Toxicology is the science of poisons; when applied to medicolegal proceedings, the terms forensic toxicology or analytical toxicology are often used. A forensic toxicologist is concerned with the detection of drugs or poisons in samples and is capable of defending the result in a court of law. This distinction from an ordinary analytical toxicologist is important, as a conventional toxicologist is mainly concerned with the detection of substances, and may not understand the specific medicolegal requirements in forensic cases.
The process of conducting toxicology is similar to other analytical disciplines, in that sufficiently suitable analytical techniques need to be employed, which are appropriately validated. The conduct of suitable quality assurance is important to assure the analyst and clients of the quality of the result. These issues are discussed in this overview, while in other articles specific issues of techniques, specimens and interpretation are further discussed.

Applications of Forensic Toxicology

Forensic toxicology has a number of applications. Traditionally, it is used in death investigations. It provides clinicians with information of a possible drug taken in overdose, or authorities investigating a sudden death or poisoning with information on the possible substances(s) used. Ultimately toxicology testing results will assist the medical practitioner, coroner or medical examiner in establishing the evidence of drug use, or by refuting the use of relevant drugs.
Toxicology testing is also important in victims of crime, or in persons apprehended for a crime. Drugs may have been given by the assailant to reduce consciousness of the victim, such as in rape cases. These drugs include the benzodiazepines (e.g. Rohypnol) and y-hydroxybutyrate (GHB). Toxicology also establishes if any drug was used by the victim and which may have affected consciousness or behavior. Defendants arrested shortly after allegedly committing a violent crime may be under the influence of drugs. It is vital, therefore, that toxicology testing is conducted (on relevant specimens) to establish the extent of drug use, as allegations of drug use and its effect on intent or clinical state may be raised in legal proceedings.
Forensic toxicology is also used in employment drug testing and in human performance testing. The former category relates to the detection of drugs of abuse in persons in a place of employment, prior to being hired by an employer, or even a person in detention, such as in a prison. Human performance testing relates to the detection of drugs that might have increased (usually) performance in athletic events. This may even apply to animals such as horses. Specimens used in these cases are usually urine, although hair is increasingly used to provide a longer window of opportunity.

Initial Tests and Confirmation

The foremost goal in forensic toxicology is the need to provide a substantial proof of the presence of a substance(s). The use of conventional gas chromatography (GC), thin-layer chromatography (TLC) or high-performance liquid chromatography (HPLC) would not normally be sufficient to accept unequivocal proof of the presence of a chemical substance. Two or more independent tests are normally required, or the use of a more powerful analytical test, such as mass spectrometry (MS) is often preferred. Because of the need to perform a rigorous analysis, the analytical schema is often broken up into two steps. The identification stage is termed the screening or initial test, while the second analytical test is the confirmation process. The confirmation process often also provides a quantitative measure of how much substance was present in the sample, otherwise a separate test is required to quantify the amount of substance present in the specimen (see later). In all processes it is important that no analytical inconsistency appears, or a result may be invalidated (Fig. 1). For example, in the identification of codeine in a blood specimen, an immunoassay positive to opiates is expected to be positive when codeine is confirmed. The apparent detection of a drug in one analytical assay, but not in another, means that the drug was not confirmed, providing both assays are capable of detecting this drug.
Table 1 lists common techniques used in screening and confirmation assays. While MS is the preferred technique for confirmation of drugs and poisons, some substances display poor mass spectral definition. Compounds with base ions at mass:charge ratios of less than 100, or with common ions such as m/z 105 and with little or no ions in the higher mass range, are not recommended for confirmation by MS alone. Derivitization of a functional group to produce improved mass spectral properties can often be successful. Common derivatives include perfluoro-acyl esters, trimethyl silyl ethers, etc. Alternatively,reliance on other chromatographic procedures can provide adequate confirmation. It is important when using any chromatographic procedure (HPLC, GC, capillary electrophoresis (CE), etc.), that the retention time of the substance being identified matches with that of an authentic standard.
Identification, confirmation and quantification processes in forensic toxicology.
Figure 1 Identification, confirmation and quantification processes in forensic toxicology.

