Real-time, accurate assessment of islet viability is critical for avoiding transplantation of nontherapeutic preparations. Measurements of the intracellular ADP/ATP ratio have been recently proposed as useful prospective estimates of islet cell viability and potency. However, dead cells may be rapidly depleted of both ATP and ADP, which would render the ratio incapable of accounting for dead cells. Since the DNA of dead cells is expected to remain stable over prolonged periods of time (days), we hypothesized that use of the ATP/DNA ratio would take into account dead cells and may be a better indicator of islet cell viability than the ADP/ATP ratio. We tested this hypothesis using mixtures of healthy and lethally heat-treated (HT) rat insulinoma cells and human islets. Measurements of ATP/DNA and ADP/ATP from the known mixtures of healthy and HT cells and islets were used to evaluate how well these parameters correlated with viability. The results indicated that ATP and ADP were rapidly (within 1 hour) depleted in HT cells. The fraction of HT cells in a mixture correlated linearly with the ATP/DNA ratio, whereas the ADP/ADP ratio was highly scattered, remaining effectively unchanged. Despite similar limitations in both ADP/ADP and ATP/ DNA ratios, in that ATP levels may fluctuate significantly and reversibly with metabolic stress, the results indicated that ATP/DNA was a better measure of islet viability than the ADP/ATP ratio.Islet cell transplantation is emerging as a promising therapy for the treatment of type 1 diabetes. [1][2][3][4][5] Despite recent advances, the transplantation of islets poses a unique challenge with respect to achieving a consistent clinical outcome. Part of this challenge is being able to reliably and rapidly assess clinical islet quality through the quantification of viability and function before transplantation. Current viability assays are limited in their ability to accurately predict transplantation outcomes in vivo. 6 Consequently, to improve the clinical islet transplantation outcomes, it is imperative to develop more accurate viability assays.A proposed method to assess islet viability and potency before transplantation is quantification of the ADP/ATP ratio. 6 -10 This ratio has been specifically applied in discriminating islet preparations that are suitable for clinical transplantation from those that are not. 6 However, this application may be problematic under certain conditions. Intracellular ADP and ATP levels fluctuate rapidly because these high-energy phosphate molecules are rapidly produced and consumed in many intracellular biochemical reactions. It is widely known that viable cells turn over entire ATP stores on the order of minutes, and that dead cells are incapable of replenishing To better account for dead cells and to more accurately assess the viability of an islet preparation, we suggest the use of an ATP/DNA ratio. DNA does not degrade as rapidly as ADP or ATP in a dead cell. Therefore, using direct measurements of DNA, dead cells in an islet preparation...