Median lethal dose

This article is about the toxicological term. For the album by Mudvayne, see L.D. 50 (album). For other uses, see LD50 (disambiguation).

In toxicology, the median lethal dose, LD₅₀ (abbreviation for “lethal dose, 50%”), LC₅₀ (lethal concentration, 50%) or LCt₅₀ (lethal concentration & time) of a toxin, radiation, or pathogen is the dose required to kill half the members of a tested population after a specified test duration. LD₅₀ figures are frequently used as a general indicator of a substance's acute toxicity. The test was created by J.W. Trevan in 1927. The term semilethal dose is occasionally used with the same meaning, in particular in translations from non-English-language texts, but can also refer to a sublethal dose; because of this ambiguity, it is usually avoided. The U.S. Food and Drug Administration has begun to approve alternative methods to LD₅₀ in response to research, cruelty concerns, and the lack of validity/sensitivity of the test as it relates to humans.

Conventions

The LD₅₀ is usually expressed as the mass of substance administered per unit mass of test subject, such as grams of substance per kilogram of body mass. Stating it this way allows the relative toxicity of different substances to be compared, and normalizes for the variation in the size of the animals exposed (although toxicity does not always scale simply with body mass). Typically, the LD₅₀ of a substance is given in milligrams per kilogram of body weight. In the case of some neurotoxins such as batrachotoxin, one of the most deadly toxins known, the LD₅₀ may be more conveniently expressed as micrograms per kilogram (µg/kg) or nanograms per kilogram (ng/kg) of body mass.

The choice of 50% lethality as a benchmark avoids the potential for ambiguity of making measurements in the extremes and reduces the amount of testing required. However, this also means that LD₅₀ is not the lethal dose for all subjects; some may be killed by much less, while others survive doses far higher than the LD₅₀. Measures such as "LD₁" and "LD₉₉" (dosage required to kill 1% or 99%, respectively, of the test population) are occasionally used for specific purposes.

Lethal dosage often varies depending on the method of administration; for instance, many substances are less toxic when administered orally than when intravenously administered. For this reason, LD₅₀ figures are often qualified with the mode of administration, e.g., "LD₅₀ i.v."

The related quantities LD₅₀/30 or an LD₅₀/60 are used to refer to a dose that without treatment will be lethal to 50% of the population within (respectively) 30 or 60 days. These measures are used more commonly within Radiation Health Physics, as survival beyond 60 days usually results in recovery.

A comparable measurement is LCt₅₀, which relates to lethal dosage from exposure, where C is concentration and t is time. It is often expressed in terms of mg-min/m³. ICt₅₀ is the dose that will cause incapacitation rather than death. These measures are commonly used to indicate the comparative efficacy of chemical warfare agents, and dosages are typically qualified by rates of breathing (e.g., resting = 10 l/min) for inhalation, or degree of clothing for skin penetration. The concept of Ct was first proposed by Fritz Haber and is sometimes referred to as Haber's Law, which assumes that exposure to 1 minute of 100 mg/m³ is equivalent to 10 minutes of 10 mg/m³ (1 × 100 = 100, as does 10 × 10 = 100).

Some chemicals, such as hydrogen cyanide, are rapidly detoxified by the human body, and do not follow Haber's Law. So, in these cases, the lethal concentration may be given simply as LC₅₀ and qualified by a duration of exposure (e.g., 10 minutes). The Material Safety Data Sheets for toxic substances frequently use this form of the term even if the substance does follow Haber's Law.

For disease-causing organisms, there is also a measure known as the median infective dose and dosage. The median infective dose (ID₅₀) is the number of organisms received by a person or test animal qualified by the route of administration (e.g., 1,200 org/man per oral). Because of the difficulties in counting actual organisms in a dose, infective doses may be expressed in terms of biological assay, such as the number of LD₅₀'s to some test animal. In biological warfare infective dosage is the number of infective doses per minute for a cubic meter (e.g., ICt₅₀ is 100 medium doses - min/m³).

Limitation

As a measure of toxicity, LD₅₀ is somewhat unreliable and results may vary greatly between testing facilities due to factors such as the genetic characteristics of the sample population, animal species tested, environmental factors and mode of administration. Another weakness is that it measures acute toxicity only (as opposed to chronic toxicity at lower doses), and does not take into account toxic effects that do not result in death but are nonetheless serious (e.g., brain damage). There can be wide variability between species as well; what is relatively safe for rats may very well be extremely toxic for humans, and vice versa. In other words, a relatively high LD₅₀ does not necessarily mean a substance is harmless, since its relative harmfulness depends on its usual dose, but a very low one is always a cause for concern.

When used to test venom from venomous creatures, such as snakes, LD₅₀ results may be misleading due to the physiological differences between mice and humans. Many venomous snakes are specialized predators on mice, their venom may be adapted specifically to incapacitate mice. While most mammals have a very similar physiology, LD₅₀ results may or may not be directly relevant to humans.

Examples

NOTE: Comparing substances (especially drugs) to each other by LD₅₀ can be misleading in many cases due (in part) to differences in effective dose (ED₅₀). Therefore, it is more useful to compare such substances by therapeutic index, which is simply the ratio of LD₅₀ to ED₅₀.

The following examples are listed in reference to LD₅₀ values, in descending order, and accompanied by LC₅₀ values, {bracketed}, when appropriate.

