Studies have shown that donor DNA in blood transfusion recipients persists for a number of days, sometimes longer, but its presence is unlikely to alter genetic tests significantly. Red blood cells, the primary component in transfusions, have no nucleus and no DNA. Transfused blood does, however, host a significant amount of DNA-containing white blood cells, or leukocytes—around a billion cells per unit (roughly one pint) of blood. Even blood components that have been filtered to remove donor white cells can have millions of leukocytes per unit.

Investigators have detected donor DNA after transfusion with a process called polymerase chain reaction (PCR) that amplifies minuscule amounts of genetic material for detection and identification of specific genes. Studies using PCR to amplify male genes in female recipients of transfusions from male donors have demonstrated that donor DNA endures in recipients for up to seven days. And a study of female trauma patients receiving large transfusions showed the presence of donor leukocytes for up to a year and a half.

All these results, however, were found using very sensitive techniques whereby donor DNA was selectively amplified over the more plentiful recipient DNA. In studies where genes common to both donors and recipients were amplified, the results reflected the dominance of the transfusion recipient’s own DNA, showing the donor’s DNA to be a relatively inconsequential interloper.

Note: This question was submitted by W. McFarland, Winter Springs, Fla. and printed in the February 2009 issue of Scientific American