13.2: Blood Analysis

Blood is a vital complex biological fluid, containing red cells, which is present in vertebrates and may be shed during accidental, intentional, and criminal acts. Red cells, called erythrocytes, are the primary method of transporting oxygen through the body through the circulatory system. Red cells are incredibly small, approximately 7 to 8 microns in size, which allows them to pass through the capillaries and tributaries of the circulatory system. In fact, they are bi-concaved in shape, which maximizes the surface area in a small size. Two to three drops of blood may contain one billion red cells. Although the presence of mitochondria has been detected in human blood using electron microscopy, red cells themselves have no mitochondria and therefore they have no method of producing energy. Thus, they have a very short lifespan of approximately four months. Three million red cells expire every second and are passed through the body as waste. Blood is separated into three components: plasma volume, the buffy coat, and red cells. Plasma makes up approximately 55% of the volume of blood and is 90% water and 10% solids. Plasma is the transportations system for the circulation of nutrients, proteins, and hormones through the body. It also collects and removes waste. The buffy coat, so called because if you spin blood in a test tube in a centrifuge, the buffy coat will appear as a thin whitish layer between the plasma and the red cells, contains the white cells, called leukocytes, the primary components of the immune system, and platelets, which are cell fragments that are necessary for clotting. Leukocytes and platelets make up less than 1% of the volume of blood. Red cells make up approximately 45% of the volume of blood. There are approximately 60 milliliters of blood per kilogram of body weight, or two ounces per 2.2 pounds of body weight. The average adult male has about 1 1/2 gallons of blood, whereas the average adult female has 1 1/8th gallons of blood.

Prior to the advent of DNA typing in forensic science, it was common to include or exclude a suspect based on blood typing analysis. Blood typing was accomplished by mixing blood specimens recovered from the crime scene with antibodies to see if the blood cells would clump together. This process is called agglutination, which indicates the presence of specific antigens on the red cells. This tested for the A, B, AB, or O system, and for the positive of negative Rh factor. The technician would then conduct a follow-up “back typing” step to confirm the antigen result by inspecting the liquid part of the blood for antibodies against known exemplars of blood types. This way, if blood from the crime scene was typed as O negative and the suspect was typed as AB, then they could be excluded as having contributed the specimen. However, if the blood type matched the specimen from the scene, then it could be argued, especially when based on the rarity of the blood type, that the suspect was a potential contributor. By example, only 2% of the population are B- and Rhnull (no antibodies) is the rarest blood type. O+ is the most common blood type with approximately 38% of population being of that blood type. It is unfortunate that the criminalistic laboratories have stopped conducting blood typing, because typing could be conducted much faster than DNA typing and there are times when a bloodstain pattern analyst could easily determine which participant contributed blood to certain patterns. Also, blood typing could be used to exclude suspects as noted above, which would be a great time saver to the investigation.

In the infancy of DNA typing, a blood specimen about the size of a quarter dollar coin; however, today all that is needed could fit on the tip of a sewing pin. DNA is found in the nucleated cells in leukocytes, in small fragments also found in plasma called cell-free DNA, as well as in mitochondria that may be found in the blood. Red cells lack a nucleus and therefore do not contact DNA. Again, a discussion on the proper techniques to collect blood specimen will be in the next chapter.