My NAD+ assay is giving inconsistent results when repeated-do you have any suggestions as to why?

Cell lines vary in their requirements. You may need to adjust your preparation or protocol to reflect these variations. Liver cells, for example, often require more care due to high enzyme levels. 

Confirm that you are using an appropriate protocol for your experimental requirements. Manufacturer recommendations may need to be adjusted for your needs.

Quality-tested assay kits may be an easy solution if you are using an in-house procedure or a kit that you haven't previously tried.