*Purpose: Donor derived cell-free DNA (dd cf-DNA), this new technology allows for non-invasive monitoring of graft function. Two most commonly used assays are Prospera (Natera®, San Carlos, California) and AlloSure (CareDx®, Brisbane, California). We report our experience in comparing simultaneous use of these tests.

*Methods: A series of 15 paired-samples from individual kidney transplant recipients using Prospera and AlloSure test were included. We used the company cut off of >1% to indicate active rejection for Prospera, and 0.5% or 1% for Allosure. Acute rejections were all defined by kidney histology.

*Results: The paired-results are shown in Table 1. There were concordant results for all 15 paired-samples when using Allosure cut off level of 0.5%. A single discordant result was observed when using Allosure cut off level of 1%. Kidney biopsies were performed in seven of the cases where dd cf-DNA was elevated. Rejection was found in six of biopsy results (see in table 2). When cut off level of 1% was used, Prospera identified 80% (4/5) of T-cell mediated rejections (TCMR) compared to 60% for Allosure (3/5). Both assays recognized the single case of antibody mediated rejection. When using cut off level of 0.5% for Allosure, all 6/6 rejections were identified.

*Conclusions: There were concordant results between the two using cut off of 1% for Prospera and 0.5% for Allosure. The accuracy of the result was confirmed by the kidney biopsy (5 active rejection diagnosis + 1 non-rejection). Prospective, head-to-head trials are needed to compare the accuracy and predictive values of these two clinically available dd-cfDNA assays.

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