Originally Posted by JTF
As I have explained previously, what you posted is highly misleading. The K-M test is more specific (in the statistical sense of the word) than luminol is, yet it is still only a presumptive test. Even a positive result from a KM test would not demand the conclusion that blood were present; that would require a confirmatory test. Yet the garage gave a negative K-M test. It has been known since at least 1999 that the K-M test can detect blood that has been cleaned with bleach. See attached image, which is a portion of Table 1 from the paper by Gross and coworkers that I previously cited. The first three columns refer to the phenolphthalein test and are B, W, and D, respectively. We do not know what the luminol-positive substance is, but we know what it is not.
If you wish to read up on forensic testing for the presence of blood, I suggest:
Forensic Science Reform Protecting the Innocent
Elsevier, 2017. ISBN: 978-0-12-802719-6, Edited by Wendy J. Koen and C. Michael Bowers
Chapter 8: Presumptive and Confirmatory Blood Testing, Christopher Halkides and Kim Lott.