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Welcome back to The Deep Dive.
Today we're tackling a fundamental question in forensic science.
You find a suspicious red stain at a scene.
How do investigators actually prove its blood?
Yeah, it sounds simple, but getting a definitive answer involves some pretty fascinating science.
We've been digging into the foundational texts, specifically Richard Lye's Forensic Biology, to really map out how this works step by step because, you know, telling the difference between blood and, say, rust or ketchup is obviously critical.
Exactly.
And we're going to unpack that process.
We'll look at the underlying biology, then the quick screening tests, and finally the methods that give you that courtroom -ready confirmation.
It's all about balancing speed, sensitivity,
and certainty.
Okay, sounds good.
So let's leave the groundwork first.
What are the biological basics we need to know about blood itself?
Right.
So blood makes up about maybe 8 % of your body weight, and it's basically got two main parts.
There's the liquid bit, the plasma that's over half the volume, and then there's the cellular fraction.
This is where the key components for forensics are.
And what's in that cellular part?
Three main things to focus on.
First, erythrocytes, or red blood cells, RBCs.
Their job is oxygen transport using hemoglobin.
Hemoglobin, right.
That contains iron.
It does.
And here's the critical forensic point.
Mature human red blood cells, they don't have a nucleus.
Same goes for platelets, which help with clotting.
No nucleus.
Meaning they lack nuclear DNA.
That's huge.
If you only have RBCs or platelets, you're not getting a standard DNA profile from them.
Oh, okay.
So if RBCs don't have the DNA, what does?
That would be the leukocytes, or white blood cells, WBCs.
They're your immune system's defenders, and they have nuclei.
Got it.
So WBCs are the source for nuclear DNA testing.
Precisely.
They're the main source when we talk about DNA profiling from a blood sample.
So it sounds like there's a split strategy here.
WBCs for DNA.
But what about just identifying the stain as blood in the first place?
You mentioned RBCs are key there.
Yes.
Because of what's inside the RBCs.
Even without a nucleus.
It comes down to one molecule, really.
Heme.
Heme.
That's part of hemoglobin.
Exactly.
It's this organic ring structure with an iron ion.
Phypinase Plus, right in the middle.
That iron is what binds oxygen.
It's also, unfortunately, what binds things like carbon monoxide or cyanide, causing asphyxia.
So this Heme molecule is the target for most identification tests.
It really is.
It's the active ingredient, you could say.
But there's a catch.
Heme isn't unique to human blood.
You find very similar structures in animal blood, and even in related proteins like myoglobin in muscle or neuroglobin in the brain.
So finding Heme tells you it could be blood, but not necessarily human blood.
Right.
That makes sense.
It's a starting point, not the final answer.
Exactly.
Which leads us nicely into the first type of tests used in the field.
The presumptive assays.
Okay, presumptive.
Meaning, they give a strong indication, but not absolute proof.
That's the idea.
They're designed for speed and high sensitivity, finding even tiny traces.
They work because Heme acts as a catalyst.
A catalyst for what?
For an oxidation -reduction reaction.
Basically, you add hydrogen peroxide, that's the oxidant.
Heme speeds up the reaction between the peroxide and a colorless chemical substrate.
And when that reaction happens?
The substrate gets oxidized and changes.
It produces a color, or it might glow, which is chemiluminescence, or even fluoresce under special light.
And you said they're sensitive.
Incredibly sensitive.
We're talking detecting blood diluted down to one part in a hundred thousand or even one part in a million.
That's crucial when you're searching a large area or dealing with cleaned up scenes.
Wow.
And importantly, do these tests mess up the sample for later DNA testing?
Generally, no.
Most common presumptive tests don't interfere with getting a DNA profile later, which is a huge plus operationally.
Okay, so what are some of these tests?
Let's start with the ones that change color.
Sure.
A classic one is the phenolphthalein assay, better known as the Kasselmeyer test.
You have the reagents, and if Heme is present under alkaline conditions, this colorless chemical, phenolphthalein, turns bright pink.
Pink means possible blood.
Simple enough.
Pretty much.
There's also leukomalachite green, LMG, which turns green under acidic conditions.
Historically, benzidine -based tests were common when they gave a blue color.
Historically, not used anymore.
Right.
Benzidine itself is a known carcinogen, so it's prohibited.
Orthotolidine was used too, but also a potential carcinogen.