Table 1 Screening and confirmation techniques

Screening tests Confirmation tests
Immunoassays MS (LC, GC, CE)
Spectroscopy (UV, F, etc.) Second chromatographic test
AAS, colorimetric tests

AAS, atomic absorption spectroscopy; CD, conductivity detection; CE, capillary electrophoresis; DAD, photodiode array detector; ECD, electrochemical detector; F, fluorescence; FID, flame ionization detector; GC, gas chromatography; HPLC, high-performance liquid chromatography; ICP-MS, inductively coupled plasma MS; LC, liquid chromatography; MS, mass spectrometry; NPD, nitrogen-phosphorus detector; TD, thermionic detector; UV, ultraviolet.
Some apparent analytical inconsistencies may provide important forensic information. For example, if a result for opiates is negative in urine, but positive in blood, it is possible that heroin (which is rapidly metabolized to morphine) was administered shortly before death, and therefore metabolites had not yet been excreted. This situation is often found in heroin users suffering an acute sudden death, in whom substantial urinary excretion has not yet occurred.

Common Drugs and Poisons

The most common drugs and poisons are clearly the initial targets of any forensic toxicological analysis, particularly if no specific information is available to direct the investigation. The most common substances can be categorized as fitting into four classes: alcohol (ethanol), illicit drugs, licit (ethical) drugs, and the nondrug poisons. An example of the distribution of drugs in various types of coroner’s cases is shown in Table 2. These data are likely to be similar throughout developed countries.
Alcohol is the most frequent finding in many countries, and, when detected, can play an important role in any investigation because of its ability to depress the central nervous system (CNS). At best, alcohol will modify behavior, causing disinhibition and possible aggression; at worst, it can cause death, either by itself, or in combination with another drug.
Illicit drugs include the amphetamines, barbiturates, cocaine, heroin and other opiates, cannabis,phencyclidine, designer fentanyls and lysergic acid diethylamide (LSD). It should be borne in mind that some illicit drugs also have medical uses in some countries. Cocaine is used in some forms of facial and nasal surgery, amphetamine is used to treat narcolepsy and attention deficit syndrome, and cannabis is used (among other indications) to reduce nausea following chemotherapy.

Table 2 Incidence of drugs in various types of death (%)s

Type of death Ethanol Opioids” Benzodiazepines Stimulants0 Cannabis Antipsychotics
Natural death 15 13 9.4 1.4 2.3 2.6
Homicides 38 11 11 4.0 16 0
Drivers of motor vehicles 27 6.2 4.3 4.3 16 <1
Nondrug-related suicides 33 10 21 2.9 13 2.1
Licit drug deaths 40 41 59 3.2 8.0 13
Illicit drug deaths 35 96 61 7.1 38 5.4
All cases 27 20 20 3.1 12 3.2

Ethical drugs include the whole range of prescription and over-the-counter drugs used in the treatment of minor to major ailments. Those of most interest include the antidepressants, major tranquilizers, narcotics and other forms of pain relievers, anticonvulsants, etc. Since these drugs are widely prescribed, this is by far the most common drug category encountered in toxicology. Each country will have its own list of registered drugs, hence laboratories will need to consider these as a matter of priority over other members of a particular class available elsewhere. For example, most countries only have a relatively small number of benzodiazepines registered for medical use, whereas over 35 are available throughout the world. From time to time laboratories will be required to consider drugs not legally available in their own countries because of illicit supplies arriving in, or tourists visiting, their country.
The nondrug poisons include, most commonly, organophosphates and other pesticides, carbon monoxide, hydrogen cyanide and cyanide salts, and volatile substances (petrol, gas, kerosene, etc.). (Carbon monoxide and hydrogen cyanide are gases emitted by fires and are therefore frequently detected in victims of fire.) Other poisons include heavy metals (arsenic, mercury, thallium, etc.), plant-derived poisons (hyos-cine from belladonna, coniine from hemlock, etc.), strychnine, and toxins such as venoms. Performance-enhancing drugs such as the anabolic steroids may also be considered in some instances. Clearly this list is potentially unending, although some chemicals are more readily available to certain occupational groups than others and are dependent on national regulations within countries. In a review of 10 years of forensic cases from Victoria, Australia (Victorian Institute of Forensic Medicine toxicology laboratory 1989-1998), the distribution of unusual poisons was as shown in Table 3. Clearly, the distribution of unusual drugs and poisons will vary from country to country.

