*Purpose: The time necessary for dd-cfDNA to reach baseline levels following transplantation has been reported in kidney, heart, and liver as 14-21 days. We explored the time required for dd-cfDNA to reach baseline levels (<0.5%) after lung transplant.

*Methods: Eight patients were followed with serial dd-cfDNA measurements for the first 6 months post-transplant. Dd-cfDNA measurements (AlloSure, CareDx) were binned within time groups (week or month). The median across all timepoints within a given bin across all patients was calculated. A best-fit line was derived to define dd-cfDNA over time. Pulmonary function reflected by changes in FEV1 was measured from the time of discharge and compared to dd-cfDNA.

*Results: Dd-cfDNA was very high immediately following transplantation. The levels gradually decreased, with a median of 5.8% at 6 days post-transplant and 1.5% at 18 days post-transplant. The median level did not reliably stay below 0.5% until after day 70 post-transplant (Figure 1). Individual patient data demonstrate variability in this time course across patients (Figure 2).

*Conclusions: Lung transplant recipients have dd-cfDNA levels well above baseline for 60-90 days compared to 14-21 days reported after kidney and heart transplants. The initial high levels of dd-cfDNA do not appear to reflect injury that would influence the post-transplant course, based on the stability and/or improvement of FEV1 of these patients. The initial variation in decreasing dd-cfDNA levels to baseline among patients reflects the need for serial testing for an individualized patient monitoring approach.

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