The World of Forensic Laboratory Testing

Forensic Pathology and Post-mortem Examinations (Autopsies)

Pathology involves the study of changes in the body caused by disease or injury. Forensic pathology involves the evaluation of pathology issues that arise in public forums such as coroner’s and criminal investigations, and in civil litigation. Forensic pathologists are experts in histopathology, which deals with structural alterations of the human body, and may become experts in related areas such as forensic toxicology, which is concerned with poisoning and the effects of poisons on the body.

During a post-mortem, the forensic pathologist first conducts a "gross examination." This involves detailed assessment and documentation of physical characteristics such as height, weight, colour of hair/eyes/skin, any physical markings (scars, tattoos, wounds, etc.), or any other physical anomalies. The post-mortem also includes dissection and measurement of the internal organs. From these tissues, samples may be taken for microscopic examination. Other samples may include blood, fluid from the eye (vitreous humour), urine, bile from the gallbladder, stomach contents, body hair, nail, and indeed solid organs such as liver, brain, and lung. Such samples may be used for a range of investiagtions including toxicology testing, DNA typing, blood grouping, culture for infectious disease, pregnancy testing, and various other chemical tests.

The fluid from the eye (vitreous humour) can be useful for helping ascertain a cause of death as it can be tested for a number of different substances, including alcohol, drugs and some other poisons, glucose, and electrolytes. The eyeball is a protected environment and thus this sample is usually available if death has followed severe trauma, for example. Vitreous humour is easy to collect and is quite useful in that some changes in concentration of substances that normally occur rapidly after death in blood, for example, take place less rapidly in vitreous humor. The results of vitreous humour analysis may aid in the diagnosis of certain conditions such as diabetic ketoacidosis, dehydration, and renal failure.

Forensic Toxicology

Forensic testing for exposure to poisons or drugs can be important in a criminal investigation. While knowledge of poisons spans several centuries, the ability to test for certain common poisons reliably in biological specimens was not available until the mid-19th century. Today, forensic toxicology routinely involves alcohol and drug testing.

Post-mortem toxicology
A forensic toxicologist might get involved in a post-mortem investigation if poisoning was not suspected initially as the cause of death. Murder by surreptitious administration of a poison still occurs. Toxicology testing is also undertaken in an attempt to ascertain if either a suspect, or the deceased might have been under the influence of drugs at the time of death. For example, drug testing may help assess whether a driver was impaired because he or she had drunk alcohol or taken illicit drugs prior to a fatal incident.

Forensic toxicologists may not only perform testing for drugs of abuse (including alcohol), but also prescription and other medication. Alcohol testing is routinely performed in nearly all traumatic deaths such as those occurring in motor vehicle accidents. Detection and measurement of drugs given in treatment may also be important . For example, measuring post-mortem blood concentrations of anticonvulsant medication in a patient known to suffer from epilepsy and who is thought to have died as a result of a seizure may help assess whether the deceased had been taking their medicine as prescribed prior to the fatal event.

Laboratory analysis of different types of sample from the deceased may first require separating the substance of interest from the body fluid or tissue sample before identifying any substance(s) present. The general rule used to be that two independent analytical methods should be used before reporting a result, but this is not always possible since the amount of sample available is limited, and in any event use of highly sensitive and selective mass spectroscopic methods can now give unequivocal results without the need for confirmatory analyses. However, the presence of a particular substance in a post-mortem sample does not necessarily mean it caused the death. The laboratory must if possible measure the concentration of the drug or poison in one or more samples, and then take into account all the available evidence in assessing the role of the substance in the death

Other uses of toxicological analysis
Whilst forensic toxicology often deals with post-mortem specimens, it may also involve issues related to the effects of drugs on performance or behaviour in non-fatal cases. Toxicology testing not only includes investigations of driving under the influence of alcohol or drugs, but also testing for illicit or performance-enhancing drugs in athletes, and testing for substance abuse in the workplace.

Illicit drug use continues to be a big medical and social problem in the UK. Employees in certain occupations (particularly the armed forces and the transport industry) may be required to be tested for illicit drug use before starting a new job, or as part of their contract of employment in order to maintain that job. Drug testing may be required when securing new insurance policies, for example. Prisoners may also be tested for illicit drug use, most frequently screening for the presence of amphetamines, marijuana (cannabis), cocaine, and opiates (morphine, heroin). Urine is still the most commonly used specimen in testing for illicit drug use, but blood, hair, sweat, and saliva may also be used.

For more detail on this, see the articles on Drugs of Abuse Testing and Emergency and Overdose Drug Testing.

Genetic Tests and DNA Typing

More recently, genetic testing has been added to the forensic pathologist's toolkit. Molecular testing of DNA from cells in a particular biological sample can be analysed to ascertain the unique genetic make-up of any one person. Each individual inherits a set of genes (two copies, one from each parent) that is unique and distinctive as a fingerprint. In clinical settings, genetic testing is used most often to detect chromosomal mutations that may be present to help diagnose disease, or used to ascertain a person's predisposition to develop a particular disease. In forensic settings, DNA typing analyses the genetic material from two or more sources and compares the genetic sequences to determine the likelihood that the two samples are from the same person, or from a relative of the person. This can be applied in "identity" and "paternity" testing, and may be used in civil or criminal proceedings.

DNA typing can be used to uniquely identify an individual and can be done on a very small amount of sample. Often, a swab of cells from the inside of the cheek (buccal swab), a drop of blood or a bloodstain, or a small amount of tissue can be enough to isolate a sample of DNA. Also, DNA is relatively stable and is not easily degraded by heat, cold, or drying. It is ideal for identity and parentage testing because an individual’s DNA does not change during their lifetime and is the same for all the cells in the body. Except for identical twins, DNA genotype is different in everyone.

