Blood dries over time. How quickly this happens depends on the surface on which the blood landed, how much blood the spatter contains, and the heat and humidity at the crime scene, but as a rule the outer edges of the stain dry first. Consequently, after the interior portion flakes off or is smeared by an object, a dry blood spatter can skeletonize, that is, leave behind a ring similar in appearance (if not color) to a water ring on a coffee table. Patterns of drying help analysts determine how long an assault went on, detect whether it took place all at once or in stages, and nail down possible crime scene contamination. Clotting patterns in blood provide similar information and can help nail down the time factor if analysts arrive at the scene before blood can dry. Clotting begins within 3 to 15 minutes, but actual times vary by amount, surface type and environment. Mixed levels of clotting can indicate that multiple blows or gunshots occurred over time
Dry blood samples can detect both antibodies and hepatitis virus RNA successfully. It involves obtaining a few drops of blood by fingerprinting, and letting them dry on a specially specific absorbent paper card.
