If you’re a forensic technician or similar with access to a fully equipped laboratory, what can you realistically find out from a blood sample recovered from a crime scene, provided you have no idea who it’s from?
Obviously you can find out the blood type, whether the subject is HIV positive and so forth, but what else? Can you establish gender? Age?
Also, how old can the blood sample be before recovering information becomes difficult? How long afterwards can you see things like presence of toxins/alcohol?
Well, speaking from a genetics standpoint, you could theoretically sequence the entire genome. Gender would be extremely simple - just look and see wether there’s a Y chromosome or not.
Yes to gender, no to age. I don’t know what is done routinely, but it is a possibility that one could identify certain population haplotypes, hemoglobin and cell surface marker isotypes, and other factors and make a reasonable guess at the person’s ethnicity, although it would certainly be a probability thing not a definite thing.
One could in theory sequence a genome from a blood sample but it would be highly impractical at present.
You couldn’t even tell the difference between infant blood and octogenarian blood?
Another thing: When testing for the presence of e.g. drugs, you can’t actually just test “for drugs”, can you? You have to do one test for each drug you want to look for, right?
I think you would be able to if you wanted to. For one, not that many infants are taking geriatric drugs. I would guess prolonged aspirin use would be visible, as well as cholesterol drugs, etc. Plus there might be a chromosomal test for age, I don’t know.
The cool thing about blood and forensics is 99% of the blood is worthless for DNA analysis. Yes, that’s right, folks, red cells are the only common cell in the body that have no DNA, because they have no nucleus. I think about that sometimes when I’m squirting a few precious cc’s from a baby autopsy onto the two DNA spot cards we use. It would save blood for tox, and be more useful, if I could just rub liver on the spots. But the people who process them aren’t set up to recognize the value of liver. They have this silly idea, if the spots aren’t soaked through to the back of the card, they’re not quality enough for processing.
In my mind, the red cells in blood serve as a marker for where the technician should look to find the white cells, which have DNA in them.
You don’t need a whole genome sequence to look for male versus female. You don’t even really need a karyotype. Ever heard of Barr bodies? Anyone with XX has one of the X’s inactivated in every cell. It forms a Barr body that is easily distinguished on light microscopy. Rehydrate your stain, or scoop it up on a glass slide if there’s lots of it as we so often see, and I can tell you in ten minutes if it’s got a Barr body. No one ever asks me, of course. The dead body is usually near by.
But if it’s a tiny stain, they won’t waste it on rehydration. DNA is the way to go. Right now they are using RFLP’s (pronounced rufflups) which are a really kludgy and old fashioned way to go; ask Colibri if you want an explanation. Eventually they will sequence everybody. Right now sequencing takes too long, but they’re working on automating it and it’ll get faster. Before I retire, everybody’s blood will get sequenced, and that way they’ll know exactly what genome you have; can match it exactly to your previously set aside blood spot. Or to hairs from your hairbrush. Or cells from your toothbrush. Those being what we ask family for now, when we have to ID an unidentifiable body through DNA. Those results right now take five to ten weeks.
What nobody here has mentioned in re little blood spots at a crime scene is the wonders of blood spatter (never “splatter”) analysis. High velocity, medium velocity, low velocity spatter, anyone?
Oh yeah - forgot to say - red cells useless for age OR gender. First, in a stain, most of the red cells have clotted and/or burst, and are valueless. Second, once you get past the first three days of life, in which you might have a few circulating nucleated erythrocytes, baby RBC = octogenarian RBC. That is because you make all new RBC’s all the time; if you are one of the fortunate normals with a properly functioning bone marrow, they have a 120 day life span, after which they are retired by the spleen. Every day, every night, as you sit here reading this, your skull, ribs, breastbone, spine, and hips are oozing out fresh red cells to keep up the supply. So no help in forensics at all.
Sorry to hog thread. Wanted to address the OP’s questions. You probably cannot find out HIV status from a dry smear; you’d need several CC’s for testing. Blood type might need more than a half cc. Gender we said was easy. Age, impossible.
How old can the blood sample be – If it has dried, very old. The blood spots the US military is now doing on every member admitted (no more Unknown Soldiers, ever) will keep in a filing cabinet for fifty years. Conceivably longer. We often get forensic information from the cold files on cases 10 or 15 or 20 years old. Preserve it dry, we can work with it.
