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Tolerable Contact Limits for Ethylene Oxide Residual Testing

By David Simmons, Study Director over Ethylene Oxide Residual Testing, Member of the AAMI Working Group over Residuals

The use of ethylene oxide (EO) remains one of the most common forms of medical device and packaging sterilization. Its properties as a gas allow it to effectively penetrate various types of packaging and materials and subsequently be removed leaving behind a sterile product inside a sterile barrier.

In the same way that it is important to ensure effective penetration of the sterilant gas to properly sterilize the product it is also essential to ensure that the gas is judiciously removed from the product to mitigate any risk to the patient from the toxic and irritating effects of the EO gas.

Residual limits have been established in the form of a US and International consensus document ANSI/AAMI/ISO10993-7:2008. This standard recognized by the FDA, provides residual limits of both EO and Ethyl Chlorohydrin (ECH) a breakdown product of EO. Limits are provided for both the toxicity aspect of exposure to EO as well as the irritation effect. Tolerable Contact Limit or TCL is the analytical measure of the value of residual EO or ECH that may be on a device and still exhibit negligible irritation.

The TCL residual limit for EO is 10µg/cm2. This is a change from the weight-based method from the previous revision of the standard where the test required to show less than 250 parts per million to be considered non-irritating. Additionally, an ECH limit of 5 mg/cm2 was added to the standard.

Nelson Labs follows the new standard as outlined in ISO 10993-7. A simulated use aqueous extraction is used to determine residual levels. The TCL for a permanent device is determined by summing the residual values over a 72-hour period, then dividing by the area of the extracted portion of device, measured in centimeters squared. TCL for devices that are considered limited use devices or devices that have less than 24 hours of patient exposure are determined by the total residual from the determined extraction time divided by the total centimeters squared of the extracted portion of the device. This extraction time may vary anywhere from 1 hour to 24 hours.

Understanding the new TCL requirements ensures medical device manufacturers are releasing safe products following EO sterilization. Nelson Laboratories can assist manufacturers in determining the correct method to use in establishing safe residual limits in their sterilization processes.