If I committed a crime the day after having a full blood transfusion and left some of my blood at the scene of the crime, would forensic scientists be able to detect my DNA in it? Or would their analysis result in confusion? I hasten to add that it is unlikely that I would feel well enough to commit any crime after a transfusion, and I also have no intention of trying this out.
Blood has three cellular components: red blood cells, white blood cells and platelets. Only the white cells are complete cells with nuclei and DNA. These three elements are typically separated when donated blood is processed. The blood used for transfusion only contains red cells, which lost their mitochondrial and nuclear DNA during development.
Even a complete exchange transfusion would not result in the donor鈥檚 DNA being left at a crime scene. Large numbers of white cells are located outside the bloodstream. On the day after a transfusion, the recipient鈥檚 blood would pick up a considerable number of these cells. So the recipient could still be identified.
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鈥淐omplete exchange transfusion would not result in the donor鈥檚 blood being left at a crime scene鈥�
Whole blood is rarely used in transfusions. When it is, the white cells are attacked by the recipient鈥檚 immune system, making it highly unlikely that any of the donor鈥檚 DNA would remain the following day.
The only exception to this is a bone marrow transplant. The stem cells in the transplant populate the recipient鈥檚 blood with the donor鈥檚 DNA and, as their own bone marrow has failed, little or none of their DNA remains. This might indeed confuse the evidence at a hypothetical crime scene.
There is a simpler way for master criminals to throw the police off the scent. If they have blood samples from other people these could be placed at the scene. Or anonymous donor red blood cells could be mixed with DNA amplified from a hair from someone they want to frame to create a blood-like residue.
Picking up an ashtray of cigarettes stubs from a public place and leaving these at a scene would be the simplest way to create confusing DNA evidence.
Ian Flitcroft, Mater Misericordiae Hospital, Dublin, Ireland
A full blood transfusion, in which 100 per cent of the patient鈥檚 blood is replaced, is not possible. Even an exchange transfusion, like that given to some sick newborn babies, can only replace 60 per cent of the patient鈥檚 blood. So if the questioner had an exchange transfusion and left a bloodstain, it would still contain some of his DNA.
A more interesting question is whether such a bloodstain would also contain the donor鈥檚 DNA soon after a transfusion. Probably not. True, after receiving a transfusion some of the blood in a bloodstain will be from the donor, but red blood cells do not have a nucleus.
White blood cells, which do contain DNA, don鈥檛 survive when cooled to 4 掳C, the standard storage temperature for red cells. So the relatively few white blood cells present in a transfusion are almost all dead by the time the blood is given. These remnants, and any miraculously surviving white cells, are quickly recognised as foreign by the recipient and are eliminated.
The day after a transfusion, there won鈥檛 be any circulating donor white cells left, either dead or alive.
Nicolas Slater, Consultant haematologist, London, UK