Serum, or blood serum, is a useful medium for a range of forensic analyses, as well as for laboratorial diagnostic assays, due to its biological contents. Pure serum, however, does not contain blood cells, platelets, or fibrinogen (coagulation factors). The sticky consistency of serum is due to albumin, a protein that provides the proper density for blood, and prevents it from leaking through cell vessel walls. The main function of serum is to moisten the surfaces of cell membranes and to transport to organs and tissues diluted water-soluble nutrients such as blood red cells (erythrocytes), hormones, fat-soluble nutrients (chyle), white blood cells, and antibodies present in the lymphatic fluid as it enters the blood circulation. Serum is also present in seminal fluid and lymphatic fluids , and exudates from wounds and blisters as a clear watery substance. The presence of these and other contents in serum allowed the development of several types of analytical assays (tests) useful for both clinical and forensic purposes, such as for detecting tumor markers, detecting antibodies for specific infectious agents, anti-doping tests, blood typing, and DNA tests. Serological tests are also used in postmortem identification of poisons or illegal drugs in the body fluids of corpses.
Blood plasma is formed by serum and lymphatic fluid, and contains suspended leukocytes (white blood cells), erythrocytes, coagulation factors, electrolytes (e.g., mineral ions), gases, proteins, glucose, water, and micronutrients essential for cells. Plasma may contain poisonous metabolites resulting from enzymatic transformation of drugs, poisons, allergenic substances, or environmental pollutants, known as exogenous metabolites. Additionally, serum transports endogenous toxic metabolites resulting from cellular and enzymatic processing of gases and nutrients, which are released in blood plasma to be excreted through urine, sweat, and feces. Serum and plasma contents can therefore be analyzed for various purposes: the forensic determination of environmental toxins in outbreaks of illness due to either water or food contamination, DNA screening for the identification of suspects and/or victims, paternity tests, and drug screening.
The most common serological immunoassays used by forensic technicians are: enzyme-multiplied immunoassay technique (EMIT), radio-immunoassay (RIA), fluorescence polarization immunoassay (FPIA), cloned enzyme donor immunoassay (CEDIA), enzyme linked immunosorbent assay (ELISA), gel diffusion analysis (immunodifusion), serum protein electrophoresis (immunoelectrophoresis), and Western immunoblotting. EMIT, RIA, FPIA, and CEDIA allow the screening of very small amounts of antibodies against all classes of drugs and the identification of specific drugs as well. As some small drug metabolites do not trigger natural antibody formation, specific commercial kits are used to recognize these potential antigens in suspects of drug abuse. Antigens are proteins and substances recognized either by the immune system as foreign to the host (such as viral and bacterial particles or allergens) or by an assay antiserum reagent. The following specific drugs can be detected in serum through the above immunoassay tests: benzoylecgonine, a cocaine metabolite, phentanyl, methadone, phenylciclidine, and propoxyphene. Additionally, the following classes of drugs can be identified: amphetamines , barbiturates , benzodiazepines, cannabinoides, and opiates. ELISA is more commonly used for HIV and hepatitis C diagnosis, although this method can also detect drugs in the serum or in urine. ABO blood grouping and Rh factor typing is usually performed through gel diffusion analysis (immunodiffusion) or serum protein electrophoresis, known as immunoelectrophoresis. These assays can also determine whether a blood sample has a human or animal origin. Serum proteins can also be sorted by immunoelectrophoresis, and the resulting protein complexes can be further screened for specific antibodies against HIV antigens or very small drug metabolites through the Western immunoblotting technique.
Another immunoassay technique, the antibody profile assay (APA), was developed in the late 1990s, and has proved to be useful in forensics. The test design is based on the fact that the serum of each individual contains a variety of antibodies, each one specific to a given foreign protein to which a host was exposed in different periods of life. Therefore, each individual has a unique antibody serological profile, consisting of all the different antibodies that he or she carries. At birth, babies have in their blood plasma only the antibodies received from the mother; gradually, each will develop his or her own particular antibody profile through exposure to infectious agents and other antigens during life. APA determines the unique set of individual specific antibodies (ISA) by embedding a paper strip containing antigenic proteins into blood samples, which causes specific immune reactions by the antibodies present in the sample. The strip is then stained to reveal a unique ISA pattern of bands that identifies the donor of the sample. Several hospitals use APA to identify newborns and their respective mothers.
The Wyoming State Crime Laboratory has collaborated with APA researchers in testing APA specificity for forensic application in identifying contaminated and tainted samples. The laboratory received ten blood samples and proceeded to subject them to all possible conditions found in crime scenes: tainting, contaminating, mixing the samples with other human or animal blood samples, urine, or gasoline, and putting them in upholstery, car hoods, and side walks. Researchers also exposed some samples to a variety of temperatures, ranging from 68°F ( 56°C) to 140°F (60°C). The resulting 422 tainted samples were then returned to researchers for APA testing, and resulted in 91% correct identifications.
see also Amphetamines; Antibody; Antigen; Autopsy; Barbiturates; Blood; Bloodstain evidence; Commercial kits; DEA (Drug Enforcement Administration); Death, cause of; FBI crime laboratory; Fluids; Homogeneous enzyme immunoassay (EMIT); Illicit drugs; Immune system; Narcotic; Paternity evidence; Pathogens; PCR (polymerase chain reaction); Poison and antidote actions; Rape kit; RFLP (restriction fragment length polymorphism); Saliva; Semen and sperm; Serology; Toxicological analysis; Toxins; Vaccines.
se·rum / ˈsi(ə)rəm/ • n. (pl. se·ra / ˈsi(ə)rə/ or se·rums ) an amber-colored, protein-rich liquid that separates out when blood coagulates. ∎ the blood serum of an animal, used esp. to provide immunity to a pathogen or toxin by inoculation or as a diagnostic agent.
Blood serum is blood plasma without the fibrinogen. When blood clots, the fibrinogen is converted to fibrin, which is deposited in strands that trap the red cells and form the clot. The clear liquid that is exuded is the serum. See also blood clotting.