Re: EO (Ethylene Oxide) and ECH (Ethylene Chlorohydrin) Residual as per ISO 10993-7:2
I have been informed by a laboratory in Sydney that there are quite a few organisation who release the product based on "discounting factors". Apparently, a calculation can be made depending upon the intended use and period of contact. I tried to find something like this in the standard ISO 10993-7:2008 but came across nothing. Upon my query to the Laboratory Manager, I was explained that the amount of EO residual present after 24 hour areation could be divided by the maximum number of device use. For example if the EO residual after 24 hrs exhaustive study is 50mg/device and the intended period of use is 3 hours then 50 divided by 3. Her rationale behind this is that 50mg of EO wont be tranferred into the patient because the dissipation time or retention period of EO varies due to size (Gas Chromatography).
I'm not familiar with this terminology or approach. Per Figure C, my understanding is that except for Special Situations, Categories and Devices, a device must meet the requirement for total residual content for its use-period category (as opposed to its actual use period, but based on an extraction for its use period); and for surface contact and implantable devices, the TCL limit for its amount of contact area.
Perhaps there's a published guidance that I'm not familiar with, though. What does your lab say is the source-document and section for its explanation?
4.4.6.2 says that simulated use extraction, i.e. for the actual period of use, is the reference method. 4.4.6.3.1 says that exhaustive extraction is an acceptable alternative; however, the standard specifically states that
"...because such an extraction precludes measurement of dose as a function of time, it does not ensure that the mass of residue is not delivered to the patient on the first day (...) of exposure."
It sounds to me as if the lab is doing an exhaustive extraction, then factoring it by use-exposure time as a fraction of the 24 hour Limited period, with an implied justification that the mass of residue will be delivered on a time-linear basis. That technique seems to me to be directly in conflict with the quoted statement above, which I read to state that it is impermissible to assume delivery on a time-linear basis. In particular, if a simulated use extraction for three hours (in your example) would yield a higher extracted value than the lab's method, then my judgement would be that the lab's method is non-conforming.
Also, do we have to validate the test method individually on our specific products or the validation of the test method which has been done by the lab internally is sufficient?
My interpretation is that the lab's test method validation is sufficient, but that it should have been done with test devices that are morphologically and chemically similar to, and defensibly as challenging as, your production devices. I would think they would have been aware of that expectation, and would have developed their processes using a worst-case-challenge-device approach, which they could document for you. 4.4.6.1 may apply.