Scope of Testing Protocols

As the previous sections indicate, cases may involve a variety of ethical and illicit drugs, or unusual poisons. Worldwide experience also shows that forensic cases often involve more than one drug substance. A survey of drug-related deaths shows three or more drugs are present in more than 70% of cases. High rates of multiple drug use are also found in perpetrators and victims of violent crimes and suicides, and often also in accidents and road crashes.

Table 3 Incidence of poisons in coroners’ cases

Poison 10 year incidence11
Organophosphates 19
Butane and other hydrocarbons 12
Other pesticides/herbicides 12
Solvents (methanol, chloroform, etc.) 9
Strychnine 7
Potassium cyanide 6
Plant-derived poisons 5
Ethylene glycol 3
Heavy metals 2
Potassium 2
Others 11

It is also well known by forensic toxicologists that the information provided to the laboratory concerning possible drug used may not accord with what is actually detected. It is therefore strongly recommended that laboratories provide a systematic approach to their toxicology cases and include as wide a range of common ethical and illicit drugs as feasible. This approach is termed systematic toxicology analysis (STA). A laboratory using this approach would normally include a range of screening methods, often incorporating both chromatographic and immunological techniques. Drug classes such as alcohol, analgesics, opioid and nonopioid narcotics, amphetamines, antidepressants, benzodiazepines, barbiturates, cannabis, cocaine, major tranquilizers (antipsychotic drugs) and other CNS depressant drugs would be included.
The incorporation of a reasonably complete range of drugs in any testing protocol is important because many of these drugs are mood-altering, and can therefore affect behavior as well as affecting the health status of an individual. Persons using benzodiazepines, for example, will be further affected by cocaine, amphetamines, and use of other CNS depressant drugs. The toxic concentrations of drugs are also influenced by the presence of other potentially toxic drugs. For example, the fatal dose for heroin is affected by the concomitant use of alcohol and other CNS depressant drugs.


It is essential that the relevant specimens are taken whenever possible, as re-collection is rarely an option. The preferred specimens collected in forensic toxicology will, of course, depend on the nature of the case. In general, a blood specimen is a minimum requirement, although specimens such as urine can be useful for screening and to checkfor the use of drugs two or more days prior to sampling.
Hair can provide an even longer memory of drug intake, lasting up to several months, depending on the length of hair. Drugs are usually incorporated into the growing root and appear as a band in the hair shaft when it externalizes from the skin. This process can therefore provide a history of when exposure to a drug or poison has occurred. Most drugs and poisons are incorporated into hair, although the extent will depend on the physiochemical properties of the substance. Basic drugs are often found in higher concentrations than acidic drugs, and invariably the parent drug is present rather than metabolites. For example, cocaine and the heroin metabolite 6-monoacetylmorphine are more likely to be found in hair of cocaine and heroin users than their corresponding metabolites found in blood and urine (benzoylecgonine and morphine). Unfortunately, some drug will be absorbed into the hair from skin secretions adjacent to the hair follicles, and may even be incorporated from external contamination. Care and treatment of hair, hair color, such as washing, use of dyes and bleaches, etc., will also affect the concentration of drug in hair. Consequently, any interpretation of drug content in hair needs to take these factors into account. The advantages of other specimens are described elsewhere.
Courts and other legal processes usually require proof that the laboratory has taken all reasonable precautions against unwanted tampering or alteration of the evidence. This applies to specimens and to physical exhibits used by the laboratory in their toxicology investigations. (The term ‘exhibit’ applies both to specimens and physical items, such as tablets, syringes, etc.) Consequently, it is essential that the correct identifying details are recorded on the exhibit or specimen container, and an adequate record is kept of persons in possession of the exhibit(s). When couriers are used to transport exhibits, the exhibit must be adequately sealed to prevent unauthorized tampering.
Procedures are available to assist laboratories in establishing suitable chain-of-custody records.

