The Microscopic Slide.

A Potential DNA Reservoir

Detectives of the Baltimore County, Maryland, Police Department began pursuing a lead in a sexual-assault homicide case, which had remained unsolved for 19 years. In an effort to identify DNA evidence from the victim, officers requested the microscopic slides, made during the autopsy, from the chief medical examiner's office. Because the process of extracting DNA results in the loss of cellular material on the slide, the medical examiner photographed and delivered the slides, through the proper chain of custody, to the Baltimore County Police Department, who, subsequently, delivered them to a nationally recognized DNA laboratory. In turn, the laboratory staff extracted sufficient DNA from the slides to produce a satisfactory DNA profile to assist in solving the case.

DNA, also known as a "genetic fingerprint," exists in biological materials (e.g., blood, semen, skin cells, bone, saliva, and perspiration). Individuals leave DNA evidence on many items, such as cigarette butts, facial tissues, and eyeglasses. Law enforcement officers only need a few cells from an individual to recover DNA.

DNA testing has identified perpetrators years after they have committed a crime. [1] Evidence retained in adjudicated cases may exonerate an individual accused wrongly and identify the true perpetrator. DNA material from homicide investigation scenes provides valuable evidence. Therefore, law enforcement officers must inform investigators when DNA material exists and obtain, secure, and preserve the DNA evidence properly.

Obtaining and Securing DNA Material

Evidence obtained decades ago can contain forensically valuable DNA material. However, environmental factors that exist at a crime scene (e.g., heat, sunlight, moisture, bacteria, and mold [2]) can affect DNA and render it useless. If not properly obtained and secured, DNA evidence can degrade. Officers should ensure that DNA evidence remains dry and at room temperature and secured in paper bags or envelopes, sealed and labeled properly. Officers should never place DNA evidence in plastic containers, direct sunlight, or the trunk of a car.

When identifying, obtaining, and handling DNA evidence, officers must ensure that they do not contaminate the material. Contamination occurs when the evidence comes in contact with another individual's body fluids through actions, such as sneezing, coughing, or touching. Contamination becomes a critical issue because today, laboratories use the polymerase chain reaction (PCR) technique to replicate DNA. This technique involves extracting DNA from a small evidence sample and then replicating it through a complex operation of repeated heating and cooling cycles and exposure to an enzyme. Because each cycle doubles the quantity of DNA, experts can replicate the original extraction several million times within a short period. [3] But, the PCR process cannot distinguish between DNA from a suspect and another source. Therefore, any substantial contamination to the DNA material will result in a confusing result.

Biological material may contain hazardous pathogens that can cause potentially lethal diseases. Law enforcement officers always should contact their laboratory personnel or evidence collection technicians when they have questions about obtaining or securing DNA evidence. After officers follow careful procedures to obtain and secure DNA material, they must take appropriate steps to preserve and process DNA evidence correctly as well.

Preserving and Processing DNA Evidence

Today, investigators can reexamine evidence from preserved DNA samples using newer and more sophisticated technologies unavailable when they obtained the evidence. For DNA evidence to meet legal and scientific requirements for admissibility in court, investigators must follow certain procedures to preserve the DNA evidence properly and to prevent decomposition and deterioration. First, as with any evidence, agencies must maintain a proper chain of custody when handling DNA material. When transferring DNA evidence by direct or indirect means, the material remains on surfaces by absorption or adherence. [4] Liquid biological evidence (e.g., blood, semen, and urine) absorbs into surfaces, and solid biological evidence (e.g., hair, bones, and teeth) adheres to surfaces. The proper way to preserve DNA evidence depends on the liquid or solid state and the condition of the evidence. [5]

Medical examiners and forensic pathologists permanently maintain microscopic slides that contain biological evidence from a swab of a body cavity taken at the time of an autopsy. Experts have extracted DNA material successfully from medical examiners' offices, hospitals, or rape kits. [6] The critical number of sperm on a slide for successful DNA testing appears to exist in the range of at least 100. [7] For example, after experts properly fix, process, and stain a specimen of sperm on a microscopic slide, this evidence retains its DNA characteristics. But, the specimen must remain sequestered from exposure to environmental effects, which could result in decomposition and deterioration. If the material transferred to the slide contains a sufficient number of sperm, experts can use the PCR technique to identify the male's DNA profile, regardless of the time interval from when they made the slide.

Smear preparation of DNA material, which includes drying by air, immersing in several liquids, and mounting on various kinds of material, does not degrade the genetic material. Similarly, DNA tissues preserved in paraffin blocks, such as surgical specimens, often remain suitable for PCR testing. Research over the past 10 years has demonstrated that tissues stored in formalin/formaldehyde may be suitable for DNA testing using PCR. [8]

Advancing Technology

Recent efforts at the local, state, and federal levels of government to employ advanced information system technologies emphasize the importance of preserving DNA samples. For example, the FBI created a National DNA Indexing System (NDIS) that uses computer applications and forensic science practices to match DNA samples across the country. [9] In this effort, scientists analyze DNA samples and store profiles of the biological evidence in a computer database. Public forensic laboratories throughout the United States use these profiles to exchange and compare DNA profiles electronically. [10] If an electronic match results, the FBI notifies the submitting jurisdiction and further investigation ensues. Additionally, the design of the FBI's Combined DNA Indexing System (CODIS) allows state and local law enforcement crime laboratories to exchange and compare DNA profiles electronically as well. DNA samples in CODIS represent both DNA profiles from convicted felons and evidence from unsolved cases.

