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re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST

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Offline JGW


Joined: Sat Dec 26, 2009 4:25 pm

Posts: 5

Location: Florida, USA

PostPosted: Sun Dec 27, 2009 5:59 am   Post subject: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Just for the record, phenolphthalein is a compound that is colorless in an acid environment (pH < 7.0) and turns pink in an alkaline one (pH > 7.0). At pH 7.0 (neutral) it imparts a slightly pinkish tinge. As an indicator it's worth is only qualitative: the reaction of its environment is either acidic, alkaline (basic), or neutral. Beyond that phenolphthalein tells you nothing.

Human physiological pH is nominally 7.4 and has a very narrow range of variability under normal circumstances. Human peroxidase, like many enzymes native to humans, functions optimally at a pH of 7.4 which is alkaline. Thus when added to blood plasma or blood serum, phenolphthalein will turn pink. The pink color reaction will occur despite the presence or absence of blood, peroxidase, or any other constituent of that suspension of cells and proteinaceous fluid known as blood.

The notion that phenolpthalein is a reliable presumptive indicator for the presence of blood is problematic at best.

Carbonic anhydrase is an enzyme occurring in human red blood cells as well as in numerous other life forms. It reacts with carbonic acid (carbonated water) to produce carbon dioxide gas and water. If one were to add lysed red blood cells to club soda, seltzer, Pepsi, etc. the liquids would immediately foam and fizz as carbon dioxide was released. However, since carbonic anhydrase occurs in nature so commonly, the fact that when I add something to a carbonated liquid and carbon dioxide is released it is not a presumptive indication that human blood is present.

In Euclidean geometry, this is known as "indirect proof" and is fraught with pitfalls for those careless individuals who attempt to use this method to prove a point. Consider this very simple example of erroneous application of indirect proof.
1) All geniuses have poor handwriting.
2) I have poor handwriting.
3) Thus I am a genius!
Clearly, making a definitive inference from a general premise is a pitfall best avoided. The exact same applies to phenolpthalein as a presumptive indicator for the presence of blood.

Oh, BTW, phenolpthalein is also a very effective purgative. I do not know if it turns pink or not when used in this manner.
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Offline Fly by Night


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Joined: Fri Sep 26, 2008 1:09 pm

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PostPosted: Sun Dec 27, 2009 5:35 pm   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

JGW wrote:
In Euclidean geometry, this is known as "indirect proof" and is fraught with pitfalls for those careless individuals who attempt to use this method to prove a point. Consider this very simple example of erroneous application of indirect proof.
1) All geniuses have poor handwriting.
2) I have poor handwriting.
3) Thus I am a genius!
Clearly, making a definitive inference from a general premise is a pitfall best avoided. The exact same applies to phenolpthalein as a presumptive indicator for the presence of blood.


We're far more accustomed to having people quote William of Ockham than Euclid in trying to solve the murder of Meredith Kercher. Surely you realize that while the principles of axiomatic systems can be readily applied in some aspects of forensics work, in forensics it is also common to rely upon inferences formulated from a combination scientific principles and lengthy experience in the field. When these inferences are then backed up with a host of corroborating evidence they become galvanized nails-in-the-coffin.
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Offline Skeptical Bystander


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Joined: Fri Sep 26, 2008 5:36 pm

Posts: 7006

PostPosted: Sun Dec 27, 2009 5:56 pm   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Fly by Night wrote:
JGW wrote:
In Euclidean geometry, this is known as "indirect proof" and is fraught with pitfalls for those careless individuals who attempt to use this method to prove a point. Consider this very simple example of erroneous application of indirect proof.
1) All geniuses have poor handwriting.
2) I have poor handwriting.
3) Thus I am a genius!
Clearly, making a definitive inference from a general premise is a pitfall best avoided. The exact same applies to phenolpthalein as a presumptive indicator for the presence of blood.


We're far more accustomed to having people quote William of Ockham than Euclid in trying to solve the murder of Meredith Kercher. Surely you realize that while the principles of axiomatic systems can be readily applied in some aspects of forensics work, in forensics it is also common to rely upon inferences formulated from a combination scientific principles and lengthy experience in the field. When these inferences are then backed up with a host of corroborating evidence they become galvanized nails-in-the-coffin.


Actually, this is not a good example of indirect proof. It is simply a garden variety logical fallacy. And it is easy to fix, because the first premise would be easy to disprove. All it would take is one counterexample (i.e., a genuis with great handwriting).

