Talking of that fine website devoted to a balanced, rational discussion of the Kercher case, PMF: its "co-head-honcho", Michael, wrote a response yesterday to something that I'd posted on JREF, and I thought I'd like to offer a counter-reply since I was simultaneously amused and amazed at his "logic".
My original post is here:
http://www.internationalskeptics.com/forums/showpost.php?p=6678788&postcount=22083
(It was posted back on 20th December, so I'm guessing that Michal might be a slow reader...)
And Michael's response is this:
Now, personal insults aside, Michael seems to have a fundamental misunderstanding of blood and DNA. This is a very important point in relation to this case: if Michal's answer were correct, then the DNA result from the knife might make some sense. But fortunately he's wrong, and the DNA result makes no sense.
Here's why he's wrong: he seems to be under the impression that human blood is composed of "blood cells", all of which are essentially the same as each other. But human blood is in fact a "soup" containing may different constituent elements. The two most prevalent elements are red blood cells (or erythrocytes) and white blood cells.
Now for the important "science" part:
1) White blood cells contain DNA, but red blood cells do not
2) For every white blood cell in human blood, there are around 800-1000 red blood cells
Next, remember that red blood cells are the ones needed for presumptive tests for blood (such as TMB or Luminol). And white blood cells are needed for DNA testing.
So....if the knife had Meredith's DNA on it, let's suppose for a moment that it came from her blood. Therefore the DNA would have had to come from a white blood cell (or cells). But even if there were only ONE white blood cell on the knife (unlikely to give a positive DNA result, but let's run with it in a "reductio ad absurdum" argument), then statistically there ought to have been 800-1000 red blood cells also present (to preserve the proper relative proportions of human blood). And if there were 800-1000 red blood cells on the knife (and remember, this is based on there only being ONE DNA-containing white blood cell being present), then a presumptive test for blood - which requires only around 30-50 red blood cells to produce a positive result - would have been positive.
But the blood test came back negative. Therefore the DNA on that knife did not come from a deposit of Meredith's blood on the blade.
And this is also why every medical/forensic text book says - with confidence - that (in the absence of a few very specific mitigating factors such as intense heat or fire) if a presumptive blood test on an unidentified spot or stain comes back negative, then it's not worth doing a DNA test. If there are not enough red blood cells for a positive blood test, then there are assuredly not enough white blood cells for a positive DNA test.
So, next time a person-who-believes-that-Knox_and-Sollecito-were-correctly-and-safely-convicted you happen to be debating with declares the material on the knife might have been blood despite the fact that the blood test proved negative, you now have the answer for them, all courtesy of the Great JREF Windbag himself