Simple Chemical Tests

Chemical tests were once the mainstay of postmortem toxicology.21,124-127 Today, many have been abandoned and replaced by automated broad range screening procedures. However, some are still useful as supplemental tests to rapidly and easily detect drugs and poisons that are not detected by the other screening tests.

Virtually all clinical and forensic toxicology laboratories utilize micro-color tests to indicate the possible presence of drugs or toxins — the so-called spot tests.21,125,127-133 Micro-color tests are performed by adding a single or multiple reagents to a specimen or extract and observing the color produced. Color tests are specific examples of the more general method of qualitative organic chemistry, relying on functional group reactions with the reagents. These tests require some expertise and familiarity to use, but are inexpensive and relatively rapid. They are nonspecific. However, in conjunction with other confirmatory tests, they can be used as rapid diagnostic aids.

Color tests can be combined with visible or spectrophotometric methodologies for qualitative to semiquantitative answers. Drugs including phenothiazines, salicylates, acetaminophen, carbamates, ethchlorvynol, and imipramine may be detected by colorimetric methods. Positive test results are confirmed and quantified by another technique. The greatest advantage of color tests is their ease of use. They can be performed directly on urine or a protein-free filtrate of blood or tissues. A negative color test precludes the need for any further work on that drug, assuming the detection limit of the test is acceptable. For example, the color test for phenothiazines is useful if an overdose has occurred, but is not sensitive enough to test for patient compliance with the drug.

3.2.1 Useful Color Tests

Some or all of the following tests may be incorporated in a routine screening protocol or some may be reserved for cases requiring more comprehensive poison screening.

3.2.1.1 Trinder's Reagent

Trinder's test is a simple color test that detects salicylic acid in urine or serum. It does not detect acetylsalicylic acid in gastric contents without prior hydrolysis by boiling with dilute HCl. Trinder's reagent is a mixture of ferric nitrate and mercuric chloride in dilute hydrochloric acid. It immediately produces a violet color when mixed with an equal volume of sample containing salicylate. Phenothiazines also give a positive reaction.125,126,129,133,134

3.2.1.2 Fujiwara Test for Trichloro Compounds

A mixture of 1 mL 20% sodium hydroxide and 1 mL of pyridine at 100°C yields a red or pink color with chloral hydrate or other compounds with at least two halogens bound to one carbon.124-126,131,135 Trichloroethanol gives a yellow color. Contamination of the laboratory atmosphere with chlorinated solvents will give "false" positive results. Metabolites of carbon tetrachloride may also give a positive result with this test, but carbon tetrachloride is only partially metabolized to trichloromethyl compounds and the test may fail to detect this agent. A blank sample and a control (trichloroacetic acid) should be tested at the same time, with both blank and control solutions treated in similar fashion to the sample.

3.2.1.3 Diphenylamine Test for Oxidizing Agents

A solution of 0.5% diphenylamine in 60% sulfuric acid added to the sample or sample filtrate in a porcelain spot plate or a test tube immediately gives an intense blue color if an oxidizing agent is present. This test detects hypochlorite, chlorate, bromate, iodate, chromate, dichromate, nitrate, nitrite, permanganate, vanadate, lead (IV), or manganese (III, IV, or VII).125,126

3.2.1.4 Ethchlorvynol (Placidyl) Test

In this test 1 mL of sample (urine or sample filtrate) is mixed with the reagent and allowed to stand for 20 min. If ethclorvynol is present, the solution turns red or pink. The reagent consists of 1 g diphenylamine dissolved in 50 mL concentrated sulfuric acid and added slowly with stirring to 100 mL of 50% (V/V) acetic acid. The test is sufficiently sensitive to detect therapeutic concentrations.125,126,136

3.2.2 Other Color Tests That May Be Included in a Screen

Other color tests that may be included in a screen can be found in various toxicology references.124-128

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