I would imagine that not too many murder cases have been solved using pure inductive reasoning. Inferences, otherwise known as hunches, can result in leads, some of them promising. Murder also often contains elements that "make no sense" but which are nonetheless true.

By the way, last night CBS 48 Hours revisited the case of a guy convicted of slaying and then mutilating a woman ten years ago. The evidence against him was circumstantial. A lawyer who believed he was innocent took up his cause when it was discovered that a known sex offender lived near where the body was dumped. The original investigating officer had saved everything, including the victim's clothing. Tests were done for DNA in places where she would have been picked up and dragged. The tests excluded the young guy who was in prison for the crime, but no DNA of the sex offender was found. However, DNA on the woman's underpants and under her arms was found to belong to her ex-boyfriend. The wrongly convicted guy was released from prison, but so far no one has filed any charges against the ex-boyfriend.

I found it interesting that the DNA evidence had been preserved for so long without the contamination issue being raised. Also, the Dutch specialists who did the testing explained how vigorous the contact would have to be for DNA to be deposited on clothing or other objects. It made me think of both the knife and the bra clasp, which contained copious amounts of RS's DNA.

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Offline Greggy


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Joined: Mon Feb 09, 2009 11:10 pm

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Location: Southern USA

PostPosted: Tue Dec 29, 2009 3:35 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Phenolpthalein is an old school detection system for blood and tests that use a peroxidase-linked secondary reagent. I don't think many lab people use it in the USA for these purposes anymore due to its limitations and lack of sensitivity compared to newer methods. When I initially saw the pictures of the bathroom in the MK case, I assumed what I was looking at was blood when it was actually well-aged phenolpthalein. Phenolpthalein turns non-specifically pink in about 15-20 minutes; the specific reaction appears immediately and needs to be either photographed or xeroxed, as we often did in the old days with immunoblots. Luminol detection of blood is far more sensitive than phenolpthalein and doesn't display the short specific reaction window that phenolpthalein does. The usage of phenolpthalein by the Italian Police Technicians in the bathroom begs the question though of why did they use phenolpthalein at all? Why not just stick with a Luminol detection system? I can only suppose that the Italian Police technicians believed that DNA recovery would be better using blood bound to the phenolpthalein reagent rather than to the luminol detection reagent. Perhaps Nicki can comment on this forensic aspect.
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Offline Michael

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PostPosted: Tue Dec 29, 2009 3:49 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Hi Greggy. I think the answer is that since it was clear the killer had used the bathroom they would have a high chance of finding their DNA in there and wanted to ensure that any DNA that was left wouldn't be damaged. Therefore, they used Phenolpthalein in the bathroom which wouldn't damage any DNA instead of luminol which would. The luminol they could use in places such as the corridor, since they would have felt it unlikely that the killer would have made skin contact with the floor and so left DNA. Of course, until they laid down the luminol in the corridor they didn't realise that two of the killers had gone barefoot, but that's not something one would reasonably predict.

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Offline nicki

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Joined: Fri Sep 26, 2008 9:27 am

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PostPosted: Tue Dec 29, 2009 10:47 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Skeptical Bystander wrote:
Fly by Night wrote:
JGW wrote:
In Euclidean geometry, this is known as "indirect proof" and is fraught with pitfalls for those careless individuals who attempt to use this method to prove a point. Consider this very simple example of erroneous application of indirect proof.
1) All geniuses have poor handwriting.
2) I have poor handwriting.
3) Thus I am a genius!
Clearly, making a definitive inference from a general premise is a pitfall best avoided. The exact same applies to phenolpthalein as a presumptive indicator for the presence of blood.


We're far more accustomed to having people quote William of Ockham than Euclid in trying to solve the murder of Meredith Kercher. Surely you realize that while the principles of axiomatic systems can be readily applied in some aspects of forensics work, in forensics it is also common to rely upon inferences formulated from a combination scientific principles and lengthy experience in the field. When these inferences are then backed up with a host of corroborating evidence they become galvanized nails-in-the-coffin.


Actually, this is not a good example of indirect proof. It is simply a garden variety logical fallacy. And it is easy to fix, because the first premise would be easy to disprove. All it would take is one counterexample (i.e., a genuis with great handwriting).

I would imagine that not too many murder cases have been solved using pure inductive reasoning. Inferences, otherwise known as hunches, can result in leads, some of them promising. Murder also often contains elements that "make no sense" but which are nonetheless true.

