If you want to see what snake venom — or at least Russell's viper venom — does to blood take a look at this chilling video. A drop of venom, and a quick swish of the dish of blood, and you get big hunk of solid blood jelly.
Russell's vipers do a number of unpleasant things to their victims, including sending them back through puberty by taking out their pituitary gland. But that takes months. The immediate effects of the bite are pretty unpleasant as well, as we can see in this video. After snake handlers milk the snake of venom, we see a little demonstration: a drop of viper venom in a dish of blood, a swish of the dish, like the blood's a fine wine, and there's a blob of semi-solid blood jelly.
What you're seeing there is a very unofficial version of a medical test. Russell's viper venom causes blood to coagulate as long as the blood is healthy. In the presence of anticoagulants related to lupus, blood exposed to viper venom will clot much more slowly, if at all. The Dilute Russell's Viper Venom Time test is a preliminary test for lupus; this may be what the snakes in the video are being milked for in the first place.