Abstract: Provided herein are methods for quantifying foreign cell-free DNA (cfDNA) via SNP profiling of low-volume blood sample. The methods allow for monitoring the status of organ transplant rejection through analysis of small volumes of patient capillary blood samples collected non-invasively with fingersticks or other devices. The methods also allow for guiding the dosage of immunosuppressant and for preparing for a new organ transplant in case of imminent organ failure.