General Techniques

The techniques available for the detection of drugs in specimens collected post mortem are essentially identical to those for specimens collected ante mortem. These range from commercial kit-based immunoassays (enzyme multiplied immunoassay technique (EMIT), fluorescence polarization immunoassay (FPIA), cloned enzyme donor immunoassay (CEDIA), radioimmunoassay (RIA), etc.), traditional TLC, to instrumental separation techniques, such as HPLC, GC and CE. MS is the definitive technique used to establish proof of structure of an unknown substance, and can be linked to GC, HPLC and more recently to CE.
The use of appropriate extraction techniques is critical to all analytical methods. Three main types of extractions are used: liquid-liquid, solid-phase and direct injection. Traditionally, liquid techniques, in which a blood or urine specimen is treated with a buffer of an appropriate pH followed by a solvent capable of partitioning the drug out of the matrix, have been favored. Solvents used include chloroform, diethyl ether, ethyl acetate, toluene, hexane, various alcohols and butyl chloride, and mixtures thereof.
The solvent is then isolated from the mixture and either cleaned up by another extraction process or evaporated to dryness.
Solid-phase techniques are becoming increasingly favored, as mixed phases offer the ability to extract substances of widely deferring polarity more readily than with liquid techniques. Often less solvent is used, or simple hydroalcoholic systems can be employed, rather than potentially volatile or inflammable solvents.
Direct injection techniques into either GC or HPLC instruments bypass the extraction step, and can offer a very rapid analytical process. In GC, solid-phase microextraction (SPE) is most commonly used, while HPLC tends to require the use of precolumns, which are back-flushed with the use of column switching valves.

Quality Assurance and Validation

An essential part of any form of toxicological testing is validation and quality assurance. It is important that the method used is appropriately validated; that is, it has been shown to accurately and precisely identify the substance(s) detectable, there is little or no interference (from other drugs or from the matrix) with the specimens used, and that a useful detection limit has been established. Moreover, it is essential that the method is rugged and will allow any suitably trained analyst to conduct the procedure and achieve the same results as another analyst. To achieve these aims, it will be necessary to test the method in the laboratory over several assays, with varying specimen quality, before claiming that a full validation has been conducted.
It is recommended that internal quality controls with each batch of samples be included to enable an internal check of the reliability of each assay. These controls contain known drugs, at known concentrations. Suitable acceptance criteria are required for these controls before results of unknown cases can be accepted and released to a client. Acceptance criteria vary, depending on the analyte and application. For example, blood alcohol estimations have acceptance criteria less than 5%, while postmortem blood procedures may be 10-20%. (Normally the coefficient of variation of the mean (CV) is calculated as a standard deviation divided by the mean of the result.)
An important feature of analytical assays in forensic toxicology is the use of internal standards. These are drugs of similar chemical and physical characteristics as the drug(s) being analyzed, and, when added at the start of the extraction procedure, provide an ability to negate the effects of variable or low recoveries from the matrix. Hence, even when recoveries are low, the ratios of analyte and drug are essentially the same as for situations of higher recovery. An ideal recovery marker is when the internal standard is a deuterated analog of the analyte. When deuterated internal standards are used, it may not be necessary to match the calibration standards with the same matrix as the unknown samples. It is important, however, that absolute recoveries are reasonable, i.e. at least over 30%. This ensures less variability between samples and optimizes the detection limit.
From time to time it will be important to run unknown samples prepared by another laboratory, or a person not directly involved in laboratory work, to establish proficiency. These are known as proficiency programs or quality assurance programs. These trials are often conducted with many other laboratories conducting similar work, and provide an independent assessment of the proficiency of the laboratory to detect (and quantify) specific drugs. The performance of the laboratory should be regularly assessed from these results, and any corrective action implemented, if appropriate. This process provides a measure of continuous improvement, an essential characteristic of any laboratory. There are a number of collaborative programs available throughout the world. The College of American Pathologists (CAP) organizes an excellent series of proficiency programs in forensic toxicology.
The international (TIAFT) and American (SOFT) societies of forensic toxicology provide guidelines on the conduct of analytical assays and quality assurance of assays.

