Human blood is a rich source of DNA. The DNA is found within the nuclei of white blood cells. Interestingly there is no DNA in the red blood cells, which do not have nuclei.
Presumptive tests for blood are highly sensitive, but not specific. Further characterization of possible human bloodstains includes appearance, presence of human DNA, and presence of human hemoglobin. The age of a bloodstain currently cannot be determined. If blood is detected on an item of evidence and a mixture of DNA from multiple sources is present, it is not necessarily the case that all DNA present is from blood.
Determining the handler of weapons such as knives and guns may be possible by swabbing for DNA. If an item is simply handled, DNA on hands from sources such as skin cells and perspiration may transfer in detectable amounts. Complicated DNA mixtures and/or incomplete DNA profiles are often obtained from swabs of weapons.