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This entire article is interesting....
http://www.crimelibrary.com/forensics/serology/3.htm
According to Henry C. Lee, a forensics expert who has assisted law enforcement in over 6,000 major criminal investigations—including that of O. J. Simpson---blood evidence is found most often in "crimes of violence such as homicide, assault, and sexual assault." It may be in the form of fresh liquid, coagulated, dried, or as a small drop or stain, and each form involves a different method of preservation and collection.
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There are several tests—presumptive tests used strictly for screening---that will differentiate between blood and other substances, but if other chemicals are present at the scene to which the test chemicals are sensitive, the tests may be vulnerable to corruption. For that reason, these tests are done with great care. A positive result from any of them is an indication to go ahead and use other tests to confirm.
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The Kastle-Meyer Color Test uses a solution of phenolphthalein and hydrogen peroxide on a piece of filter paper, and when blood of any quantity is present, it turns pink. However, it also turns pink in the presence of potatoes or horseradish, so care must be taken at the scene.
http://www.thesandiegochannel.com/sand/news/stories/news-152281920020620-130651.html
Three stains tested positive for the presumptive presence of blood, the criminalist testified. They were on the front right middle, the front right shoulder and the neck portion of the jacket, he said.
http://www.cnn.com/US/9609/18/simpson.trial/
The judge ordered a special hearing to determine the admissibility of presumptive blood tests. Those tests indicated the presence of blood in the drains of Simpson's bathroom sink and shower, and on the air conditioner by the guest house at his estate. Simpson's lawyers counter that since no further testing was done to prove the stains were human blood, the jurors should not hear any of it. The presumptive test results were barred at the criminal trial.
presumptive presence of blood
http://www.crime-scene.com/ecpi/serology.shtml
Phenolphthalein is one of the most commonly used tools both in the field and laboratory to indicate the presence of blood.
:Blood Tests These presumptive blood tests are designed to be a simple one-shot test for the presence of blood.
JJ i pinged u cuz I thought you might still be interested in the blood test.
Yes Kim. JJ & I hashed this out many threads ago.
The motorhome carpet was tested with the phenolphthalein reagent test. The vD home and the green jacket got the Hemostix testing, IIRC. Hemostix has an even worse false positive rate than phenolphthalein, but LE uses them because the test is simple and cheap.
A conclusive test such as precipitin anti-body or a hemochromagen (Takayama test) was not done, or at least no testimony of it being done.
The DNA is there, no doubt in my mind.
What is questionable is: Is it blood or some other substance that contains DNA ?
Or if you consider the "salting" theory, Det Ott with knapkin and ketchup pack from MickyD's ?