We thank Drs. Gupta and Kaplan for their insightful comments. As we noted in our Research Letter, this preliminary report of the novel two-threshold algorithm had a small cohort size and limited confirmatory biopsy specimen data.1 We are currently in the process of applying this two-threshold algorithm to a larger cohort as part of a prospective, >30-site study with full biopsy matching. This study will contain more cases of biopsy specimen–proven acute rejection than the combined initial validation studies of the two clinically available donor-derived cell-free DNA (dd-cfDNA) fraction assays,2,3 addressing concerns of inadequate sample size.
We agree that the clinical utilization of diagnostic testing should be evidence driven. The two-threshold algorithm disclosed in our Research Letter did identify several rejections with elevated cfDNA and borderline dd-cfDNA results, and thus may be useful in patients with borderline dd-cfDNA fractions. As such, Natera intends to include both the dd-cfDNA fraction and the absolute dd-cfDNA quantity in our clinical reports. This will allow transplant physicians and surgeons to continue using the dd-cfDNA fraction cutoff as usual, while also having access to the two-threshold algorithm, which may prove valuable in key cases.
We are excited about the promise of biomarkers, especially dd-cfDNA, in kidney transplant surveillance, and agree that the implementation and interpretation of biomarker assays are best supported by solid clinical evidence.
S. Bunnapradist reports consultancy agreements with CareDx, Taekeda and Natera; research funding from Astellas, CareDx, Merck, Angion, Vitaeris and Natera, Angion; honoraria from BMS, Veloxis, Caredx, and Sanofi; and speakers bureau from Eurofins, TGI, Veloxis, Caredx, Natera, and Sanofi. H. Tabriziani reports current employment, consultancy agreements, ownership interest, scientific Advisor or membership, and speakers bureau with Natera; and other interests/relationships with HossMed, Inc.
1. Bunnapradist S, Homkrailas P, Ahmed E, Fehringer G, Billings P, Tabriziani H: Using both the fraction and quantity of donor-derived cell-free DNA to detect kidney allograft rejection. J Am Soc Nephrol 32: 2439–2441, 2021
2. Bloom RD, Bromberg JS, Poggio ED, Bunnapradist S, Langone AJ, Sood P, et al.; Circulating Donor-Derived Cell-Free DNA in Blood for Diagnosing Active Rejection in Kidney Transplant Recipients (DART) Study Investigators: Cell-free DNA and active rejection in kidney allografts. J Am Soc Nephrol 32: 2221–2232, 2017
3. Sigdel TK, Archila FA, Constantin T, Prins SA, Liberto J, Damm I, et al.: Optimizing detection of kidney transplant injury by assessment of donor-derived cell-free DNA via massively multiplex PCR. J Clin Med 32: 19, 2018