing parameters and the product release specification. The validation parameters are established through a performance qualification (dose mappings), which is typically run three times using the maximum packaging size.

 

Performance monitoring

Once the validation has been established, it is important that the components in their packaging continue to be stacked as per the arrangement carried out during the validation. Hence, once the validation has been established it is important that the composition of the pallet stays the same as in the validation.

This is necessary because if the orientation alters, then this can cause changes to the density mix and thus the effectiveness of the irradiation. Once the validation parameters are established, they are adopted for routine processing. This includes controlling time and dose, with sterilisation measured using suitable indicators. No parameter permitted to vary by more than ten percent from the measures recorded during the validation(7).

Re-validation

Once established, it is prudent to re-assess the validation parameters on periodic basis, established by risk assessment. This includes the supplier assessing bioburden of the product quarterly. This is necessary to determine that the gamma radiation process remains effective, and a regular assessment of representative product is important as the microbial numbers and species may vary. Those involved in the selection of sterile consumables should check with the supplier that regular bioburden assessments are being conducted.

It is also necessary that any future changes in product, its package, or the presentation of product for sterilisation are assessed to ensure that the sterilisation process remains effective (7).

 

Regulatory concerns

The importance of undertaking correct sterilisation for cleanroom consumables and single-use, sterile devices is indicated from recent recall notifications and warning letters. Many of these relate to either the absence of validation being conducted, validation not being conducted according to the international standard, or the failure to execute revalidation in the context of a process parameter having changed. The most common finding is not executing radiation according to the validated load plan. The second most important finding is damage to either the consumable or the outer packaging from the radiation process(8-11).

Further examples can be reviewed by searching the FDA medical devices recall data base at: https://www.fda.gov/drugs/enforcement-activities-fda/warning-letters-and-notice-violation-letters-pharmaceutical-companies

 

Summary

This article has looked at the sterility assurance of cleanroom consumables and single-use items and the importance of validation, process controls, and revalidation. Failure to follow any aspect of these leads to the risk of using a non-sterile or damaged item. When selecting cleanroom consumables and sterile devices, cleanroom managers should assess whether the sterilisation has been performed correctly by the supplier, that it is controlled, and that the process is periodically reassessed (particularly product bioburden assessments).

 

 

 


References
  1. Holmes, S. (2021) An overview of current surgical instrument and other medical device decontamination practices. In Walker, J.(Ed.) Decontamination in Hospitals and Healthcare, Elsevier, Oxford. pp443-482
  2. Sandle, T. and Saghee, M. R. (2011): Some considerations for the implementation of disposable technology and single-use systems in biopharmaceuticals, Journal of Commercial Biotechnology, Vol. 17, No. 4: 319–329
  3. Dendy, P. P.; B. Heaton (1999). Physics for Diagnostic Radiology. USA: CRC Press
  4. Vértes, A., Nagy, S. and Klencsár, Z. (2003) Handbook of nuclear chemistry, Volume 3. Amsterdam: Kluwer Academic Publishers
  5. ANSI/AAMI/ISO 11137-1: 2006. Sterilisation of health care products-Radiation-Part 1: Requirements for development, validation, and routine control of a sterilisation process for medical devices
  6. Sandle, T. (2012). Utilisation de la gamma, Salles Propes, Issue 81, pp42-54
  7. Sandle, T. (2013). Sterility, Sterilisation and Sterility Assurance for Pharmaceuticals: Technology, Validation and Current Regulations, Woodhead Publishing Ltd.: Cambridge, UK (ISBN 1 907568 38 7)
  8. Potential failure of process to effectively sterilise items -
    https://www.gov.uk/drug-device-alerts/national-patient-safety-alert-supply-disruption-of-sterile-infusion-sets-and-connectors-manufactured-by-becton-dickinson-bd-natpsa-slash-2021-slash-001-slash-mhra
  9. Radiation dose inappropriate. Incorrect load pattern -
    https://www.accessdata.fda.gov/scripts/ires/?Product=183727
    https://www.accessdata.fda.gov/scripts/ires/?Product=183681
  10. Product degradation following gamma radiation -
    https://www.accessdata.fda.gov/scripts/ires/?Product=183728
  11. Gamma sterilisation validation testing indicated the sterilisation dose for the product was not adequate -
    https://www.accessdata.fda.gov/scripts/ires/?Product=161789