*Purpose: Living donor KTx (LDKT) provides the best option for patients with end-stage kidney disease (ESKD). ESKD patients who have an incompatible living donor still have an option to be transplanted through kidney paired exchange (KPE) programmes. In the UK KPE programme, the donor organ travels to the recipient centre. In KPE programmes where the kidney travels rather than the donor, prolonged cold ischaemia times (CIT) are seen, which may impact on transplant outcomes. This study examines the impact of CIT on LDKT outcomes in the UK, comparing KPE and non-KPE LDKT, and focusses on the effect of CIT within the KPE cohort.

*Methods: All UK LDKT between 2007 and 2018 were analysed. Data was acquired via NHSBT including patient characteristics and transplant outcomes from 27 kidney transplant centres. We compared outcomes of KPE versus non-KPE LDKT, and studied the effect of a CIT longer than four hours within the KPE LDKT.

*Results: a total of 9956 LDKT were included in our study, of which 1396 (14%) were included via the KPE programme. Compared to the non-KPE group, KPE LDKT had a significantly higher incidence of delayed graft function (DGF) (4.08% versus 6.97%, P < 0.0001), a worse 1-year graft survival (survival probability = 0.96 versus 0.98, p < 0.01) and lower graft function at 1 year (eGFR 57.90 versus 55.25, p = 0.04) and 5 year (eGFR 55.62 versus 53.09, p = 0.01). There was no difference in 5-year graft survival, but we found DGF (HR = 2.7 [1.8 - 3.9]), recipient age < 40 years (HR = 1.7 [1.4 - 2.1]) and >3 years on dialysis (HR = 1.7 [1.3 – 2.5]) as most important risk factors for significantly lower 5-year graft survival. Within the KPE-cohort, a CIT >4 hrs resulted in a significantly higher incidence of DGF (9.26% versus 4.80%, p = 0.02), and lower graft function at 1 year (55.9 versus 54.6, p = 0.03), but no difference in 1-and 5-year graft survival compared to a CIT <4 hrs. DGF (HR = 5.6[2.0 - 15.6]) and a third graft or more (HR = 5.1[1.5 - 17.3]) are risk factors for 1-year graft survival.

DGF (HR = 3.0[1.3 – 7.0]) and >2 grafts (HR = 3.3[1.3 – 8.6]) were risk factors for significantly lower 5-year graft survival.

*Conclusions: Our study shows that KPE LDKT had a higher incidence of DGF and worse 1-year graft survival and function. Within the KPE, a CIT of more than four hours does impact on DGF and graft function, but not on graft survival. We recommend examining whether including CIT in the KPE matching algorithm could positively impact on outcomes by reducing CIT. Another suggestion is to study whether machine perfusion should be employed when kidneys in KPE with long-anticipated CIT, or with more risk factors are transported to the recipient centres.

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