By the way, last night CBS 48 Hours revisited the case of a guy convicted of slaying and then mutilating a woman ten years ago. The evidence against him was circumstantial. A lawyer who believed he was innocent took up his cause when it was discovered that a known sex offender lived near where the body was dumped. The original investigating officer had saved everything, including the victim's clothing. Tests were done for DNA in places where she would have been picked up and dragged. The tests excluded the young guy who was in prison for the crime, but no DNA of the sex offender was found. However, DNA on the woman's underpants and under her arms was found to belong to her ex-boyfriend. The wrongly convicted guy was released from prison, but so far no one has filed any charges against the ex-boyfriend.

I found it interesting that the DNA evidence had been preserved for so long without the contamination issue being raised. Also, the Dutch specialists who did the testing explained how vigorous the contact would have to be for DNA to be deposited on clothing or other objects. It made me think of both the knife and the bra clasp, which contained copious amounts of RS's DNA.

C'mon, don't tell me they don't know that DNA flies... Mua-)

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Offline JGW


Joined: Sat Dec 26, 2009 4:25 pm

Posts: 5

Location: Florida, USA

PostPosted: Wed Dec 30, 2009 4:21 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Nicki,
No DNA does not fly, but people cross contaminate. Do you purport to say that the lab in Rome is the only one in the world where contamination of samples could not possibly happen? Further, that dragging hair in the collection field is not a potential source of contamination only in Perugia? That moving an object around in a room can't compromise any evidence contained in/on it? I am amazed how people in this forum seem to lack any objectivity. It's like watching a replay of the events in Salem, Massachusetts Colony, in 1692. They were Brits too .......
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Offline JGW


Joined: Sat Dec 26, 2009 4:25 pm

Posts: 5

Location: Florida, USA

PostPosted: Wed Dec 30, 2009 4:33 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

Fly-By-Night:
While I am admittedly unable to quote William of Oakham, I can quote Yogi Berra: "You can observe a lot by just watching".
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Offline Fly by Night


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Joined: Fri Sep 26, 2008 1:09 pm

Posts: 1014

Location: Seattle

PostPosted: Wed Dec 30, 2009 8:02 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

JGW wrote:
Fly-By-Night:
While I am admittedly unable to quote William of Oakham, I can quote Yogi Berra: "You can observe a lot by just watching".


Yes, and we watched closely as they documented a surprisingly large sample of Raffaele Sollecito's DNA on Meredith’s sliced bra clasp, even though no one could find any other source for his DNA anywhere in the cottage, saving for a sparse sample found on a cigarette butt in the kitchen. The place was well-cleaned before the police got there. And shouldn't we have found Rudy Guede's DNA where the bra was violently cut from Meredith's lifeless body with a knife? Guede's DNA was not there. JGW, your best bet is to simply suggest that the police deliberately placed Raffaele's DNA there to frame him - the contamination theory can fly no further than DNA.

While the bra clasp could have been better handled it remains as a solid piece of evidence, as it would in any court. Lawyers argued about it as they should, but no one in that courtroom could come up with a viable explanation for such a relatively large DNA sample being found in a sealed crime scene. That kind of sample is exceedingly unlikely to have come from lab contamination – you can suggest contamination all you want but only a paid expert witness will agree with you. Raffaele was exceedingly lucky that it turned out to be such relatively "weak" evidence because he got the full and fair trial he deserved. However, even weak forensic evidence gains incredible weight when taken into consideration with the multitude of outright lies Raffaele told and admitted that he told of his own volition.

And where Raffaele went, so went Amanda.
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Offline Greggy


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Joined: Mon Feb 09, 2009 11:10 pm

Posts: 208

Location: Southern USA

PostPosted: Thu Dec 31, 2009 3:15 am   Post subject: Re: re: PHENOLPHTHALEIN PRESUMPTIVE BLOOD TEST   

The DNA evidence on the bra clasp was solid and would have stood up in any world court because there was so much DNA found on the bra clasp (i.e., nanogram amount). DNA does "fly" a little bit in the form of shed skin cells, body secretions, and shed eyelash and hair follicles, but the amounts of DNA that are usually recovered from these sources are far less than was found on the bra clasp.

We are fortunate that oleaginous RS had little experience with women and had to forcibly grasp MK's bra clasp to release it so that his DNA could be found in a generous amount and help convict him of her murder. There are some men whom, in their halcyon youth, develop a single-twist wrist technique to quickly remove a woman's bra. These rakes, often now reformed, would probably not had left enough DNA to ensure a conviction.
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