LD50 (thing)

A measure of the acute toxicity of chemicals, developed in 1927; LD stands for lethal dose. The amount of a substance which when administered to test animals, usually rats or mice, causes the death of half of them. Expressed as weight of the chemical administered per kilogram body weight of the test animal, and animal used and method of administration or exposure:

LD₅₀ (oral, rat) - 5 mg/kg
LD₅₀ (dermal, rabbit) - 10 g/kg.

Useful because different poisons or medicines may have effects of similar intensity but on different tissues or organ systems, so a test is needed that produces an identical effect, namely death.

An LD50 test is performed by administering gradually increasing dosages to successive test populations over a set time frame, usually 14 days, until 50% of the subjects die, after which surviving animals are killed.

The LC50 test is another measure of acute toxicity, but of concentrations in air or water rather than an administered dosage, and is expressed in parts per million (ppm) or milligrams per cubic meter, and the test animal, and the duration of exposure:

LC₅₀ (rat) - 500 ppm/4 hr
LC₅₀ (rabbit) - 5 mg/m³/2 hr.

LD50 is controversial today, for several reasons. The test consumes a large number of animals and to those not inured to laboratory use of animals, seems cruel. It gives little or no information about cause of death. Its reliability is very sensitive to test conditions. Most telling, variations in species-to-species sensitivity call into question the test's relevance for human toxicity. For example, acetaminophen has an LD50 in mice of about 300 mg/kg due to liver necrosis, but in rats the LD50 is about 1000 mg/kg with little liver damage evident.

Nevertheless, use of the LD50 test is widespread not only as a measure of chemical toxicity, but to:

classify substances for regulatory purposes including safe transportation and labeling

provide information for treatment of acute intoxications

standardize certain biological products

set dose levels for subsequent toxicity studies

provide comparative information on dose response curves

provide data for evaluation and validation of alternative test methods.