Timing of Biomarker Sample Collection During Transplant Kidney Biopsies

There has been a significant recent interest in the use of noninvasive blood and urinary biomarkers for monitoring kidney transplant recipients. One significant biomarker gaining increasing clinical use is donor-derived cell-free DNA (dd-cfDNA), an allograft injury marker. Assessing dd-cfDNA has been shown to improve the detection of rejection in addition to standard care in a larger group of kidney transplant recipients.1 Changes in gene expression often precede many biological processes; therefore, measurements of gene profiles have the potential to be very early markers of allograft rejection or the signature of a healthy graft.2 One of the blood gene transcripts investigated is a 5-gene classifier (AlloMap Kidney, CareDx) that differed significantly between quiescence and rejection.3 The 3-gene urinary cell signature consisting of CD3ε mRNA, CXCL10 mRNA, and 18S rRNA was shown to be diagnostic and prognostic for acute rejection in kidney transplant patients.4 The UroMap (CareDx) score was calculated on the basis of these 3 urinary cell signatures.5 Those biomarkers are currently being studied at the time of kidney transplant biopsies for research purposes or as clinically indicated. A previous study by Kyeso et al6 analyzed dd-cfDNA levels before, and 20 min, 2 h, and 24–48 h after transplant kidney biopsy in a small number of patients to investigate the effect of biopsy procedure on dd-cfDNA levels. The authors reported an increase in dd-cfDNA exceeding 61% from baseline in 8 of 15 patients early after the biopsy, which is normalized 24–48 h after the biopsy, suggesting some leakage to blood during a biopsy. In this issue of Transplantation, Li et al7 conducted a prospective study to investigate the impact of ultrasound-guided percutaneous core needle biopsy on urinary cell 3 gene signature score (UroMap), peripheral blood gene expression score (AlloMap Kidney), and plasma dd-cfDNA (Allosure, CareDx) in 41 biopsies from 39 patients. The biospecimens were collected within 2 h before the biopsy and within 4 h after the biopsy. The authors reported that UroMap and AlloMap Kidney scores are not affected by the biopsy procedure. Neither the purity nor the yield of total RNA is impacted significantly by the biopsy procedure. However, dd-cfDNA increases after the biopsy procedure (0.61% versus 0.26%, P = 0.004). A significant increase in absolute values of dd-cfDNA after the biopsy was also documented (median, 55 [interquartile range, 6–128] versus 41 [interquartile range, 13–30] copies/mL; P = 0.01). A similar higher postbiopsy percentage was replicated in an independent cohort of 119 postbiopsy plasma collected from 105 patients with no rejection biopsies. To normalize the biopsy-associated increase, a correction factor of –0.36% was derived by subtracting the prebiopsy dd-cfDNA percentage from the postbiopsy percent. This is an important study documenting the importance of collecting samples before the biopsy for dd-cfDNA but not UroMap and AlloMap. There are circumstances in which the biospecimens would be collected after the biopsy procedure. For example, a patient may have emptied their bladder before the biopsy or may prefer to have the blood collected only once after the biopsy when the blood is being collected to check for hematocrit. It is important to investigate whether the kidney allograft biopsy itself perturbs the isolation of RNA from urinary cells and peripheral blood cells and whether the biopsy procedure alters gene expression patterns in those RNA samples in addition to the expected possible increase in dd-cfDNA levels. The limitation of this study is that there is considerable heterogeneity in histological diagnosis with a limited number of T cell–mediated and antibody-mediated rejection samples, which precludes further analysis of different pathologies. In summary, this study suggests checking dd-cfDNA levels before the biopsy or using a correction factor if the sample is drawn after the biopsy. AlloMap Kidney and UroMap scores are not impacted by the kidney allograft biopsy.