För att se till att det faktiskt är blod, och (i vissa fall) identifiera vilken typ av blod det är / informellt blodspridning anyalysis
Som anges i TV Tropes-posten är Ketchuptestet var:
a character touches a blood stain with his fingers, usually in order to make sure that it actually is blood. Sometimes, it turns out to be ketchup or strawberry jam (something that's red, goopy and fake), especially if another character has been doing a Chicken Little impersonation over the supposed bloodstain.
Being a vampire or a cyborg may also help for analysing the type of blood. In Real Life, it's possible that some animals (bloodhounds, brown bears, tyrannosaurs, larger sharks) may have a sufficiently powerful sense of smell, this (or simply sniffing the blood) might actually allow them to determine the species or even the individual victim involved.
See also Sniff Sniff Nom, Fingertip Drug Analysis.
Det finns många exempel på ovanstående länkar av karaktärer som använder smaken som ett sätt att främja deras förståelse av en situation.
Det kan också vara att det rör sig om blodet gör att karaktären identifierar koagulering som kan hjälpa till att identifiera hur länge sedan blodet kom i kontakt med ytan och från vilken del av kroppen:Blood is in a liquid state when inside the body, and when it exits the body, it does so as a liquid. But as anyone who has had a cut or a scrape knows, it doesn’t remain a liquid for long. Except for people with hemophilia, blood will begin to clot within a few minutes, forming a dark, shiny gel-like substance that grows more solid as time progresses.
The presence of blood clots in bloodstains can indicate that the attack was prolonged, or that the victim was bleeding for some time after the injury occurred.