Understanding the VDmax Method

How are cleanroom garments validated as sterile for use in an aseptic cleanroom using the VDmax method?

The validation protocol is specified in the ANSI/AAMI/ISO 11137-1:2006 document entitled “Sterilization of health care products – Radiation – Part 1: Requirements for development, validation and routine control of a sterilization process for medical devices” and ANSI/AAMI/ISO 11137-2:2006 “Sterilization of health care products – Radiation –Part 2: Establishing the sterilization dose.

These documents address both Method 1 and VDmax (Verification Dose Maximum) methods to determine the device bioburden and perform a verification dose experiment. Specifically, the VDmax methods described in 11137-2:2006 are for selected sterilization doses of 25 kGy and 15 kGy. The method for 25 kGy is applicable to products having an average bioburden less than or equal to 1,000 CFUs (colony forming units) per device. The method for 15 kGy is applicable to products having an average bioburden less than or equal to 1.5 CFUs per device.

Under a new AAMI document for VDmax (AAMI TIR 33:2006), sterilization doses can be validated from 15 kGy to 35 kGy in 2.5 kGy increments. The VDmax method is based on the same concept as Method 1 and is now more commonly used than Method 1 in the United States. It can be applied to any type of product that can use Method 1 and is widely accepted by the FDA.

Both gamma irradiation sterilization validation protocols begin with determining the bioburden on the sample item proportion (SIP). The SIP is generally 10% of a medium cleanroom coverall because the product is too large for the precise performance of the required tests of sterility. This would also be the justification for the gamma sterilization validation of other cleanroom items that are too large for accurate performance of the required tests.

In the VDmax25 method described in Worked Example Table 31, a minimum of 40 samples are required. Ten devices from three separate production lots are washed with other contaminated garments, dried and packaged with these garments, and are submitted for bioburden determination. It is recommended that 20 samples be prepared from each lot in the event that the variance in bioburden between lots requires subsequent dose verification of a specific lot.

The 30 samples are sent to a contract laboratory to undergo exhaustive extractions for both the bioburden and bioburden validation testing. If the bioburden per SIP is less than 30–50 CFUs per sample, another validation bioburden validation approach may be necessary. These two tests are often performed simultaneously. Each sample is extracted multiple times (usually four) and the extracts or rinsates are filtered. The filter from each extraction is placed on an agar culture medium such as tryptic soy agar and incubated allowing the total aerobic bacteria and fungi CFUs to be counted.

In the validation stage, the counts from the first extract for each sample are divided by the total from all extractions to determine the percentage or efficiency of recovery in the first extraction. A bioburden recovery factor is then calculated as the inverse of the percentage of recovery. This value is used to calculate the bioburden for each of the thirty samples and an average for each of the three lots is obtained. These numbers are later multiplied by 10, if the SIP is 10%, to determine the number of CFUs for the total device (coverall).

As long as no single lot has an average more than twice, the overall average then the verification dose experiment can use 10 samples from any of the three lots. A chart in the “ANSI/AAMI/ ISO 11137-2-2006 Sterilization of health care products – Radiation – Establishing the sterilization dose” then indicates an SIP dose reduction factor that should produce a biological reduction equal to that of the Method 1 sterilization dose.

The ten additional samples previously prepared along with the thirty samples tested for bioburden are then sent to the contract sterilizer with an instruction to irradiate at the calculated verification dose for the SIP samples. The samples are then subjected to a test of sterility and cultured in tryptic soy broth. If there are one or zero positive samples per the ten sample lot, then it can be accepted and substantiate 25 kGy as the sterilization dose. If two positives are obtained then a confirmatory verification dose test is performed. In this test, ten additional samples are irradiated and tested exactly as performed previously.

If there are zero positives in this confirmatory test, the validation passes and 25 kGy is substantiated. If any positives are obtained in the confirmatory test and no errors in testing or dosing can be confirmed, then 25 kGy is not substantiated and a higher dose must be validated (i.e., 27.5 kGy in AAMI TIR 33-2006). Additionally, at the time of validation, a one-time bacteriostasis/ fungistasis test is performed with product to assure an appropriate sterility test method was used.