Concrete and other similar porous surfaces are difficult simply because it is difficult to separate the blood from the concrete. Soil is an almost impossible substrate from which to obtain usable DNA because current extraction methods are not capable of releasing the DNA from soil. Blood stains may be mixtures of blood from two different people and can produce DNA profiles that are more complex than those from a single individual. DNA profiling may be the only way to determine if a given stain is a mixture. DNA testing can also determine if specimens are from individuals of a different sex.
Gender is the only physical characteristic that can be determined by forensic DNA analysis.
Specimens drawn from suspects or victims are called known exemplars and usually consist of liquid blood. The simplest and most reliable samples to work up are liquid whole blood. While the best storage method for blood is frozen, DNA has been successfully isolated after refrigeration for several months. The most common way in which forensic blood samples are handled, following collection, is as a series of dime-sized stains on washed cotton sheeting. This method makes for convenient transport and storage of large numbers of samples.