Well, it depends. Ardent Sensor may not give full CD, but Karma gets really low cd on her abilities naturally. The Ardent Censor is good because it makes Karma's shields and heals stronger. Nashor's tooth just gives AP and more CD. So, do you want to deal a tad more damage or have more sustain and protection? Another good item would be the Unholy Grail. It gives similar things of the Ardent, but more cd. Karma's prowness in lane comes from her ability to heal and shield up damage from these items. Just my pov.

would nashors tooth be a good replacement for ardent sensor, cause it gives that max CDR and has overall more AP, and its passive allows you to alot of burst dps with your auto. What do you think?