RQA GLP Committee
GLP requires that all study activities are pre-planned and documented in the study plan. SOPs are generally used for procedures that are standard i.e. Use of a balance, use of a pipette etc and supplement the study plan. In your situation fully document how you intend to conduct the study in the study plan and approve it before the study starts. If during the study you need to alter how you intended to do something, this must be done via amending the study plan, prior to doing the activity. As long as everything is documented in this way, the lack of an SOP for a procedure will be okay. The key thing is that from the study plan and any amendments, it is possible to follow how the study has been conducted and the study records confirm you have followed the plans and amendments.