Safety regulations really pushed for alternatives.
So what's the standard now?
The current go -to is tetramethylbenzidine, or TMB.
It gives a green to blue -green color, it's much safer, and it's used in handy field kits, like hemastics.
You might have even seen those used in medical settings too.
Okay, TMB.
Now what about the tests that produce light?
You mentioned those are good for finding hidden stains.
Exactly.
These are great for latent stains, maybe where someone tried to wash blood away.
The most famous is probably luminol.
Luminol, yeah, I've seen that on TV shows, the blue glow.
That's the one.
Heme catalyzes luminol's oxidation,
and it emits light chemiluminescence.
It creates this eerie blue glow.
But here's the thing, you can only see it in the dark.
And it's very useful for revealing patterns,
maybe faint shoe prints in blood or white marks on a wall.
But the glow is temporary, it fades fast, so you need to photograph it immediately.
Right.
Documentation is key.
Is there an alternative?
There is.
Fluorescent.
Similar idea, heme catalysis causes it to react, but instead of glowing on its own, it fluoresces.
Meaning you need a special light source.
Yes, you need an alternate light source, typically in the blue -green range, around 425 to 485 nanometers to excite it.
Then it emits this really intense yellowish green light.
And the advantage.
Light lasts longer than luminol's glow, which gives you a bit more time to work and document.
Okay, these sound like powerful tools for finding evidence, but you mentioned trade -offs earlier.
Yes, and here's a big one.
While luminol and fluorescent are great for finding fainter, clean stains,
spraying these liquids can dilute the sample.
Dilute it how?
Well, you're essentially adding more liquid to a potentially very small, maybe already washed stain if you dilute it too much.
You might not be able to get enough DNA later.
Exactly.
The very act of finding the stain could compromise the ability to get a full DNA profile.
It's a real dilemma sometimes.
Wow, okay, that really highlights why these are presumptive.
They point you in the right direction, but they aren't the end of the story.
And they can be wrong sometimes.
They can.
Because they rely on that general catalytic property of heme, other things can sometimes trigger the reaction.
This leads to false positives.
What kind of things cause a false positive?
The most common culturates are strong oxidants.
Think household bleach hypochlorite ions are strong oxidizers.
Or even some cleaning products, hair dyes that contain peroxide.
These chemicals can basically kickstart that oxidation reaction themselves, without any heme needed.
So you could get a positive result on a clean floor just because someone used bleach.
Potentially, yes.
Copper or nickel salts can also interfere.
So how do investigators deal with that?
How do you know if it's blood or just bleach residue?
There's a neat trick, a procedural fix.
It's called a two -step catalytic assay.
Instead of mixing everything together, you apply the chemical substrate first, before adding the hydrogen peroxide.
Ah, I see.
If it changes color just with the substrate?
Then you know some other oxidant is present,
doing the work before the peroxide gets involved.
It tells you right away it's likely interference, not necessarily blood.
Clever.
What else can cause false positives?
I think you mentioned plants earlier.
That's right.
Some plants, like horseradish is a classic example, contain enzymes called peroxidases.
These can mimic heme's catalytic activity.
So a salad spill could test positive?
Potentially, yes.
But there's a way to differentiate.
Plant peroxidases are usually sensitive to heat.
So you heat the sample?
You can, yes.
Gently heating the sample often inactivates the plant enzymes.
Heme, being part of a stable iron complex in hemoglobin, is much more heat resistant.
If you heat it and retest and the reaction doesn't happen anymore, it points towards plant interference.
Okay, that makes sense.
What about the react?
That can happen too, though it's maybe less common.
Strong reducing agents can interfere.
Certain metal ions, like lithium or zinc, can actually inhibit the oxidation reaction needed for the color change or light emission.
So the chemistry has to be just right.
It really does.
Which is why, once a presumptive test comes back positive,
the sample goes to the lab for the next stage.
Confirmatory assays.
Okay, confirmatory.
Now we're moving from maybe blood to definitely blood.
That's the goal.
These tests aim for much higher specificity.
They work by actually identifying characteristic crystalline forms of heme derivatives.
Crystals.
Like tiny microscopic crystals.
Exactly.
They're looking for specific shapes under the microscope that only form when heme is treated with certain chemicals.
What are the main crystal tests?
The two big ones are the Takeyama and Teichman tests.
