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.
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