Medical Pharmacology Topics   

Toxicology: Forensics

Forensic toxicology is mostly concerned with alcohol and other substances of abuse, like other CNS depressants, stimulants, hallucinogens, inhalants, and narcotic analgesics.

There are two types of alcohol beverages: non-distilled and distilled. Non-distilled beverages like wine and beer are prepared by fermentation and have a maximum alcohol concentration of 14% by volume. Distilled beverages include liquors and wiskeys made by distilling wine or beer and may have up to 100% alcohol by volume (alcohol concentration x 2 = proof, i.e. 100 proof = 50% alcohol).

Alcohol Pharmacokinetics

About 20% of the ingested alcohol can be absorbed directly from the stomach. The upper intestine normally absorbs about 80% of the ingested alcohol in less than an hour. The lower intestine and lower bowel readily absorb alcohol, but most of it is absorbed before reaching the lower intestine. Alcohol has not been demostrated in the blood as a result of absorption through the skin or urinary bladder. If it is absorbed thru the skin, the rate of absorption is lower than the rate of metabolism.

Peak blood alcohol concentration (BAC) levels are reached 30-40 minutes after absorption. Foods will delay the absorption of alcohol by lowering the peak BAC.

Since alcohol is water-soluble, it will distribute throughout the body, achieving higher concentrations in the water-rich tisues and organs. Uniform distribution is complete 30 minutes to 1 hour after absorption. The average water content is 68% in males and 55% in females. If one gram of alcohol per kilogram of body weight is unifomly distributed, the concentration in the entire body would be 100 mg% , as described by Widmark's r (rho) factor:

mg% = milligrams of ethyl alcohol in 100 milliliters of blood

Most of the absorbed alcohol is metabolized by the body (90-95%). The rest is excreted in urine, sweat, tears, milk and breath. The body gets rid of alcohol at a relative constant rate known as Widmark's b factor: 15 mg% per hour (0.015% per hour). This is an average rate that may vary among individuals in a rage of 0.010% - 0.030%, depending on the individual's metabolic enzyme status. Experience drinkers will metabolize/eliminate alcohol faster than others.

Alcohol Physiological Effects and Testing

There are four primary types of alcohol induced impairment: loss of judgement and self-control, impaired vission and hearing, clumnsiness of voluntary muscles, and decreased awareness of the surroundings.

Alcohol will leave the blood as it passes through th lungs and become part of the expired breath. It distributes between blood and breath from the deepest part of the lung according to Henry's Law: in a close container, at a given temperature and pressure, a material in solution will be in equilibrium with the air in the space above. Therefore, there is a ratio between the concentration of alcohol in the blood and that in breath (alveolar air). This ratio is about 2100:1, i.e. 2100 mL (2.1 L) of alveolar air contain the same amout of alcohol as 1 mL of blood.

Since alcohol distribute to water, the average ratios in body fluids as compared to blood are also known: saliva/blood = 1.10, serum/blood = 1.19, and urine/blood = 1.34. Alcohol presense in urine is not indicative of impairment, but provides a rough estimate of recent use. Only blood levels are indicative of impaiment. In the case of vehicle accidents, when there are little remains and is imposible to test blood, a forensic investigator may sample the eye's vitrous humor to determine blood alcohol concentration at the time of death.

Alcohol levels may be measured using different chemical techniques: wet chemistry (colorimetric, not used any more), electrochemical (hand-held units), infrared spectroscopy, and gas chromatography (preferred forensic lab method, most acurate). Regarding DUI enforcement, several tests may be used: field sobriety test (behavioural test), roadside chemical breath test, evidentiary breath test (infrared spectroscopy ?), and forensic blood test (gas chromatography). In most jurisdictions, 0.08% BAC is indicative of impairment in driving performance of a motor vehicle. Lower limits are enforced for airplane pilots and minors (technically 0.0 % BAC in both cases, but alowances are made for medical, higiene-related or other natural exposures to alcohol).

Other Substantces of Abuse

Stimulant drugs of abuse include amphetamine, methamphetamine and cocaine. In general, they increase heart rate, respiration and blood pressure. Behavioural symptoms include restlessness, talkativeness, euphoria, confusion and insomnia.

Cocaine's onset of symptoms occur within 15-30 second of administration and lasts for 60-90 minutes, and is detectable in the urine for 2-4 days and in the blood for 4-8 hours. Amphetamines take 30-40 minutes to show an effect that may last 2-4 hours, and are detectable in the urine for 1-2 days, and in blood for 6-12 hours.

The hallucinogens like mescaline, psilocybin, peyote and LSD will produce anxiety, exhilaration, hallucinations, patranoia, incoordination and dificulty speaking. Thei effect is seen 5-10 minutes after administration and may last 8-12 hours. They are detectable in urine for 2-3 days and in blood for 6-12 hours.

Marijuana (delta-9-tetrahydrocannabinol, THC) impairs attention and the ability to performe tasks. Symptoms include bloodshot eyes, memory loss, loss of inhibitions, and limited attention. Effects are seen 10 seconds after inhalation and may last 2-4 hours. Since it is very lipid soluble and chronic use will lead to storage in fat, marijuana detection is variable according to total exposure. The use of 1-2 joints is detectable in urine 2-3 day, and a similar amount ingested orally is detected in urine for 1-5 days. In a chronic user, marijuana may be detected in urine 20-30 days after las exposure. Marijuana is detectable in blood 4-8 hours after exposure.

Inhalant include toluene, methyl ethyl ketone, TCE, nitrous oxide, ether, chlorophorm, and gasoline. Symptoms include confusion, slurred speech, dizziness, drowsiness and disorientation. The effects are felt sseconds after administration andmay last 1-2 hours. They are not detectable in the urine and are detectable in blood only for a few minutes, or up to 1-2 hours by gas chromatography.

The narcotic analgesics are the opiates: morphine, codeine, heroine and methadone. Symptoms nclude droopy eyelids, dry mouth, depressed reflexes, euphoria, nausea, sedation and respiratory depression. Effects are seen 15-30 minutes after administration and may last 4-6 hours (12-24 hours with methadone). They are detectable in urine for 2-3 days and in blood fpor 4-12 hours.

Other than alcohol, CNS depredsants of abuse include barbituates and benzodiazepines. Intoxication with this egents mimics alcohol intoxication. Other symptoms include sedation, slurred speech, lethargy, ataxia and nystagmus. Effects are seen 15-30 minutes after administration, and dependiong on the agent may last less than 10 hours, 10-24 hours or more than 24 hours. They are detectable oin the urine for 2-3 days and in blood for 6-24 hours.

There are two types of forensic tests to determine the use of controlled substances: screening tests and confirmatory tests. Screening tests are qualitative, designed to rapidly distinguish negative specimens from presumably positive specimens. Confirmatory tests are quantitative, and use different chemical principles than the screening tests to confirm the presence of drug with greater sensitivity and specificity. The most generally used specimens for drug testing are usine, serum (medical/diagnostic use) and whole blood. Saliva and har specimens may be used experimentally. All sample must follow a proper chain of custody, a procedure that documents all individuals that handled the specimen, demonstrates lack of tampering and documents that the speciment belongs to a specific subject.

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