RQA GLP Committee
It is unclear as to why you wouldn’t want to conduct the ADA phase to GLP. As you say, it is easy to find a facility that can conduct this analysis to GLP. If it is because you haven’t sufficiently validated the method, or are having issues with validation or because you are trying to avoid a QA audit then this is not a valid reason to conduct this work Non-GLP.
It would depend on how critical this analysis was to your study, and that you made it clear from the start of the study, but there doesn’t appear to be a good reason why your should not conduct this work to GLP if you can.
if it is part of a regulated study then it should be conducted to GLP. However, if you are still at the investigative stage as to whether or not you suspect there to be an ADA then you might consider removing the samples from the study and conducting the work as a separate study.