If there's a bloodstain on the carpet, what can you learn from it?

October 11, 2013 3:31 PM

Say someone broke into your house and dripped some blood on the carpet. What does the forensics guy do with it? How long would the tests take? And what information could be gleaned from it?

(I'm writing a Call of Cthulhu adventure. Really.)

posted by hishtafel to Science & Nature (9 answers total)

Bloodstain pattern analysis is a thing. So is laboratory blood analysis, though if you want to do DNA profiling on the blood, it takes a good deal longer than it does on CSI.
posted by DarlingBri at 3:34 PM on October 11, 2013

The patterns can say a lot: how fast someone is moving while bleeding, what direction they're going, how much/how fast they're bleeding, etc.

Or so they tell me.
posted by Madamina at 3:37 PM on October 11, 2013

Best answer: If you're thinking of getting some sort of DNA match, that would be hard. The problem is that the area is already littered with dead cells which contains DNA, so it would be hard to separate out the DNA from the blood. (Red blood cells don't contain DNA; it's only the white cells that do.)

Humans (and dogs and cats) shed dead skin cells constantly; that turns out to be the primary component of household dust. It's all through your carpet.

If the blood landed on linoleum, or tile, that would be a different matter; you could probably get an uncontaminated sample from there.
posted by Chocolate Pickle at 4:18 PM on October 11, 2013

Best answer: My sister works in a crime lab, she is not a scientist. Whenever people ask these sorts of questions and I ask her about them she's always like "It's not like CSI here!" So there are things that can be done and things that likely will be done. If this is just a random break in, there is a good chance that no one will do anything with any of the blood, especially if nothing major is stolen and no one was injured (if someone was injured, see DarlingBri's link and this one from HowStuffWorks talking about spatter analysis, it's interesting stuff). They will also not take fingerprints. This sort of thing happens in big deal cases and in small town cases where the police have the time and energy to do this sort of stuff and/or give a shit. You can read a sample crime scene processing outline from the Minnesota department of Public Safety's Bureau of Criminal Apprehension here. You don't get the Crime Scene Team for burglaries. So, if you want realism, you might want to figure that in.

Here's some more about what that crime scene team does.

- checks if those stains are blood
- checks if those stains are human blood
- types the blood

[here's more about what those tests look like - please enjoy reading about the Sperm Hyliter™]

Like fingerprints, one of the big things with dealing with blood at the scene is the whole "Ruling people out because of their blood type" So if someone broke into your house and they have O+ blood but you also have O+ blood, that's going to make identification trickier. There is more subtyping that can be done to eliminate you from the possible bleeders/suspects. The DNA stuff is totally out of my wheelhouse but hope this is helpful.
posted by jessamyn at 4:18 PM on October 11, 2013 [1 favorite]

If you can see the bloodstain, you can almost certainly get a DNA profile from it, unless it's been bleached/ground into the floor by someone's dirty boot/otherwise degraded. The target input for our DNA profiling method is 1 nanogram - that's 1 billionth of a gram - so any stain that's visible should have way more than enough DNA. There might be some background types from whatever skin cells etc. were already on the floor, but the predominant profile would be from the blood.

(Also, Sperm Hyliter is pretty much just as fun to use as you think it might be. They gave me a shirt with glow-in-the-dark sperm around the time of the product launch, but alas, I never did find an appropriate place to wear it.)
posted by Flannery Culp at 4:47 PM on October 11, 2013

Nothing.

I doubt that cops would bother even collecting fingerprints if it was just a break in.
posted by Good Brain at 5:29 PM on October 11, 2013

If there was blood at the scene of a residential burglary, we would absolutely test it. Solving nonviolent crimes and getting those profiles into the national DNA database is how you catch repeat offenders before they escalate to crimes against people.

[see: NIJ (PDF paper)]
posted by Flannery Culp at 5:35 PM on October 11, 2013

To add to what Flannery said....few years back NYPD lab was doing a pilot program they called 'biotrax' which was grant funded and was used to collect DNA samples from robberies (blood drops definitely but also screwdrivers, hammers, knives, other tools.....cops even submitted a half eaten banana one time ) the theory they were testing was that suspects committing robberies may in time move up to assaults/murder so tracking and maybe catching them early could do some good.

Looks like that paper that was linked too was sort of the phase 1 almost 10 years ago. Not sure if they are still doing it in NYC. On the one hand the cost involved in doing a DNA test has come WAY down in the last 10 years....on the other hand case load/sample volume has also gone WAY up so most of the time they focus on violent crime (homicide, sexual assault, ect)

Bottom line...could they get useful DNA from blood drops on your carpet? Definitely! Would they take the time,effort,cost....probably not...but depends on where the crime occurred and how the department/crime lab is set up.
posted by Captain_Science at 5:45 PM on October 11, 2013

Right. My lab will test body fluids and foreign objects from burglary scenes, but we won't do, say, touch DNA swabs from a doorknob or car steering wheel that has been handled by who knows how many people. Your lab may vary.
posted by Flannery Culp at 5:55 PM on October 11, 2013