I hope this topic hasn’t been covered before, but this occurred to me the other day as I was giving blood. What would be the status of the DNA that police might collect at the scene of a crime or other investigation when the person whose blood it is has just recently been given a blood transfusion. If that person bled somewhere, would their DNA show up as well as their donors, or would it just be theirs?
The first thing to realise is that red blood cells are the most numerous cells in blood and they contain no DNA. Assuming your blood is used for whole blood transfusion that leaves a fairly small number of WBCs to contribute foreign DNA.
The next thing to note is that blood cells have a short lifespan, so you’d need to recover form the accident and transfusion fairly fast to get out committing crimes.
But allowing for all that your DNA will show up in there. However it’s unlikely to be enough to confuse the test. Most samples are slightly contaminated anyway.
In the genetics lab I work for, we ask that people wait 6 months after a transfusion before submitting a sample for testing to minimize the risk of an iffy result. Everyone I’ve talked to seems to agree that that seems like a long time, but that’s our policy at the moment.