If it decomposes before it dries, much more difficult. Maybe and maybe not. DNA degrades when bacteria get hold of it.
You need more than a cc to do alcohols and you might need several ccs to do drugs (is that what you mean by “toxins”)? If I want cocaine, opiates, fentanyl, amphetamines, alcohol, methanol, and a base and acid neutral screen, I probably need to send the lab 20 cc. So if you’re talking about a mere fingerprint’s worth of blood smeared on a wall, we can’t send that for tox. DNA will be the way to go.
Again, if the blood is decomposed, it’ll have alcohol in it made by bacteria, and the value will be meaningless. If it’s not, if it got put in a fridge or was in a cabin up North that was cold all winter, it can be tested almost as well the year after it was collected as the day it was collected. Cocaine will go to BE (benzoylecgonine), but that’s all.
Addendum: You can get an idea of the ethnicity of the spatter-producer from the 30 or so proteins that are on the outside of a red cell. It is possible to extract DNA from cells other than leukocytes, but it isn’t as easy.
RFLP: you make a bazillion copies of a section of DNA, and then cut it up with an enzyme that only cuts at a specific sequence. For instance, the enzyme AvaII cuts only when the DNA base sequence is GACC, and DpnII only cuts when the sequence is GATC. The result is a bunch of cut up DNA strands of several specific lengths that form a distinct pattern when separated on an agarose gel. If you perform RFLP analysis on the right part of a chromosome with the right enzyme, you can tell two or more individuals apart based on their RFPL patterns.
I’m not sure what kind of cells you’re talking about here, but we extract DNA from tissue samples all the time in our lab. It’s actually slightly easier than doing it from blood, because you don’t have to lyse the RBCs and pellet the WBCs first. It does take longer, because there’s generally more stuff to digest, but it’s pretty simple.
The situation is this: I’m writing a role-playing scenario and trying to keep it as realistic as possible. The players find a very bloody kitchen knife and have no idea how old the blood is or whose it is, but it has spent that time in roughly room temperature. Could you find out anything meaningful from such a sample? Gender? Drugs (probably not, due to small quantity)? Approximate time since the stabbing? If they ask you to simply “tell us as much as you can”, what could you do?
First, is there a body? Or a person missing? Because lots of times, knives are used for butchering animals.
Let’s assume it’s a murder of a human being. The blood will have dried, so it will work just fine for DNA. But let’s stop and do the obvious first: fingerprints. Blood is a great medium for preserving the assailant’s fingerprints. Very few people wear rubber gloves to a crime. Stabbings are crazy wild moments in which success is not guaranteed, violence takes over without common sense in charge, and in the vast majority of cases, the other person defends themselves before the fatal blow. In situations like that assailants just can’t guarantee there will be no prints on the knife. If they do the usual thing, cast the knife away in the moments after they’ve won, they won’t even think about their fingerprints. If they did, they would wipe the knife, which screws your scenario.
Second: DNA. Two kinds. Victim (copious) and assailant (not guaranteed, but quite possible!). Assailants cut themselves more often than you would believe. Vertical shallow cuts along the edge of the palm are classic assailant accidental self-cuts (long cuts across the palm are classic defense cuts). We’d definitely look for assailant DNA. There might be some non-bloody assailant DNA on the handle, too. Wouldn’t be much if any, but PCR does wonders.
If your story is in the present day, victim and assailant blood don’t do you much good until the police develop suspects. Because there is no central bank of DNA-typed humans to whom to compare it, and because they don’t sequence it. So it won’t tell you much, although you could do gender. If your story is in the not so far future, it could tell you ethnicity possibly, particularly if your victim or assailant came from one of those little groups that has odd quirks in their blood, but it won’t tell you race. It won’t tell you height. It won’t tell you eye color. It won’t tell you age. It won’t tell you the way the face looks. So its major use is to match up to the DNA of either a dead body (or living but wounded person), or to the DNA of a guy who swears he was never there, never seen the place, never met her in my life, Officer. And to do that, you have to find the guy first.
If you are working in the slightly farther but not so far off future, everybody born in the USA will have been karyotyped from the umbilical blood, and they’ll just stroll down to the databank plug, feed in the sequencing, and ask it to pick out your man. Of course, if he was born in Russia or Mexico or Peru, you might be out of luck.
So the answer so far is: Not much, but scrape that dried blood and hang onto it, and we’ll see whose it tells us it is.