half-life

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half-life

 [haf´līf″]
the time required for the decay of half of a sample of particles of a radionuclide or elementary particle; see also radioactivity. Symbol t½ or T½.

half-life

(haf'līf),
The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance, such as a drug in serum, whose quantity decreases exponentially with time. Compare: half-time.

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.
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.

half-life

(hăf′līf′, häf′-)
n.
1. Physics The time required for half the nuclei in a sample of a specific isotopic species to undergo radioactive decay.
2. Biology
a. 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.
b. The time required for the radioactivity of material taken in by a living organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay.

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 biological half-life, effective half-life.
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 (e.g., serum half-life)
Haematology The time that cells stay in the circulation—e.g., red blood cells, 120 days, which increases after splenectomy; platelets, 4–6 days; eosinophils, 3–7 hours; neutrophils, 7 hours
Immunology The time an immunoglobulin 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
Nuclear medicine The length of time required for a radioisotope to decay to one-half of the original amount having the same radioactivity; a radioisotope’s effective T1/2 is either the time of decay—physical T1/2—or the time to elimination from a biological system. See Biological half-life
Physiology The time that it takes for half of a molecule’s activity to decay
Research See Cited half-life, Citing half-life
Therapeutics The amount of time it takes for the serum concentration of a drug to fall 50%, which reflects its rate of metabolism and elimination of parent drug and metabolites in the urine and stool

half-life

T1/2 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  

half-life

(haf'līf)
1. The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance whose quantity decreases exponentially with time.
Compare: half-time
2. Time required for the serum concentration of a drug to decline by 50%.
Half-lifeclick for a larger image
Fig. 188 Half-life . X = half-life. Note that the time taken to reach zero amount is not 2 x X.

half-life

the time required for half of the mass of a radioactive substance to disintegrate. For example, the half-life of 14C is 5,700 years.

Half-life

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

half-life

(haf'līf)
1. The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance whose quantity decreases exponentially with time.
Compare: half-time
2. Time required for the serum concentration of a drug to decline by 50%.

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.

half-life

the time in which the radioactivity usually associated with a particular isotope is reduced by half through radioactive decay.
References in periodicals archive ?
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The fluoxetine peak, while present 6-10 hours post dose, was much less pronounced because of the drug's long halflife.
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Notably, the rate of chemical transformation in humans was a much greater influence on guideline effectiveness than the assumed degree of advisory compliance, because effectiveness increased as human transformation halflife decreased.
5) David Lehman, "Foreword," The Best American Poetry 2008, edited by Charles Wright (Scribner-Simon and Schuster, 2008), xiv; Charles Wright, Halflife, 5.
More than six million tuned in on Saturday night to watch Philip play mysterious ex-CIA agent Rupert, now a crusader in a war between humans and the halflife in modern-day London.
Vampire uses the same technology as Halflife 2 so the games shimmers with the same loveliness.
Because of the shorter biological halflife of BNP compared with NTproBNP (6), we hypothesized that BNP would show a faster response to hemodynamic improvement during intravenous levosimendan therapy.
Also, unfractionated heparin is metabolized by the liver, making clearance from the body and halflife variable.
For the years after working at the plant, serum estimates were decayed at a rate of 18% per year, based on a presumed halflife of 3.