One criticism of kidney paired donation (KPD) is that easy-to-match candidates leave the registry quickly, thus concentrating the pool with hard-to-match sensitized and blood type O candidates. We studied candidate/donor pairs who registered with the National Kidney Registry, the largest US KPD clearinghouse, from 1/2012–6/2016. There were no changes in age, gender, BMI, race, ABO, or PRA of newly registering candidates over time, with consistent registration of hard-to-match candidates (59% type O and 38% PRA≥97%). However, there was no accumulation of type O candidates over time, presumably due to increasing numbers of non-directed type O donors. Although there was an initial accumulation of candidates with PRA≥97% (from 33% of the pool in 2012 to 43% in 2014, p=0.03), the proportion decreased to 17% by June 2016 (p<0.001). Some of this is explained by an increase in the proportion of candidates with PRA≥97% who received a deceased donor kidney transplant after the implementation of the Kidney Allocation System, from 8% of 2012 registrants to 17% of 2015 registrants (p=0.02). In this large KPD clearinghouse, increasing participation of non-directed donors and the Kidney Allocation System have lessened the accumulation of hard-to-match candidates, but highly sensitized candidates remain hard-to-match.
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