*Purpose: Despite the new Kidney Allocation System and Kidney Paired Donation, renal allograft candidates with a >99.9% cPRA still have extremely low rates of transplantation and thus remain candidates for desensitization (Clinical Transplantation, 2019). However, many of these candidates have very high levels of alloantibody, and reducing their cPRA to a level at which they will be able to receive compatible kidney transplants may be extremely difficult. It was shown that MFI values alone do not have good correlation with antibody strength. The goal of this study was to determine the level of alloantibody using serial dilutions.

*Methods: Using the LABscreen Single Antigen assay (One Lambda), we retrospectively studied 20 patients with cPRA 99.9% from two institutions. Using an MFI of 1000 as the cutoff for negative, we determined the 2ⁿ dilution that led to a cPRA <98%. Two serum samples at least 6 months apart were tested in triplicate, over 2 days, for inter- and intra-assay reproducibility; and to assess stability of dilution titer over time.

*Results: Reproducibility. Dilutions performed on the same sample in triplicate showed high reproducibility, with all titers within 1 dilution (40 samples performed in triplicate) at both time points. Changes in titer over time. Titers over time were generally stable with 75% (15/20) patients remaining within 1 dilution difference between the two time points analyzed. Five patients had titer changes >1 dilution over time. 2/5 showed titer increase – one pt received blood transfusion and the other received Cyclophosphamide. 3/5 showed titer decreased – one pt received belimumab, one had perforated jejunum and received a prednisone taper, one had no documented reason to explain change.

*Conclusions: Using serial dilutions, the titer needed to reduce cPRA to a “transplantable” level can be estimated with high accuracy. The assay has good reproducibility. Some patients may have changes in titer over time, mostly due to changes in immune parameters. This approach can be used as a surrogate endpoint for clinical trials aimed at determining the effect of therapy on alloantibody levels in these patients.

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