Living donor kidney exchange.

Living donor kidney exchange, also referred to as kidney paired donation (KPD), is a relatively new transplant modality that is growing by leaps and bounds in the U.S. From its first realization as an exchange of kidneys between two incompatible donor/recipient pairs, KPD has expanded to include compatible pairs, nondirected donors, three-way and larger exchanges, and living/deceased donor exchanges. Innovations both clinical (transporting organs instead of donors, and improved HLA screening) and mathematical (simulation to test policies, optimization to find better and more matches) have made this modality even more useful and accessible. There are several independent multi-center paired donation registries and many more single-center registries operating in the U.S., but incompatible pairs are most likely to match when they participate in the largest possible paired exchange pool; a single, unified KPD program in the United States would likely best serve patients in search of matches.