The word “serology”, in medical terms, refers to the study of the serum of blood. In forensic serology, it extends to looking for the presence of bodily fluids, including blood. When looking at the Serological test results of various evidence we will see the FBI lab go through a series of tests to zero in on whether a particular item, in fact, has blood on it. The first thing that is done is an ALS (Alternative Light Source) visual inspection (See Understanding ALS Returns to learn more). If an item does not fluoresce under ALS visual inspection, that doesn’t mean they automatically remove it from further serological testing, instead what this means is that the ALS inspection is part of a portfolio of serological testing techniques that are combined to look at the possibility of blood (or other body fluids) on an item. We will center on the test results looking for the presence of blood.
Serological testing for the presence of blood can be viewed as a series of “presumptive tests”. A blood presumptive test is one that either rules out the presence of blood or increases the probability that blood is present. Remember, “presumption”, from a scientific standpoint, is the belief that something could be there based on probability. So, in serological testing, because there is no single definitive test that can confirm blood presence, what is done is to continue to increase the odds that the substance testing positive is blood by decreasing the number of non-blood substances it could be. This is the “portfolio” I refer to above. As each subsequent “tightening test’ is performed and returns a positive result, the odds that the substance returning a positive result is something other than blood decreases substantially. Eventually, after the series of presumptive tests are complete, “confirmatory tests” are conducted that confirm the presence of blood by excluding all other substances but blood. At that point, the presence of blood is said to be “confirmed”.
After the ALS visual inspection the next thing that is done is to perform a phenolphthalein test. In phenolphthalein testing the chemical phenolphthalein turns bright pink when it hits a small set of certain substances, including blood. You will see results for the phenolphthalein tests in tables of the discovery entitled “Serological Examination” (as on page 1539 of the 11/9/09 discovery pdf) with a column heading “Pheno”. You will note if the “Pheno” test results are NEG (i.e. negative), then no further serological testing for blood is performed. Blood has been ruled out. (I will qualify that statement by pointing out that a “false negative” is achievable on pheno testing. In other words, blood can be there that does not react with the phenolphthalein. But remember that what the forensic serologist is attempting to do is create the “portfolio” that results in a “preponderance of odds” that this is blood.)
If the “Pheno” test results are positive, the next step in serological forensic testing is to establish if the positive return is due to the presence of blood. These types of tests are called “confirmatory tests” because they are looking to confirm the presence of hemoglobin in the blood. So what they are doing now is saying “my presumptive pheno test shows a small number of substances this can be. Only one of those has hemoglobin (i.e. blood), so if it is established that hemoglobin is present, it can be confirmed that blood is present by ruling out the other substances that could have created a positive result in the pheno testing.” (For instance, one substance that can return positive on pheno testing is potato juice – but it doesn’t have hemoglobin, so you can definitively rule it out if the confirmatory test for hemoglobin returns positive.) You will see these tests for hemoglobin results listed in the Serological Examination tables under the column heading “Hemo”.
Once the presence of hemoglobin is confirmed a series of tests to determine if the blood is animal or human are performed. To weed out animal species as the origin of the blood a precipitin test is done. This is basically taking the antibodies that particular animals create when injected with human blood (think of it as the animal is trying to reject a blood that shouldn’t be there and so their bodies create antibodies to destroy the invading blood). The antiserum that the animal’s blood creates is then placed on the suspected blood sample and if clotting takes place, then the blood is human. Once the blood is confirmed to be human in origin, if the sample size permits, blood typing and DNA profiling based on certain antigens and enzymes present in the blood can be performed.
So let’s step through what was found for the serological test results released to date that showed some form of positive return:
Q23 (spare tire cover) – ALS returns both top and bottom – pheno negative
Q24 (trunk liner) – ALS returns on top – pheno negative
Q25 (trunk liner) – ALS returns on top – pheno negative
Q45 (spare tire cover) – ALS returns on top – pheno negative
Q47 (car seat) – ALS returns – pheno negative
Q81 (pieces of “Big Trouble” shirt found with remains) – no ALS returns – pheno positive – hemo “QNS” (quantity not sufficient)
Q82 (Winnie the Pooh blanket found with remains) – no ALS returns – pheno positive – hemo “QNS”
Q84 (laundry bag found with remains) – ALS testing not listed – pheno positive – hemo “QNS”
Q104 (Koala Baby blanket found near remains) – ALS return in one area - one area tested negative on pheno testing, one area tested positive on pheno testing – neither have hemo testing results listed
Q245 (rug from Anthonys’ porch) – ALS returns – pheno negative
Q248.1 (napkin from trash sack in Casey’s trunk) – ALS testing not listed – pheno “faint positive” – hemo QNS
Q248.2 (napkin from trash sack in Casey’s trunk) – ALS testing not listed – pheno “faint positive” – hemo QNS
Q248.3 (napkin from trash sack in Casey’s trunk) – ALS testing not listed – pheno “faint positive” – hemo QNS
Q248.4 (napkin from trash sack in Casey’s trunk) – ALS testing not listed – pheno “faint positive” – hemo QNS
Valhall.
Useful references:
http://www.apsu.edu/oconnort/3210/3210lect06.htm
http://www.enotes.com/forensic-science/blood-presumptive-test
http://forensic-medecine.info/forensic-serology.html
Forensic science: An introduction to scientific and investigative techniques
Related posts:
- Review: Arpad Vass’s Work
- Decomposition Gases in Casey’s Trunk
- Caylee Anthony case: Contact DNA testing
- Some Interesting Observations on today’s Discovery
- Review: Michael Burnett’s Work
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