Postmortem Artifacts in Analysis

The event of death imparts a number of special processes that affect the collection and analysis of specimens obtained at autopsy. These include postmortem redistribution, in which the concentration of a drug in blood has been affected by diffusion of the drug from neighboring tissue sites and organs, such as stomach contents. This is minimized, but not arrested, using peripheral blood from the femoral region. Even liver concentrations are affected by diffusion from intestinal contents or from incomplete circulation and distribution within the liver. Some drugs are metabolized after death – nitrazepam, flunitrazepam, heroin, aspirin, etc. Substances such as ethanol and cyanide may even be produced by bacterial processes in decomposing bodies.

Estimation of Dose

A common request from legal counsels and police is to estimate a dose from a blood or tissue concentration. This may relate to determining likely intent from an ingestion (or injection), or simply to rationalize the circumstances to specific amounts of drugs used.
Dose can be estimated from knowledge of the volume of distribution (Vd) of drug. The calculation multiplies the blood concentration by the Vd corrected for the body weight of the person. Unfortunately, this calculation assumes one Vd for all persons, and, importantly, assumes that equilibrium has been established at the time of drug ingestion. This is rarely the case in toxicology cases, as recent drug ingestion is common. The calculation also fails to account for unabsorbed drug (and excreted drug) and may be severely affected by postmortem processes.
The variation in blood concentration at a specified time from a standard dose of drug is well known in clinical pharmacology, even in controlled situations. Therefore, estimating dose is not recommended unless these factors are considered and a range of doses is computed. Occasionally, it may be possible to compare blood (and tissue) concentrations to other cases in which doses were known, or by measuring the body burden in several tissues, including muscle and fat. Analysis of gastric and intestinal drug content will assist in this process, and also provide information on the route and time of ingestion.

Interpretation of Toxicological Results

Interpretation of any toxicological result is complex. Consideration must be given to the circumstances of the case, and in particular what significance may be drawn from the toxicology. For example, the finding of a drug in potentially toxic concentrations in a person killed by a gunshot wound to the head cannot reasonably lead to the conclusion that drugs caused the death. On the other hand, the absence of an obvious anatomical cause of death will lead investigators to consider the role of any drug use. Considerations must include the chronicity of drug use, the age of the person, the health of the person (presence of heart, liver, kidney disease, etc.), the use of multiple substances, and even genetic factors that may lead to a reduced metabolism.

Problems in Court Testimony

Forensic toxicologists and other analysts called to give evidence in court should consider that much of their technical evidence is beyond the ready comprehension of lay people in juries, legal counsel and judges. Restricting their evidence to understandable language and simple concepts is highly recommended.
A further problem relates to an assumption often made by legal counsel (and indeed other parties) that a toxicological investigation was exhaustive and all drugs and poisons were excluded in the testing processes. Most toxicology performed is restricted to a few analytical tests for a range of ‘common drugs and poisons’, unless the client (e.g. pathologist or police) has made a request to examine for (additional) specific chemicals. Analysts should make courts aware of the actual testing conducted and provide a list of substances incorporated in the investigation. Importantly, advice on any limitations applied to the interpretation of the analytical results should be supplied, e.g. poor quality specimens, postmortem artifacts, etc. Above all, toxicologists must restrict their evidence to those areas in which they claim expertise. Stretching their expertise to assist the court can lead to incorrect or misleading evidence, and damage the reputation of the expert.

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