Schematic representations of the specificity and resolution of various methods for measuring intracellular NAD+. (A) Methods are broadly ordered by their ability to both directly and specifically measure the free fraction of intracellular NAD+. Consideration of the entire methodology is taken into account. For example, analytical methods, such as LC-MS/MS, HPLC, and NMR, directly and very specifically measure intracellular NAD+; however, the preparation of biological samples for analysis may conflate free and bound fractions. (B) Methods are organized according to their ability to obtain dynamic measurements from the same locale, as well as by their spatial resolution. Abbreviations: LC, liquid chromatography; MRI, magnetic resonance imaging; MS, mass spectrometry; NMR, nuclear magnetic resonance.