Serology and DNA Evidence

Rosalind Bowman

Teri J. Labbe

Kimberly A. Mullings

Once a crime has been committed, law enforcement needs to determine what has happened (when, why, and how) and identify all parties involved. These questions can be answered by investigating closely, examining the crime scene, and analyzing the evidence recorded. Most, if not all, of these concerns can be resolved through a timely, methodical, and meticulous examination of the crime scene evidence.

Evidence, as defined by Encarta, is “the means by which disputed facts are proved to be true or untrue in any trial before a court of law.” Physical evidence, as defined by Saferstein (1), is “any and all objects that can establish that a crime has been committed or can provide a link between a crime and its victim or a crime and its perpetrator.” Testimonial evidence, obtained by skillfully interviewing witnesses to the crime and recording their eyewitness accounts, must be weighed for objectivity. It is of utmost importance that what could be physical evidence is recognized, documented, collected, and handled in the appropriate manner to ensure its integrity for forensic examination.

Forensic Serology

An investigation into a charge of sexual assault begins with examination of the primary crime scene, that is, the body of the victim. Specialized sexual assault evidence-collection kits are used by sexual assault forensic nurse examiners to provide the samples for forensic serologic analysis.

Forensic serology is the science that deals with the identification, isolation, and characterization of blood and bodily fluids from the victim, the suspect, and the environment of the crime scene for the purpose of comparison to known standards. These biologic substances, along with other trace evidence samples of hairs and fibers, may be transferred readily between the victim, the suspect, and the environment of the crime scene during the commission of the sexual assault. Sources of biologic evidence—blood, semen, saliva, urine, hair, teeth, bone, and tissue—give us deoxyribonucleic acid (DNA). Sources of trace evidence (e.g., fibers, hair, fur, paint, glass, gunshot residue, soil, explosives, and paper) must have known samples with which they can be compared.

Biologic evidence can be found in several common sites. Blood might be spattered about a scene in a pattern or found on a weapon, on clothing, or on other articles. Semen can be found in any body cavity (i.e., vagina, rectum, mouth, and colostomy opening), on clothes (particularly undergarments), on furniture, and on carpets and floors. Saliva can be found in bite marks, on cigarette butts, and on any part of the body that was licked or sucked. Other places to find saliva/DNA samples are envelopes, stamps, gum, sunglasses, phones, discarded food, and drinking containers. Epithelial cells on contact items such as gloves, caps, masks, lip
balm, cosmetics, contact lens, earplugs, gags, cords, or shoelaces can also harbor DNA.

Serologic Testing

The forensic serologist evaluating the evidence from any crime scene has an advanced degree in the sciences and has completed on-the-job training programs, which are supplemented with continued education in specialized course work offered at the Federal Bureau of Investigation (FBI) and other government and private forensic laboratories.

The serologic examination of the sexual assault evidence-collection kit begins upon receipt of the kit. The examiner must ensure that the kit is submitted in compliance with laboratory protocols: evidence must be sealed properly and identified appropriately in regard to its attached laboratory request forms. If these conditions are met, the evidence chain of custody has been established. Acceptance of the kit for examination is documented by the signature of the examiner and the date on the evidence chain of custody form.

The kit is opened and its contents are inventoried. The individual envelopes containing samples to be tested are identified with information from the victim and are labeled as to the source: vaginal, oral, anal, and defined miscellaneous. A document that accompanies the kit indicates the results of the physical examination as well as the victim’s testimonial evidence as told to the sexual assault nurse examiner.

Semen

The primary goal of the forensic serologic examination is to detect the presence of semen, as evidence of sexual activity, on the samples collected. The procedures used in this examination are predicated upon the biochemical interactions of the immune system: antigen to antibody and enzyme to substrate.

Semen is composed of seminal fluid and spermatozoa. Seminal fluid is a highly viscous fluid produced from the secretions of Cowper’s gland, the prostate gland, and the seminal vesicles. It serves as both a carrier of the spermatozoa and a buffer of protection from the acidic environment of the vagina.

An alternate light source (e.g., an argon ion laser light or a mini crime scene scope, which uses filters for wavelengths of light in the range of 390–540 nanometers) is used by the serologist to locate suspected semen and saliva stains to be tested. Exposure to certain wavelengths of light excite flavins in the biologic stain, resulting in fluorescence against the substrate background.

Small sections of the samples are chemically tested for the presence of acid phosphatase, an enzyme found throughout the body, but most highly concentrated in secretions from the prostate gland. The results of this chemical testing are compared to a known semen standard. Samples that test positive for the presence of acid phosphatase are tested further by microscopic examination to visualize spermatozoa.

The average ejaculate contains 50 to 150 million sperm cells per mL, in addition to white blood cells and epithelial cells. Specialized staining allows the serologist to visualize the distinct regions of the spermatozoa: head or cell body, neck, midpiece, and tail section. Nucleated epithelial cells are also easily visualized against the other cellular background.

The serologist documents the detection of semen on the evidence samples that test positive for acid phosphatase as an indication of seminal fluid and that contain spermatozoa. Sections of these samples are retained for further characterization of the semen stain.

In some cases, the serologist is unable to visualize spermatozoa after detecting seminal fluid, especially if the victim bathed or douched before the medical examination. If the perpetrator’s sperm is not found, he may have had a vasectomy or he may be a low sperm producer.