The Takeyama assay is often preferred.
You treat the stain extract with pyridine and glucose under alkaline conditions.
And if he means there, you get these distinctive pinkish feathery crystals of pyridine ferropetalporphyrin.
Sounds quite specific.
It is.
The other main one is the Teichman assay.
Here you use glacial acetic acid and salts and heat it.
This forms brownish rhomboid -shaped crystals of hemitin chloride.
Is one better than the other?
They both work, but the Teichman test sometimes has an edge with older, more degraded blood stains.
It seems to be a bit more robust when the heme might have broken down slightly over time.
Interesting.
But these crystal tests, they just confirm blood, right?
Not human blood.
Correct.
That's a key limitation.
They confirm the presence of heme, but can't distinguish human from animal blood.
And they're generally less sensitive than the presumptive tests we talked about earlier.
So confirming its blood is one step, but confirming its human blood is another.
Exactly.
And for that, we often move beyond crystal tests.
What other confirmatory methods are there?
Well, there are spectrophotometric methods.
These measure how hemoglobin and its derivatives absorb light at specific wavelengths.
Hemoglobin has a characteristic peak absorption around 400,
425 nanometers in the SORET band.
So using light absorption patterns.
Right.
And then to get species -specific, you have immunological methods.
Barentopodes.
Precisely.
These use antibodies that are specifically designed to bind only to human hemoglobin.
If the antibodies bind, you get a reaction, confirming human origin.
Okay.
That sounds definitive for human blood.
Is that the final word, or is there anything newer?
There is actually.
The really cutting edge approach involves looking at RNA, specifically messenger RNA or mRNA.
RNA, not DNA.
Right.
RNA.
Using a technique called RT -PCR, which is reverse transcription polymerase chain reaction, labs can detect mRNA molecules that are specific to certain cell types.
For blood, they look for mRNA unique to red blood cells, like the code for hemoglobin subunits like HbA1, or other erythrocyte -specific proteins.
And the advantage of looking at RNA?
Specificity, again.
It can potentially tell you not just that it's blood, but confirm the tissue source very precisely.
Plus, these methods can often be automated, which is great for high -throughput labs.
But RNA, isn't RNA known for being kind of unstable?
Ah, you hit the nail on my head.
That's the major limitation.
RNA degrades much more easily than DNA.
It's very vulnerable to enzymes called ribonucleases, which are pretty much everywhere in the environment.
So time is a factor.
A huge factor.
RNA -based tests work best on relatively fresh samples.
The older or more degraded the stain, the less likely you are to get usable RNA.
So while it's highly specific, it's not always a viable option, depending on the evidence condition.
Right.
So it seems like a whole toolkit is needed, depending on the situation.
Absolutely.
It's a layered approach.
You start broad and sensitive in the field, then narrow down with more specific confirmatory tests in the lab, choosing the right tool for the job based on what you need to prove and the condition of the sample.
Okay, so let's recap quickly.
We started with the basic biology understanding that RBCs carry heme, which is the target for initial tests, while WBCs carry the nuclear DNA for profiling.
Then we looked at presumptive tests.
Castlemeyer, TMB, Luminol, Fluorescent, all leveraging heme's catalytic activity for speed and sensitivity, but prone to false positives and potential sample dilution.
We covered how to navigate those false positives using procedural steps or heat treatment, and acknowledged false negatives are also possible.
And finally, the confirmatory assays, crystal tests like Takayama and Teichman prove its blood, while immunological and more recently RNA -based methods confirm if it's human blood, each with their own strengths and weaknesses, especially regarding sample age for RNA.
It's really a journey from a simple observation to complex chemical and molecular proof, balancing those needs for speed, sensitivity, and absolute certainty.
Precisely.
Understanding that balance is key to appreciating how forensic biology works in practice.
So we've figured out how science proves a stain is blood, but thinking about that RNA degradation issue raises another question.
Here's something for you, our listeners, to ponder.
If molecules like RNA break down predictably over time, could forensic scientists develop methods, maybe looking at the ratio of intact versus degraded molecules, to accurately estimate how old a blood stain is?
Not just if it's blood, but when it was deposited.
That's a fascinating area of ongoing research, trying to turn molecular decay into a forensic clock.
Definitely something to keep an eye on.
Absolutely.
A provocative thought to end on.
Thanks for joining us on this Deep Dive.