When I was younger I wondered whether you could throw investigators a loop by mixing the blood at your crime scene with animal blood. I never had the need to actually try this. I’m sure that today with all the DNA tools we have it would be readily apparent. But how fast can it be done? Can a pathologist tell the difference by looking at the samples under a microscope and see the various blood types? How about mixing several human blood samples - how do they know which blood cells belong to different individuals? Should I stop adding to my human blood collection?
There are antibody tests that differentiate human from animal blood that should be quicker than DNA testing (haven’t had occasion to do such testing in non-forensic pathology).
There’s newer infrared spectroscopy testing for rapid differentiation.
As for DNA analysis, mixed human blood samples can be differentiated so investigators should be able to tell mixed human and animal blood apart.
Slightly off topic - I once showed a biopsy section of a benign tumor from my spaniel at an informal microscope session in residency, hoping to confuse the attendees. An attending pathologist picked up on the tissue being non-human, from hair follicle density (humans aren’t as furry).
I seem to recall that, even in the days before DNA, it is possible to tell animal blood (or platelets or blood cells, maybe) from human just under a microscope?
First, your human blood collection is deteriorating as you read this. Blood banks discard whole blood after 42 days. For your nefarious purposes, maybe the shelf life is longer; but eventually it’ll be clear that some of the blood wasn’t freshly spilled.
Second, what kind of confusion are you trying to sow?
- Do you want CSI to think that several people were killed or wounded here? (Better remove all the bodies.)
- Do you want to slow down the investigation, so you can complete the rest of your plot?
- Are you just trolling the police? In other words, you want to mock up a crime scene with lots of blood from various sources.
I’m relatively new to the SMDB. Who do we call when we need to hear from a real crime scene investigator?
Need answer fast?
That would be @Jackmannii, Pathologist Extraordinaire (post #2).
So you’re saying if they run a DNA analysis on a mixed sample from two humans it will develop two DNA profiles instead of one garbled one?
Depends on the animal. Most mammals have blood cells that lack a nucleus - but virtually all other animal blood cells do have a nucleus. Try to sneak in a parrot blood cell, and it is going to be immediately obvious under a microscope. Camels, weirdly, also have strange, ovoid-shaped nucleated blood cells (an odd desert adaptation in their case). Don’t try to pass off your pet camel’s blood cells as your own.
But aside from that there are in fact chemical differences, particularly varying normal potassium and sodium levels.
I bet that any dog and some humans can tell the difference from the smell of the blood. Meat from different mammals tastes quite different. It seems likely that their blood smells different, too.
Human red blood cells have a size from 6-8 micrometers, which is observable by any expert with a microscope. The size of red blood cells varies quite a bit among mammals, and I believe that specific size is pretty unique to humans. So a quick size comparison under a microscope would expose non-human blood easily.
The explanation I remember is this:
“Not so hard. A chap – Gay, I believe – sensitized some lab animals – guinea pigs or rabbits or whatever happened to be around – to various serums. You see, if you do it right, you can inject a little serum into an animal and he’ll develop what they call antibodies for that serum. Antibody’s a substance which the blood manufactures to combat certain things that get into it but haven’t any business there. But the point is that each antibody is specific – hostile to just one certain thing. From the viewpoint of the health of the human family, that’s too bad. Be swell if you could just inject a little of anything and get a general immunity to everything. But from the viewpoint of criminology it’s useful, because if you’re smart enough, you can tell whether your suspect is lying or not about the blood on his weapon. You just dissolve your blood off the weapon, and test it against the sensitized blood from each of your known animals. When you get a reaction you know, your unknown is the same as the one which reacted to it. See?”
~Dr. John Archer, The Conjure-Man Dies by Rudolph Fisher (1932)
The detective then asked him how they can tell one man’s blood from another, and the doctor mentioned Wasserman reaction and typing. That was in the 1930s.
It seems to me that if the sample is contaminated with animal blood, as per OP, you could get false positives with the antigen tests. Maybe the spectroscopy too. But that is presuming all samples taken are contaminated, which is no sure thing.
~Max
The OP’s scenario was a plot point in LA Noire - one of the cases involved a man who tries to fake his own death by staging a murder scene in his abandoned car, splashing pig blood all around the interior. The morgue is able to identify the blood as pig blood pretty quickly, and that story is set in 1947, so if it’s meant to be historically accurate in that aspect then it must’ve been possible for quite some time.
If your blood samples come from different blood types you might get an instant reaction right then and there - incompatible types clot up pretty much immediately. Which would be a tip off something isn’t quite right.
Along similar lines, is menstrual blood easily distinguished from, uh, wound blood? Because I’ve had the pleasure of cleaning the bathrooms in a women’s dorm, and on a couple of occasions if you’d told me a murder had been committed in that toilet stall, I’d have had no reason to doubt it.
Menstrual blood is endometrium, the lining of the uterus, a large component of it is blood, but it is also lots of mucous and epithelial cells, and antibodies.
TMI: if you touch it, it feels different from wound blood. I’m sure it looks way, way different under a microscope.
This was the premise of at least one episode of Quincy ME (the source of most of my extensive medical knowledge 
). IIRC the murder weapon had previously been used to kill a butcher, so had chicken blood on it as well as human blood, which was enough information for Quincy to crack the case. So medical examiners (the fictional variety at least) were able to identify animal blood, in the pre-DNA era.
As noted chicken blood cells would be nucleated and thus it would be trivially easy to notice the contamination under a microscope. Chicken blood vs human blood at the same magnification.
And perhaps also TMI, it definitely smells different. Similar, of course, but there are some distinctive olfactory notes that menstrual blood has that wound blood doesn’t have.
Other species also have blood types that humans don’t (and vice versa). They’ve been able to distinguish human from non-human blood far longer than DNA has been known about, let alone testable for.