*Purpose: Despite an overall decline in the number of living donor kidney transplants (LDKTs) since 2005, Kidney Paired Donation (KPD) rates have increased over the past decade. Many estimates of annual KPD rates rely on SRTR data, but a recent report of the largest KPD clearinghouse in the US, the National Kidney Registry (NKR), suggested that some NKR transplants might be miscoded in the UNOS data as the initial incompatible donor-recipient relationship rather than the final KPD transplant. This suggests that KPD rates might be underestimated. The purpose of this study is to use the donor-recipient relationships in the NKR to improve national KPD estimates.

*Methods: This study used a person-level, cross-validated linkage of the SRTR and the NKR databases. We included all linked NKR transplants 2/14/2008-8/30/2018. We reported the number of miscoded donor-recipient relationships and estimated the number of KPDs correcting for these errors. We also summarized the relationship of incompatible pairs that entered the NKR system and ultimately participated in an NKR swap or chain. We defined KPD using the SRTR donor-recipient classification codes for paired donation (9), non-directed donation (10), or domino (12).

*Results: Of 2,766 NKR transplants, 482 (17%) were miscoded in the SRTR. Of those misclassified, 132 (27.4%) were coded as donors who were biologically related to the recipient, 128 (26.6%) were coded as spouses/life-partners, and the remainder used an inactive code for donor relationship or “other unrelated.” With the NKR correction, 1,024 (17.8%) of all LDKTs in 2017 were KPDs compared to uncorrected estimate of 952 (16.4%). Among incoming incompatible donors, 4% were parents of the recipient, 9% were offspring of the recipient, 7% were siblings, 4% were non-first-degree biological relations, 20% were spouses/life partners, 13% were non-directed donors, and 43% were other unrelated donors. Among those who were “other unrelated,” a plurality (N=409, 34%) were classified as a friend, with at least two donor-recipient pairs reporting that they found each other through Facebook/the Internet.

*Conclusions: At 17.8% of all LDKTs in the US in 2017, KPDs are more common than previous estimates suggested. We identified that more than 1 out of 6 KPDs in the largest national clearinghouse were miscoded in the SRTR. Most errors came from transplant center inputting the incompatible relationship instead of the compatible donor-recipient pair. This study demonstrates the value of validating national registry data with external sources, which may be particularly important for characteristics that are not vital for allocation priority, such as donor-recipient relationship. Future studies of living donors should account for this misclassification error, especially studies investigating the impact of donor-recipient relationships on post-transplant outcomes.

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