half-life (redirected from half life)

half-life (haf´līf) the time required for the decay of half of a sample of particles of a radionuclide or elementary particles; symbol t 1/2 or T 1/2.

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antibody half-life  a measure of the mean survival time of antibody molecules following their formation, usually expressed as the time required to eliminate 50 per cent of a known quantity of immunoglobulin from the animal body. Half-life varies from one immunoglobulin class to another.

biological half-life  the time required for a living tissue, organ, or organism to eliminate one-half of a radioactive substance which has been introduced into it.

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half-life

n.

1. The time required for half the nuclei of a specific radionuclide or radioactive substance to undergo radioactive decay. Also called physical half-life.

2. The time required for half the quantity of a drug or other substance deposited in a living organism to be metabolized or eliminated by normal biological processes. Also called biological half-life.

3. The time required for the radioactivity of material taken in by or administered to an organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay. Also called effective half-life.

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Half-life

The time required for half of the atoms in a radioactive substance to disintegrate.

Mentioned in: Positron Emission Tomography (PET)

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half-life (t-^½)

Etymology: AS, haelf + lif

1 also called radioactive half-life. the time required for a radioactive substance to lose 50% of its activity through decay. Each radionuclide has a unique half-life.

2 the amount of time required to reduce a drug level to half of its initial value. Usually the term refers to time necessary to reduce the plasma value to half of its initial value. After five half-lives, 97% of a single drug dose will be eliminated. See also biologic half-life, effective half-life.

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half-life,

n the time in which a radioactive substance will lose half of its activity through disintegration.

half-life, biologic,

n the time in which a living tissue, organ, or individual eliminates, through biologic processes, half of a given amount of a substance that has been introduced into it.

half-life, effective,

n the half-life of a radioactive isotope in a biologic organism, resulting from the combination of radioactive decay and biologic elimination.

half-life, physical,

n the average time required for the decay of half the atoms in a given amount of a radioactive substance.

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half-life

the time in which the radioactivity usually associated with a particular isotope is reduced by half through radioactive decay.

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half-life

T_1/2 The amount of time required for a substance to be reduced to one-half of its previous level by degradation and/or decay–radioactive half-life, by catabolism–biological half-life, or by elimination from a system–eg, half-life in serum Hematology The time that cells stay in the circulation–eg, RBCs 120 days–which ↑ after splenectomy, platelets–4-6 days, eosinophils–3-7 hrs, PMNs–7 hrs Immunology The time an Ig stays in the circulation: 20-25 days for IgG, 6 days for IgA, 5 days for IgM, 2-8 days for IgD, 1-5 days for IgE Therapeutics The time that a therapeutic agent remains in the circulation, which reflects its rate of metabolism and elimination of parent drug and metabolites in the urine and stool. See Effective half-life.

Half life in hours

Drug  Adult  Children

Digoxin  6–51  11–50

Gentamycin  2-3

Lithium 8–35

Phenobarbital  50–150  40–70

Phenytoin 18–30  12–22

Procainamide  2–4

Quinidine  4–7

Theophylline  3–8  1–8

Tobramycin  2–3

Valproic acid  8–15

Advance/Lab Feb 1995, p19