In many instances, administrative backlogs, such as processing and analyzing DNA and legal restrictions on the collection of DNA from offenders, have prevented these efforts from becoming more successful. The processing of suspected DNA evidence usually takes from 3 to 7 days. However, this time frame depends on the volume of requests, the availability of trained personnel, and the number of laboratories equipped to conduct the test. Regardless of the backlogs, the advances in DNA analysis technology and information systems technology have combined to provide a powerful law enforcement tool in solving some crimes.

Conclusion

Microscopic slides serve as valuable reservoirs of key evidence that investigators can use in DNA testing years after the commission of a crime. Efforts by law enforcement officers and forensic pathologists to comb crime scenes for possible DNA material remain crucial to solving cases. Investigators must remember to obtain, secure, and preserve DNA material properly, or the evidence will not meet admissibility requirements in court. New technology in DNA analysis allows crime laboratories to exchange and compare DNA profiles, which can help solve crimes committed many years prior to the DNA testing. Biological evidence within police departments, maintained with a proper chain of custody, can assist in the successful identification of the perpetrator, as well as in the exoneration of a wrongly convicted person.

Dr. Smialek, the chief medical examiner for Maryland, heads the Forensic Pathology Division, Department of Pathology, School of Medicine, at the University of Maryland in College Park.

Dr. Word serves as a forensic laboratory deputy director for a private firm in Germantown, Maryland.

Mr. Westveer is a violent crime specialist in The Behavioral Science Unit at the FBI Academy.

Endnotes

(1.) U.S. Department of Justice, Convicted by Juries, Exonerated by Science: Case Studies in the Use of DNA Evidence to Establish Innocence After Trial, Research Report, (Washington, DC, June 1996) and "Oops! We Forgot to Put it in the Refrigerator: DNA Identification and the State's Duty to Preserve Evidence," The John Marshall Law Review, 25:809-836, 1992.

(2.) U.S. Department of Justice, National Institute of Justice, National Commission on the Future of DNA Evidence, What Every Law Enforcement Officer Should Know About DNA Evidence, (Washington, DC, 1999).

(3.) U.S. Department of Justice, Office of Justice Programs, National Institute of Justice, Automated DNA Typing: Method of the Future, February 1997; available from http://www.ncjrs.org/txtfiles/13102-9.txt.; accessed November 10, 1999.

(4.) U.S. Department of Justice, Federal Bureau of Investigation, Handbook of Forensic Sciences: Evidence Examinations, available from http://www.jbi.gov/programs/lab/handbook/examsdna.htm.; accessed September 11, 2000. For questions concerning DNA evidence, contact the FBI Laboratory at 202-324-5436.

(5.) Ibid.

(6.) Based on a study by one of the authors who recorded several successful extractions of DNA material.

(7.) Ibid.

(8.) D. Shibata, T. Namiki, R. Huguchi, "Identification of a Mislabeled Fixed Specimen by DNA Analysis, The Journal of Surgical Pathology 14(1): 1076-1078, 1990.

(9.) U.S. Department of Justice, Federal Bureau of Investigation, "The National DNA Indexing System," FBI National Press Release, (Washington, DC, October 13, 1998).

(10.) U.S. Department of Justice, Federal Bureau of Investigation, Ensuring Public Safety and National Security Under the Rule of Law: A Report to the American People on the Work of the FBI, 1993-1998, available from www.fbi.gov/library5-year/5-year_rpt.htm; accessed September 11, 2000.

Case Scenarios

Case #1

In 1982, the Naval Criminal Investigative Service opened a homicide case that involved the rape and murder of a woman, placed in the trunk of a car and pushed into a bay. Investigators recovered little evidence from the scene except some DNA material. This case remained unsolved until 1995 when, with support from new analysis of the 14-year-old DNA, investigators reexamined the case and identified a suspect. Officers arrested and charged the suspect with the rape and murder of the female victim.

Case #2

The Richmond, Virginia, Police Department investigated a homicide with little evidence and few leads. However, investigators secured blood and semen samples from the scene. Officers identified a suspect, but DNA analysis exonerated him. Four years later, investigators examined these samples again and submitted them to their state's DNA indexing system. The DNA samples from the homicide matched samples from a 20-year-old man who had a criminal history of rape and murder. Officers charged the suspect with the 4-year-old homicide.

Contamination Precautions

To prevent contamination of DNA evidence, officers should--

* wear gloves and change them often;

* use disposable instruments or clean them thoroughly before and after handling each sample;

* avoid touching the area where DNA evidence may exist;

* avoid talking, sneezing, and coughing over evidence;

* air-dry evidence thoroughly before packaging; and

* put evidence into new paper bags or envelopes, not into plastic bags, and never use staples.

Source: U.S. Department of Justice, National Institute of Justice, National Commission on the Future of DNA Evidence, What Every Law Enforcement Officer Should Know About DNA Evidence, (Washington, DC, 1999).