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
(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.
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
Compare: half-time
2. Time required for the serum concentration of a drug to decline by 50%.
)
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.
Mentioned in: Positron Emission Tomography (PET)
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
Compare: half-time
2. Time required for the serum concentration of a drug to decline by 50%.