In contrast to medical genetic testing, forensic DNA typing does not reveal anything about a person's health or medical history. The areas of the DNA sequence that are tested have no known ability to predict health status. DNA typing is not the same as the in-depth, highly complex, full genomic sequencing often heard about in the news.

As with other types of forensic testing, DNA typing must follow strict protocols for proper sample collection, maintaining the "chain of custody", and testing.

Identity testing
Identity testing compares DNA sequences from separate sources to ascertain if they are identical. The DNA found at a crime scene or on a victim is tested and compared to that of a suspect or to that of other suspects or convicted individuals held on a database. Testing can thus be used to match a suspect to a specific crime, link a suspect to several crimes (serial crime), link crimes with no suspect in common, or exonerate someone who has been falsely accused. DNA testing on blood, saliva, tissue, hair, or bone may also be used to ascertain the identity of individuals if no other information is available, as in the case of catastrophe victims, fragmented remains, isolated body parts, and badly decomposed bodies.

DNA sequences consist of chemical units called nucleotides that vary in size and make up a chain-like structure. In humans, DNA sequences are over 99 % similar in structure, yet the small percentage that is different makes each person unique. Labs look at these DNA sequences for matches between the presented evidence and suspects based on sequences of small segments of DNA at different locations on the person's total genetic makeup (genome). Generally, a match at thirteen sites confers confidence of identity "beyond a reasonable doubt." That's because a match at 13 sites is a very rare occurrence in different individuals. Because only one-tenth of a single percent of DNA differs from one person to the next, all of the locations used for forensic DNA typing vary widely between individuals. The possibility of two people with the same DNA profile (except for identical twins) is extremely remote.

In forensic testing, obtaining DNA profile information is valuable only if there are other profiles to compare it with The UK National DNA database is the largest reference database in the world. If an individual is convicted of a crime, his or her genetic fingerprint (a select list of DNA polymorphisms) is put into the database. DNA evidence from unsolved crimes is also entered, together (in England and Wales) with DNA profiles from those arrested, but not convicted. Using this system, matches in DNA profiles have helped secure convictions in thousands of crimes as well as in helping to exonerate individuals who were wrongly suspected of committing a crime.

The segments of DNA used to create a "fingerprint" are called "short tandem repeats" or STRs. These STRs do not represent genes, but regions that occur on stretches of DNA that lie between genes. Genetic information related to genes and inheritance patterns (e.g. medical genetic testing for disease risk) is not entered into the National DNA database and the system cannot identify a person's physical traits or genetic risk of disease based on STRs.

Paternity testing
DNA is also used to establish paternity or family relationships if this information is relevant to a criminal investigation or civil litigation, as in seeking child support. In cases where biological family relationships are at issue, DNA tests can either include or exclude a presumed parent, sibling, or other familial relationship that may exist using a mathematical estimation called a "paternity" or "relationship index." The probability of the relationship is estimated through a process that combines the likelihood of the genetic test results, the physical characteristics that are the expression of gene combinations (phenotype) of the parties involved (for example, eye or hair color), with the probability of other "non-genetic events," which can include information such as the location of the alleged parent at the time of conception, etc. The resulting parentage index and probability of relationship are generally admissible as evidence in court.

See the article "The Universe of Genetic Testing: The Basics, Identity Testing, and Parenting Testing."

Testing in Cases of Abuse

Sexual Abuse Cases
Routine testing in alleged sexual assault cases can include DNA testing in addition to tests for pregnancy and sexually transmitted infections (STIs) such as syphilis, hepatitis B and C, gonorrhoea, chlamydia, and HIV. When done within a few hours of the incident, these tests provide information about the victim's health prior to the alleged assault, not health status after the incident. Due to the lag time between the initial exposure and the technical ability to obtain a positive test result, some of these tests may be controversial. Testing may be repeated six weeks to six months after the incident to help determine if the incident resulted in pregnancy and/or infection.

If the victim doesn't remember events around the time of a sexual assault, samples from the victim may be tested for "date rape drugs", including flunitrazepam (RohypnolTM) and gamma-hydroxybutyrate (GHB). Other tests may include those for alcohol and drugs of abuse. Evidence of intoxication can be used to discredit the victim in court.

Child Abuse
In suspected child abuse cases, laboratory evidence can help ascertain if an underlying health problem may be the true reason for suspicious bleeding or bruising. If a child has numerous bruises yet no history of significant trauma, a panel of tests to exclude bleeding and clotting disorders like von Willebrand disease or another clotting factor deficiency (factor VIII and IX deficiencies) may be performed. Evidence of such a problem could exclude the possibility of child abuse.

When it is suspected that a child has been sexually abused, samples may be collected for testing that may include DNA tests and tests for sexually transmitted infections as described above. Detecting the presence of an STI may aid in evaluating whether sexual abuse had taken place.

Other laboratory tests that may be useful when evaluating tissue injuries, such as abdominal, internal organ injuries, include liver function tests (alanine aminotranferase and aspartate aminotransferase), pancreas tests (amylase and lipase), and testing for blood in urine. A positive stool test for blood (guaiac test, faecal occult blood test, FOBT) can point to rectal bleeding caused by abdominal or anal trauma.

Reality of Forensic Testing

While TV has brought much attention to the laboratory profession, it often does not reflect the reality of what takes place. Because of the complexities of forensic testing, few if any laboratories can do all the types of testing that may be required. Not all laboratories provide the comprehensive testing menu that may be needed when looking for specific genetic markers. Even certain blood tests may have to be referred to a "specialty" or reference laboratory. The collection of samples, their preparation for testing, performing the tests, and evaluating all the results takes time and money to complete. While technology has greatly advanced forensic science over the past decade, many limitations still remain.