Does Sherlock Holmes' Test For Blood Mentioned in A Study in Scarlet Really Exist?
by Roopali Arora
(New Delhi, India)
Question by Roopali From New Delhi, India
Is there really a chemical test named after Sherlock Holmes?
There is this scene in the Sherlock Holmes novel 'A Study in Scarlet', in which Sherlock Holmes claims to have discovered the chemical test that works perfectly for the detection of blood in a sample.
He says he's found the compound that gets precipitated with haemoglobin only. He suggests that this test be called the 'Sherlock Holmes' test.'
Does this test really exist today?
Is it possible that Joseph Bell - the person Doyle was inspired by when writing the series - had actually discovered the test? If not, then did anyone else discover this blood test later - after Sherlock Holmes mentioned it in 'A Study in Scarlet'?
So, what you're asking is: Is there a real
test to detect blood like the one Sherlock Holmes discovers in A Study in Scarlet
? The answer is:yes and no, both!
Let's look at exactly what Sherlock Holmes says about his blood test:
"I have found a re-agent which is precipitated by haemoglobin, and by nothing else."
Holmes is saying that he's discovered a chemical that is precipitated only
by haemoglobin - a vital part of human blood. If precipitation occurs, the sample must be blood. If there's no precipitation, it's not blood. Absolutely
no doubt about it.Is there such a real reagent in the world that is precipitated only by hameoglobin?
There is no reagent on the Earth that forms precipitates only
with haemoglobin. So what Sherlock Holmes discovers in A Study in Scarlet does not exist.
However, is there a single test today that can confirm with an almost
100% guarantee that a sample is blood?
Yes. There are such tests.
But, remember how Sherlock Holmes performs the test in A Study in Scarlet
...he threw into the vessel a few white crystals, and then added some drops of a transparent fluid. In an instant the contents assumed a dull mahogany colour, and a brownish dust was precipitated to the bottom of the glass jar.
does not happen in any test that guarantees
the presence of blood.
How Do Forensic Experts Test For Blood Today?
There are generally two types of tests that forensic scientists perform: presumptive
blood tests and confirmatory
Presumptive tests tell you that a sample is probably
blood. Confirmatory tests confirm
that a sample is blood - guaranteed.Why do they need two tests?
Can't they perform the confirmatory one directly?
They can and they do sometimes. But, confirmatory tests have traditionally been pretty expensive compared to the presumptive ones.
The Guiacum test - that Sherlock Holmes mentions in A Study in Scarlet
- was one of the first important presumptive tests.
It was discovered in 1861-62. As Sherlock Holmes puts it:
"The old Guiacum test was very clumsy and uncertain."
Here's a picture of the Guaiacum plant by Forest and Kim Starr:
Just like other presumptive tests, it basically...left you hanging.
A sample could probably be
blood. Or maybe not.
It involved a colour change in the presence of haemoglobin. This means, if haemoglobin was there, a guiacum-compound would react with hydrogen peroxide and change its colour. If haemoglobin was not there, no colour change.
But: there were no precipitates in the Guiacum test.
Just like the Guiacum test, scientists discovered plenty of other presumptive
tests after A Study in Scarlet
(1887) - all involving a colour change with haemoglobin.
Here's a picture:
The Kastle-Meyer test is still
used to detect the presence of blood. Again, the reaction is similar: haemoglobin helps
phenolphthalein react with hydrogen peroxide. If it's blood, phenolphthalein's colour changes to pink.But there are no precipitates.
And there's no guarantee either that a sample definitely
is blood if it passes the Kastle-Meyer test.
Forensic experts also use luminol (1936) and fluorescein (late 90s) sprays to detect
blood if they can't find blood stains, but suspect that there are stains hidden. These chemicals react with haemoglobin to form substances that shine!
Luminol is a top favourite of all those detectives on TV - you just spray the thing and all the blood
in the surroundings lights up!
Here's an interesting picture (source:United Nuclear
) that shows what luminol can do to the crime scene:
Confirmatory Blood Tests
Just now, before I rambled on and on about all these presumptive
blood tests, I said that there were confirmatory
tests which guarantee
that a sample is blood. Just like Sherlock Holmes' test does.What are these tests?
In 1901, an amazing guy called Paul Uhlenhuth
discovered a mind-blowing
test that not just guaranteed
the presence of blood - it even freakin' guaranteed the presence of human
This test involves precipitates and it's even called the precipitin
test.So, in a way this test is the most similar to Holmes' test - because at least there are precipitates.
BUT - there's no reaction with haemoglobin here. In fact, in the preciptin test, the antigens
present in human blood react with human antibodies
that are added to them - to form white precipitates. There's another confirmation test for blood called the RSID test that's becoming quite popular these days.
This test is uber-cool
because it is extremely
easy to perform. In as little as 10 minutes you get to know with almost 100% accuracy if:
- a sample is blood and...
- if it is human
This RSID thingy detects an antigen called Glycophorin A in the blood (and not haemoglobin).
As I said, it's super-easy to perform: there's a strip and when you add the right blend of reagents to it, there are two lines formed on it - if the sample is blood.
There are also two sort-of
confirmatory tests that aren't that popular any more - the Teichman test and the Takayama test (1912).
These tests involve the formation of crystals by heating dried blood with either glacial acetic acid or pyridine.
These tests aren't used these days because they're not 'that' sure-shot.
So, What's the Final, Final Deal About the 'Sherlock Holmes Blood Test'?
Sherlock Holmes talks of a reagent that precipitates only
haemoglobin and confirms right away that a sample is blood. No confirmatory test required.
Does such a test exist?
No. There are loads of presumptive
tests which involve a change of colour. But they don't say, 'Yup, this sample is blood and nothing else.'Is there a test which confirms 100% that the sample is blood?
Yes, many! BUT - those tests don't involve precipitation - at least not the way Sherlock Holmes describes it. There is the precipitin test but
haemoglobin doesn't form precipitates there - it's an antigen-antibody reaction that happens.
And in any case this test primarily tells you if it is human
blood or not.There's RSID - but again, no precipitates.
Ah, and by the way - did Dr. Joseph Bell - the person Sherlock Holmes was based on - come up with any of these tests for blood we talk about?
No, Joseph Bell didn't discover these tests. Other people did.
Hope that helps!