We have about ten pints of blood (or up to six quarts of blood, half plasma and half blood cells) being circulated through our bodies by the circulatory system. The blood is pumped by and through the heart into vessels and veins and transports materials—nutrients such as glucose— and oxygen, to vital organs and tissues for growth and repair. It also carries carbon dioxide and waste products from the tissues for excretion. And hormones are taken to various tissues and organs for chemical signaling, all the while moving digested food from the gut to the liver, and within, too, are immune bodies that are hanging out for the prevention of infection, and which possess clotting factors to help stop bleeding to all parts of the body.
In some crime labs, forensic serologists also take on another hat and become expert in “blood pattern analysis,”—also referred to as blood spatter—which is used to recreate the violent scenarios involved in a crime scene according to the patterns displayed on surfaces around the room.
Crime scene blood is important to real life detectives (and also to mystery writers), because it is often found as the result of homicide, assault and sexual assault. Since blood can be found at different time stages, it can be present as fresh liquid pools, coagulated blood, (meaning the blood has turned to a semi-solid state), dried blood, small drops, or even swiped stains.
When any dark substance is found at the scene it is carefully collected because many things can contaminate blood and it can act as a biohazard to the criminalist collecting it. That is why crime scene technicians will often suit-up wearing latex gloves, surgical masks, eye-coverings and sometimes full coverage gowns. (Yes, the CSI people on TV all have hepatitis according to the way they dress!) And although they might not wear as much PPE (Personal Protection Equipment) as shown in the picture at the right, they will surely wear booties, gloves and maybe even a mask.
The determination of blood is best made by using a preliminary color test and for years, the most commonly used was the benzidine color test. (Benzidine was labeled a carcinogen, so it has generally been discontinued.) The Kastle-Meyer color test was used in its place and basically, the tests are based on the fact that blood hemoglobin possesses peroxidase-like activity. To simplify, these are enzymes that when tested with a reagent will cause the sample to turn a deep pink color. It’s not an absolute however, because potatoes and horseradish will also turn it pink.
Sometimes a powerful light moved across the surface of a crime scene is all that is needed for visual inspection. But if blood is suspected and not readily seen, luminol is sprayed and after about five seconds it will fluoresce in a darkened room. Luminol is prohibitive though, because it will destroy many important blood factors necessary for the forensic characterization of blood.
Microcrystalline tests can be performed on material and there are several tests available but the two most popular are the Takayama and Teichmann tests. These depend on the addition of specific chemicals to the blood so that characteristic crystals with hemoglobin derivatives will be formed. They will also react to other materials that may be present in a bloodstain.
After the blood is in the lab a test is used to find out whether it is human or animal blood. It has to do with a rabbit and chicken egg proteins and it forms a cloudy substance called a precipitin. Another test can be done using a “gel diffusion” test. The sample’s antibodies and antigens will diffuse or move toward one another on an agar gel-coated plate. The extracted bloodstain and the human antiserum are placed in separate holes opposite each other on the gel. This will then be subjected to an electrical field and a specific reaction will be expressed by a line of precipitation formed between the two. The blood is then typed. In 1925, a special percentage of the human population were found to be “secretors,” which means that certain antigens, proteins, antibodies and enzyme traits can be found in other bodily fluids such as saliva, urine, or teardrops.
Today, DNA has replaced the tests for specific enzymes and proteins and while they are more accurate, they are also more expensive, timely, and require an updated crime lab facility. Sometimes it is about expertise, resources, and budget that determines how much bodily fluids are taken into consideration.