Dataset coding for baseline endogenous substance

I am modeling a drug that is also an endogenous substance. I have found quite a bit of information on modeling practices with endogenous substances, but nothing on how to incorporate the baseline endogenous substance value (DV) into the dataset. The data is from clinical use, and the baseline values are not drawn at exactly the same times prior to drug administration. Any assistance would be appreciated! Thank you in advance... Brady S Moffett, PharmD, MPH ______________________________________________________________________ CONFIDENTIALITY NOTICE: The information in this e-mail may be confidential and/or privileged. If you are not the intended recipient or an authorized representative of the intended recipient, you are hereby notified that any review, dissemination, or copying of this e-mail and its attachments, if any, or the information contained herein is prohibited. If you have received this e-mail in error, please immediately notify the sender by return e-mail and delete this e-mail from your computer system. Thank you. ______________________________________________________________________