From http://www.drugs.com/enc/toxicology_screen.html

Toxicology screen

Definition
These are various tests to evaluate the type (and roughly measure the amount) of legal and illegal drugs a person has taken.

Alternative Names
Barbiturates - screen;
Benzodiazepines - screen;
Amphetamines - screen;
Analgesics - screen;
Antidepressants - screen;
Narcotics - screen;
Methanol - screen;
Phenothiazines - screen;
Isopropanol (rubbing alcohol) - screen;
Drug abuse screen;
Blood alcohol test

Why the Test is Performed
This test can be used to evaluate possible accidental or intentional overdose or poisoning, such as when there is a need to evaluate the type and amount of legal and illegal drugs used by a person. The test can be performed to determine the cause of acute drug toxicity, to monitor drug dependency, and to determine the presence of substances in the body (for medical and/or legal purposes).
See also: Drug Abuse First Aid.

If the test is used as a drug screen there is a finite amount of time after ingestion that the drug or any of its metabolites can be detected:

  • Cocaine
    2 to 4 days; up to 10 to 22 days with heavy use
  • Amphetamines
    24 to 48 hours
  • >>>>>>
    1 to 2 days
  • Morphine
    1 to 2 days
  • Phencyclidine (PCP)
    1 to 8 days
  • Alcohol
    3 to 10 hours
  • Benzodiazepines
    up to 6 weeks with high level use
  • Hydromorphone
    1 to 2 days
  • Tetrahydrocannabinol (THC)
    6 to 11 weeks with heavy use
  • Propoxyphene
    6 to 48 hours
  • Methadone
    2 to 3 days
  • Codeine
    1 to 2 days
  • Barbiturates
    up to 6 weeks


How the Test is Performed
Toxicology screening is most often performed on blood or urine (the specimens of choice) but can be performed on gastric contents (vomit or lavage fluids) if performed soon after the substance is ingested. Nails or hair can be tested for arsenic and mercury.

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Urine sampling can be a random sample (you are asked to urinate into a container). In some circumstances, you may need to obtain the urine sample in the presence of the nurse or technician (to verify that the urine sample came from you and was not tampered with -- see special considerations).

How to Prepare for the Test
There is no special preparation; this test is often performed as an emergency test. Inform the health care provider of any prescription and over-the-counter medications you have taken, including the amount and time of ingestion.

How the Test Will Feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

If a urine sample is used, it involves only normal urination and there is no discomfort.

Risks
The risks associated with having blood drawn are:

excessive bleeding
fainting or feeling lightheaded
hematoma (blood accumulating under the skin)
infection (a slight risk any time the skin is broken)
multiple punctures to locate veins

Considerations
Commonly found substances on a toxicology screen include:

alcohol (ethanol) -- "drinking" alcohol (see Alcohol and diet)
amphetamines
benzodiazepines
antidepressants
barbiturates and hypnotics
isopropanol -- isopropyl alcohol, rubbing alcohol (toxic) (see isopropanol overdose)
methanol -- methyl alcohol, found in antifreeze and other substances (toxic)
narcotics
non-narcotic analgesics
acetaminophen - oral
anti-inflammatory analgesics - oral
phenothiazines (antipsychotic or tranquilizing medications)
prescription medications, any type
This test is sometimes part of an investigation for drug use or abuse; special consents, handling and labeling of specimens, or other special procedures may be required.

Normal Results
"Normal" levels vary according to the institution performing the test.

Blood can be tested for the presence and levels (amounts) of medications. Urine screening is usually reported as positive (substance is present) or negative (absent), but the level of certain substances can also be measured fairly accurately in urine.

Therapeutic levels are measured for prescribed or over-the-counter medications (see the specific medication).

Alcohol, prescription medications that are not prescribed, and illegal drugs are not normally present.

What Abnormal Results Mean
The presence of illegal drugs or drugs not prescribed for the person indicates illicit drug use.

Elevated levels of alcohol or prescription drugs can indicate intentional or accidental intoxication and/or overdose.

Additional conditions under which the test may be performed:

alcohol withdrawal state
alcoholism
drug abuse monitoring
analgesic nephropathy
at risk for fetal alcohol syndrome
complicated alcohol abstinence (delirium tremens)
delirium
dementia
stroke secondary to cocaine