I don't have time to do this subject full justice today. But interested readers can click
here and
here as a good beginning. Stefanoni herself used a confirmatory test on Rep. 199 from Filomena's window area. From the second link: "Extracts are made from stained areas of interest, and from nearby unstained areas (substrate controls). Note that the use of unstained controls is a fundamental principle in forensic immunologic testing." IMO this sort of substrate control is never a bad idea.
Other types of controls can also be
run, "When a blood presumptive test is done at a crime or accident scene, an investigator must include the use of controls to ensure the accuracy of the result. This is because a blood presumptive test can be subject to what is known as a false positive result. This is when the characteristic color reaction is produced by a sample that is in fact not blood. As well, a false negative reaction is possible, where for some reason a blood sample does not produce the characteristic color change in the indicator chemical.
"Standard procedures can rule out the possibility of a false positive or negative result. However, if these controls are not run, then the accuracy of the presumptive test can be questioned. In that case, the results would not be admissible in a court of law.
"Presumptive blood tests are commercially available in a convenient form that is easily transportable to the crime or accident scene...Other containers contain the positive and negative controls."
This
presentation from North Carolina also has some interesting material:
"Defenders must obtain the underlying notes and reports of the analyst
Defenders should raise a challenge where:
Only a presumptive test was done
The mandated combination of tests was not done
Testing protocol was not followed"