Substance	Animal, Route	LD50 {LC50}	LD50 : g/kg {LC50 : g/L} standardized	Reference
Water	rat, oral	90,000 mg/kg	90
Sucrose (table sugar)	rat, oral	29,700 mg/kg	29.7
Monosodium glutamate (MSG)	rat, oral	16,600 mg/kg	16.6
Vitamin C (ascorbic acid)	rat, oral	11,900 mg/kg	11.9
Cyanuric acid	rat, oral	7,700 mg/kg	7.7
cadmium sulfide	rat, oral	7,080 mg/kg	7.08
Grain alcohol (ethanol)	rat, oral	7,060 mg/kg	7.06
Melamine	rat, oral	6,000 mg/kg	6
Melamine cyanurate	rat, oral	4,100 mg/kg	4.1
Sodium molybdate	rat, oral	4,000 mg/kg	4
Table Salt	rat, oral	3,000 mg/kg	3
Paracetamol (acetaminophen)	rat, oral	1,944 mg/kg	1.944
Delta-9-tetrahydrocannabinol (THC)	rat, oral	1,270 mg/kg	1.270
Metallic Arsenic	rat, oral	763 mg/kg	0.763
Alkyl dimethyl benzalkonium chloride (ADBAC)	rat, oral fish, immersion aq. invertebrates, imm.	304.5 mg/kg {0.28 mg/L} {0.059 mg/L}	0.3045 {0.00028} {0.000059}
Coumarin (benzopyrone, from Cinnamomum aromaticum and other plants)	rat, oral	293 mg/kg	0.293
Aspirin (acetylsalicylic acid)	rat, oral	200 mg/kg	0.2
Caffeine	rat, oral	192 mg/kg	0.192
Arsenic trisulfide	rat, oral	185–6,400 mg/kg	0.185
Sodium nitrite	rat, oral	180 mg/kg	0.18
Bisoprolol	mouse, oral	100 mg/kg	0.1
Cobalt(II) chloride	rat, oral	80 mg/kg	0.08
Cadmium oxide	rat, oral	72 mg/kg	0.072
Sodium fluoride	rat, oral	52 mg/kg	0.052
Nicotine	rat, oral	50 mg/kg	0.05
Pentaborane	human, oral	<50 mg/kg	<0.05
Capsaicin	mouse, oral	47.2 mg/kg	0.0472
Mercury(II) chloride	rat, dermal	41 mg/kg	0.041
Lysergic acid diethylamide (LSD)	rat, intravenous	16.5 mg/kg	0.0165
Arsenic trioxide	rat, oral	14 mg/kg	0.014
Metallic Arsenic	rat, intraperitoneal	13 mg/kg	0.013
Sodium cyanide	rat, oral	6.4 mg/kg	0.0064
White phosphorus	rat, oral	3.03 mg/kg	0.00303
Strychnine	human, oral	1–2 mg/kg(estimated)	0.001
Beryllium oxide	rat, oral	0.5 mg/kg	0.0005
Cantharidin	human, oral	0.5 mg/kg	0.0005
Aflatoxin B1 (from Aspergillus flavus)	rat, oral	0.48 mg/kg	0.00048
Venom of the Inland Taipan (Australian snake)	rat, subcutaneous	25 µg/kg	0.000025
Ricin	rat, intraperitoneal rat, oral	22 μg/kg 20-30 mg/kg	0.000022 0.02
Dioxin (TCDD)	rat, oral	20 µg/kg	0.00002
Sarin	mouse, subcutaneous injection	17.23 µg/kg (estimated)	0.0000172
VX (nerve agent)	human, oral, inhalation, absorption through skin/eyes	2.3 µg/kg (estimated)	0.0000023
Batrachotoxin (from poison dart frog)	human, sub-cutaneous injection	2-7 µg/kg (estimated)	0.000002
Venom of Hydrophis belcheri (Belcher's Sea Snake)	mouse, intraperitoneal	0.25 µg/kg	0.00000025
Maitotoxin	mouse, intraperitoneal	0.13 µg/kg	0.00000013
Polonium-210	human, inhalation	10 ng/kg (estimated)	0.00000001
Botulinum toxin (Botox)	human, oral, injection, inhalation	1 ng/kg (estimated)	0.000000001
Ionizing radiation	human, irradiation	3-6 Gy

Animal rights concerns

Animal-rights and animal-welfare groups, such as Animal Rights International, have campaigned against LD₅₀ testing on animals in particular as, in the case of some substances, causing the animals to die slow, painful deaths. Several countries, including the UK, have taken steps to ban the oral LD₅₀, and the Organization for Economic Co-operation and Development (OECD) abolished the requirement for the oral test in 2001 (see Test Guideline 401, Trends in Pharmacological Sciences Vol 22, February 22, 2001).

See also

• Animal testing
• Reed-Muench method

Other measures of toxicity

• IDLH
• Certain safety factor
• Therapeutic index
• Protective index
• Fixed Dose Procedure to estimate LD50
• Median toxic dose (TD50)
• Lowest published toxic concentration (TCLo)
• Lowest published lethal dose (LDLo)
• IC₅₀ (50% inhibitory concentration)
• Draize test
• Indicative limit value
• No Observable Adverse Effect Level (NOAEL)
• Lowest Observable Adverse Effect Level (LOAEL)
• Up-and-down procedure

Related measures

• TCID₅₀ Tissue Culture Infective Dosage
• EID₅₀ Egg Infective Dosage
• ELD₅₀ Egg Lethal Dosage
• Plaque forming units (pfu)

External links

• Canadian Centre for Occupational Health and Safety

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