We now know it is possible (though rare) that a person has two totally distinct sets of DNA in their bodies. Samples from the mouth and blood could be totally different.
Should crime labs always take blood samples (rather than saliva) now when getting DNA samples-because the suspect might be a Chimera?
See recent CSI episode for a storyline on this.
This is such a rare condition that it’s really not worth the added expense to worry about it - you’d be doubling the cost of every test if you’re going to do it on two sample types for every patient/suspect. If there’s a question about a result, you can always get another sample.
I seem to remember that there have only been thirty confirmed cases of human Chimeras, over the last couple of decades. I don’t know how if there’s been an estimate about how many chimeras might be alive today, worldwide, but…I’m guessing that it’s not a large enough number to have to worry about.