Hello Jason,
Theoretically Yes, practically No.
15 pounds weight loss can slightly increase the drug distribution / body weight. But you will not see any changes because of this. Sertraline is a slow acting drug. It might take few weeks the drugs to take its full effect. Moreover the drug is eliminated via liver. So the elimination kinetics differ in individuals. It could actually vary even in individuals. So loss of 15 pounds is not concern at all.
Thank you