Aim: The US Kidney Allocation System successfully exposes highly sensitized patients (pts) with a cPRA of 99 or 100% to deceased donor offers through regional and national sharing. Frequently a positive physical cross match (PXM) occurs while a kidney is being transported, requiring subsequent reallocation. It was estimated that 19.2% of kidneys transported between regions are eventually transplanted (tx) into an unintended recipient(UR), most commonly due to a positive crossmatch (AJT 2017, 17:2139).  Using a virtual cross match (VXM) strategy, we aimed to tx highly sensitized pts with kidneys from other regions, without reallocating to an UR.
Methods: We developed a VXM strategy to avoid a positive PXM.  Our VXM method includes 3 integral processes: accurate identification of HLA antibodies (Ab) using right tools and scientific principles on quarterly samples, assessment of correlation between the strength of donor specific HLA Ab (DSA) and the level of T/B cell bindings by flow XM (PXM), and listing of clinically relevant unacceptable antigens that will not impact donor offers or graft survival. When a kidney is offered a VXM is performed, and likelihood of positive PXM is estimated by the HLA lab director. If there is concern due to recent transfusions or infections treated with antibiotics a PXM is done when the pt arrives for tx, but is only required in a minority of cases. Records were reviewed of all kidneys shipped from another region since Dec, 2014 to identify kidneys not used in the intended recipient from Dec, 2014 to Sept, 2017.
Results: 252 kidneys were shipped to our center from another region based on the negative VXM (Fig 1). Of these imported kidneys 210 (83.3%) were transplanted without a pre-tx PXM, and the remaining 42(16.7%) required a PXM on admission, most commonly necessitated by a recent blood transfusion. 109 pts with 100% cPRA had kidneys imported, and 93 (85.3%) were transplanted after VXM alone. 7 kidneys went to UR, but only 3 (1.2%) were due to a pos PXM or new DSA from recent blood transfusion. The other 4 kidneys went to UR because the intended recipient had a new medical problem upon admission. Mean creatinine at 1-year was 1.30 for pts at highest immunologic risk(cPRA 100%), and 1.31(p=NS) for all other DDRT recipients. VXM alone was used in 114(86.3%) of females, and 92(79%) of re-transplant pts. Consistent with the neg VXM, retrospective PXM performed by flow cytometry for all recipients were negative (Fig 2).
Conclusions: Sharing is possible, and late reallocations can be almost entirely avoided with a strategy that relies heavily on VXM, without limiting access for highly sensitized pts on the waitlist. A centralized VXM within the allocation system would reduce transplantation into unintended recipients significantly. More frequent antibody screening, especially following blood transfusions and infections would further reduce the need for a pre-tx PXM.