Quantitation of genomic DNA in plasma and serum samples: higher concentrations of genomic DNA found in serum than in plasma

BACKGROUND: Plasma and serum samples have been used to detect cell-free gen omic DNA in serum or plasma in certain pathologic conditions such as system ic lupus erythematosus, pulmonary embolism, and malignancies, as well as in fetal cell chimerisms in maternal serum and/or plasma. In this study, base line concentrations of cell-free DNA in serum and plasma samples were evalu ated for the study of posttransfusion chimerism. STUDY DESIGN AND METHODS: DNA was extracted from fresh or stored (4 degrees C for 1-6 days) normal donor serum or plasma samples (ACD; EDTA) by using r eagents from an HIV assay kit. After incubation and washing of samples, pur ified DNA was amplified with HLA DQ-a primers (GH26 and 27) or human Y-chro mosome primers (SA and SD) to quantitate the concentration of genomic DNA. RESULTS: Fresh serum samples had concentrations of cell-free DNA that were about 20-fold higher than the concentrations in fresh plasma samples. The c oncentration of cell-free genomic DNA in serum samples increased daily, to a level more than 100 times baseline after clotted blood tubes were stored at 4 degreesC for 4 to 5 days. There was a small increase in cell-free plas ma DNA in stored ACD whole blood samples. Male WBCs, spiked into fresh nona nticoagulated female blood, were lysed during the process of clotting, with male DNA liberated into the serum samples. CONCLUSION: Most cell-free DNA in serum samples is generated during the pro cess of clotting in the original collection tube. The concentration of cell -free genomic DNA in fresh plasma is probably the same as that in circulati on. Consequently, while serum samples should not be used to monitor the con centration of cell-free DNA in a patient's circulation, serum collected fro m sample tubes containing clots (i.e., without anticoagulant), 3 to 5 days after the date of phlebotomy, could be useful as a source of DNA with which to screen for posttransfusion microchimerism.