A Simple Method to Replace Islet Equivalents for Volume Quantification of Human Islets.

Human islets come in a variety of sizes and shapes, and the total volume of islets used for research or clinical transplants must be estimated in a manner that is simple and valid. Islet equivalent (IEQ) measurements are the standard estimate of islet volume. We published a new method (the Kansas method) for estimating rat islet volume using cell numbers that was reliable and valid. Here we modified the method for human islets. We measured the dimensions of isolated human islets showing that they are not spherical and became less so in larger islets, with an average smallest/largest diameter ratio of 0.73 in large islets and 0.85 in small islets. Human islets were individually loaded into 96-well plates, dissociated into single cells, and the total cell number per islet determined with computer-assisted cytometry. Based on the counted cell number per islet, a regression model was created to convert islet diameter to cell number with a high R(2) value (0.99). Separate regression equations for male and female donors or young and old donors were not significantly different than the pooled data and did not improve the regression values. There was an inverse correlation between the cell number per IEQ and islet size. The Kansas method was validated with ATP/cell and cell viability data. Compared to the actual cell count, conventional IEQ measurements overestimated tissue volume of large islets by nearly double. Examples of differences in results obtained from the same data sets normalized to IEQ or the Kansas method included viability and insulin secretion concentrations. The implications of the error associated with the current IEQ method of volume estimation are discussed.