Does a blood donation mess up DNA evidence?

SDStaff Hawk replies:

This question just goes to show what happens when you watch too much Court TV. More educational than MTV, but, man, you’ve opened a can of worms.

First of all, the answer to your question is, “No, a person who received a whole blood transfusion would not display detectable amounts of the donor’s DNA profile.” Due to certain limitations, whole blood transfusions are rare; most blood transfusions today are done with what’s called packed red blood cells, a.k.a. erythrocytes, which perform the function of transporting oxygen and carbon dioxide to and from the cells of the body. Erythrocytes contain no DNA, however, having lost their nuclei during maturation. Hope you were paying attention, ‘cuz we’ll revisit this later.

Packed red blood cells are just that; it contains almost nothing else. This is different from whole blood, per se, which contains other components, such as serum, white blood cells, platelets, and antibodies. White blood cells, a.k.a. leukocytes, contain nuclei and are the agents that provide a DNA profile from blood. In whole blood transfusions, the whole soup is transfused from one person to another.

Be that as it may, in actual observation, the DNA profile of a blood donor has not been detected in the peripheral blood of a recipient, regardless of how much was transfused. In one study, a woman was transfused with 14 units of blood (four whole, ten packed red cells), while a man received 13 units (four whole, nine packed red cells). In both cases, neither individual had detectable levels of the foreign DNA profile, despite being tested as late as the following day. Since the average man has ten to twelve pints of blood and the average woman has eight to ten pints, you’d think there’d be something detectable, despite a complete hematological overhaul-and-then-some.

What would be even more interesting is not whole blood transfusions, but a bone marrow transplant. In the marrow of certain bones, cells called hematocytoblasts sometimes develop into rubriblasts, which eventually transform into red blood cells. Those same hematocytoblasts can also develop into white blood cells or platelets. And there lies the problem.

If a person receives a bone marrow transplant, they receive the machinery to produce, literally, someone else’s blood. Now, remember *why* a person needs a bone marrow transplant: because their own bone marrow is failing. So, on the one hand, the patient is not producing his or her own cellular elements, yet, on the other hand, the patient is producing somebody else’s. Such a person would have almost no DNA in their blood matching the DNA from other cells of the body, like spermatozoa (a standard sample in forensic cases involving rape) and saliva (typical in cases where the evidence includes an envelope that was licked sealed). This is especially disturbing, since most reference samples brought to the crime lab are blood. An erroneous conclusion could be reached because everyone assumes that the reference blood contains the person’s DNA.

Now, this isn’t to say that if a person receives a transfusion that nothing is detectable. There was a time that crime labs did without DNA analysis, where we relied on polymorphic enzymes. In those days (the 1970’s), isoenzymes were easily detected in individuals who received a transfusion. You just waited a few months for the effect to wear off and while hoping that the recipient (usually the victim of a crime) didn’t die in the meantime.

SDStaff Hawk, Straight Dope Science Advisory Board

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