These samples are then tested for the presence of the prostate-specific antigen (P30), a large protein known to come from the prostate gland. The presence of semen is confirmed by the serologist in samples that test positive with commercial reagents containing anti-P30 antibodies through comparison with a known semen standard. Samples that test positive for the presence of both acid phosphatase and P30 are retained for further characterization through DNA analysis. Samples that do not test positive for either are not tested further. Additional testing for the presence of blood and saliva may be performed by the serologist on samples from the sexual assault evidence-collection kit.

Blood

Blood is a complex mixture of cells, enzymes, proteins, and other biochemical substances, which are used by the serologist for identification of an individual or animal. For conventional serologic and DNA analysis in sexual assault cases, a liquid whole blood sample is usually necessary. For forensic analysis, the collection of whole blood samples in an ethylene diaminetetracetate (EDTA) Vacutainer (lavender capped) is recommended. Whole blood samples must be refrigerated.

The use of bloodstain evidence provides an efficient means of collecting, transporting, and storing DNA blood samples. Most programs use cotton or filter paper cards of various thickness to collect bloodstain specimens from survivors. Unfortunately, as a result of deterioration of the specimen, conventional filter paper can produce inconsistent results after storage for as little as 6 months at ambient temperature (2).

A new option, the FTA-coated paper (Whatman plc, Middlesex, United Kingdom), was developed by Dr. Leigh Burgoyne and Dr. Peter Hallsworth at Flinders University of South Australia, in response to the need to transport specimens long distances (3). This paper is chemically impregnated to kill existing microorganisms in the blood sample collected and to trap DNA in a matrix, thereby stopping bacterial and environmental degradation of the specimen. It kills human blood-borne pathogens and prevents microbial decay of nonsterile blood and other biologic samples. It also inhibits nonmicrobial decay of DNA information, such as caused by oxidation, ultraviolet light, and an acidic environment. Ambient storage, even at high humidity, is possible for years without deterioration of the specimens.

A small section of a suspected bloodstained sample is subjected to a presumptive chemical test for the presence of blood. The results of the chemical testing are compared to a known blood standard. Evidence samples that test positive are further tested to confirm the presence of blood through microscopic visualization of heme-containing crystalline structures or with a commercial reagent kit containing human hemoglobin antibodies. Sections of these samples are retained for further characterization of the bloodstain. An antihuman hemoglobin reagent (antibodies to human serum produced by injection of human serum into an animal [rabbit or goat]) is used to demonstrate the presence of blood (hemoglobin) and the origin of the blood as human (4). Buccal cells have become the current option for collecting DNA standards from individuals for comparison with evidence. Buccal swabs provide the same genetic information as blood, while being less invasive when collected and less of a potential hazard to the people who handle the samples.

Saliva

Saliva is a mixture of water, mucus (cells, salts, glycoproteins), and the digestive enzyme salivary α-amylase. Small sections of suspected saliva stained samples are chemically tested for the presence of salivary α-amylase. The results of the chemical testing are compared to a known saliva standard. Specialized staining allows the serologist to visualize nucleated epithelial cells. Sections of these samples are retained for further characterization of the saliva stain.

Other Physical Evidence

Additional evidence samples for serologic examination may be recovered from the clothing of the victim and/or suspect or from the crime scene environment. The soil on a suspect’s clothing and shoes may match the soil recovered or deposited at the crime scene. Fibrous items such as bedding, small rugs, and pillows can be used as sources of known fibers. Hair samples (head and pubic hair) are most often used for their intrinsic value in sexual assault cases because they possess identifiable characteristics regarding the origin from the body, racial group, and the manner in which the hair was removed. However, hair varies from one person to another within the same racial group. Hair may be recovered as individual shafts, portions of shafts, or as shafts with intact roots. Hair may be recovered from the scene of the incident; from objects at the scene; from clothing of the victim or suspect; or from a vehicle, weapon, or tool used in the crime. Hair found at a crime scene may corroborate the presence of the offender at the scene. In addition, the condition of the root may indicate whether hair was pulled out or naturally shed.

Documentation of Results

Upon completion of these examinations, the serologist uses an official laboratory report to document the results of all tests performed and which samples were consumed in analysis or retained for further characterization. The samples that test positive and are retained for further characterization through DNA analysis are packaged according to protocol. The contents of the sexual assault evidence-collection kit are returned to the kit, and the kit is properly resealed. The kit is released by the serologist into the custody of the evidence technician (with documentation on the evidence chain of custody form). The technician has the responsibility to transfer evidence to the evidence control section, where it remains in storage until presentation in court or until it is destroyed.

DNA Analysis

In the past, further characterization of serologic samples positive for blood, seminal fluid, semen, and saliva consisted of ABO blood grouping and enzyme assays for erythrocyte acid phosphatase and/or phosphoglucomutase. These tests were time consuming and labor intensive and were of little value regarding the source of the evidence sample (is it from the victim or the suspect?). Scientific research has advanced analytic technology so that the DNA molecule can now be used as the standard for characterization of all biologic samples.

Forensic DNA testing was first demonstrated successfully through the use of “DNA fingerprinting,” as developed by Alec Jeffreys in the sexual assault and murder case involving Colin Pitchfork. Jeffreys demonstrated that useful amounts of DNA could be recovered from old bloodstains and that forensic testing methods could be used for personal identification by comparing a known blood standard source to that of unknown evidence samples.
Restriction fragment length polymorphism (RFLP) DNA testing required high-molecular-weight and intact samples free from degradation. The usefulness of this procedure is limited because forensic samples tend to be at least somewhat degraded and yield only minute and fragmented segments of DNA. Short tandem repeat (STR) DNA testing was developed as a better alternative to address the requirements for characterization of samples typical of crime scene evidence.

Only gold members can continue reading. Log In or Register to continue