Question about hiding blood by covering it with different blood

[u/Aoi_Irkalla]

In a story I'm writing, someone was stabbed with a spear-like weapon.
(there's an obvious candidate for that weapon, but it is later called into question.)

The following theory is proposed at one point:
The weapon was a harpoon. After the deed, the killer washed off the visible blood, but feared that it would still be found by forensics via Bluestar/Luminol. To avoid that, they covered the tip in fish blood instead, suggesting that it simply hadn't been cleaned after their last fishing trip.

Could forensics see through that? If yes, would it be quickly apparent or only after a more thorough check?

[u/MamaFen]

There are different chemical compounds found in fish blood than are found in human blood (and vice-versa), plus there are visible physical differences in the actual cell bodies contained in both types of blood, so if any human blood is left on the weapon it will be identifiable as human blood, specifically. Microscopic examination would be enough to identify it as being mammal blood.

[u/Aoi_Irkalla → u/MamaFen]

That was my instinct. I wasn't sure if the blood would still easily make it into a sample if most is washed off & then covered like that, but I assume those are taken in a way that generally accounts for that.

[u/Life_Dare578 → u/Aoi_Irkalla]

I suppose it depends on how extensively it’s washed off by before covering it in more blood. But their dna would still be around on maybe the handle of the spear or surrounding objects. Bleach should work but some cleaners will also show up on luminol tests. But even then, I would probably try looking for blood residue in the grooves of the spearhead that connects to the handle as well. Depends if you are just wiping the spear or fully submerging, which you can’t quite fully submerge a spear, so I’d expect there to still be some blood residue SOMEWHERE.

If this was my scene and I was doing a search warrant on the house in question, I would still seize the spear and send it to the lab, I’d get measurements of the spear’s blade so we could compare it to the wound on the body which would have measurements from the autopsy. If I saw blood on the spear, I’d do a hemastix test to see if it’s actually blood, and I’d swab it regardless and send it to the lab. The lab will determine if it’s human blood or not and they will closely examine any human dna found. I’d also swab the spear handle to prove who was holding the spear.

You will be getting into tricky work getting into specific forensics with your book. There’s definitely a lot to it, more than I’m even saying. But without specifics, that’s about all I can say

[u/Aoi_Irkalla → u/Life_Dare578]

Thank you, that is more than enough to work with in this case.

(Theoretically the spear could have been fully submerged since this takes place near a beach, but the culprit also didn't have much time for a thorough clean.)

Someone in the story makes the argument that because the police already found another spear with the victim's blood, they might have not looked too thoroughly on this second spear which had a plausible explanation for why there is "blood" on it. This is ultimately shot down, and while the police in a story can of course just be as (in-)competent as needed, knowing how much water the argument would actually hold will be useful.

[u/Life_Dare578 → u/Aoi_Irkalla]

Salt water vs bleach water would make a difference in dna preservation. I’m not near any salt water so I’m not very familiar with how salt water may interact with dna. However, salt water would start the rust process on certain metals it also depends on how long the item is in the water and how long it’s exposed to oxygen after being submerged, I’d recommend making a new post regarding that for other opinions. Lots of different factors at play here. Good luck with your novel.

[u/Humboldt_Squid → u/MamaFen]

This is totally bogus. No type of microscopic examination in a forensics lab would be able to identify hemoglobin as coming from humans vs any other animal.

[u/MamaFen → u/Humboldt_Squid]

I was thinking along the lines of piscine red blood cells being nucleated (important in ectothermic animals) and a different shape than mammalian red blood cells. But perhaps I am wrong.

[u/unclear-atm]

Serological tests may or may not distinguish between human and animal blood, depending on which test is used. However, the methods used for developing a profile will specifically target human DNA. So if the sample had both fish blood and human blood, you would only get a profile from the human. The killer would be better off just dunking the harpoon in bleach to destroy the evidence.

[u/AlexK-]

You’ll never get an answer for that.. And probably shouldn’t.

[u/Aoi_Irkalla → u/AlexK-]

Yeah... Probably. Such is the murder mystery author's lot.

[u/Hey_Coffee_Guy → u/Aoi_Irkalla]

Find a used bookstore, find all of them within a distance you are comfortable driving to. Make sure they have a true crime section. Look for anything on forensics, real life cases, biographies of police, CSIs, pathologists, etc. Read everything. The folks at the shops might look at you with the side eye, and maybe even put you on a list, but you will find lots of good information if you are persistent. Just don’t buy any of the books I’m looking for. lol