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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
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.
Farlex Partner Medical Dictionary © Farlex 2012
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.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
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
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.
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
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.
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%.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
)
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.Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
Half-life
The time required for half of the atoms in a radioactive substance to disintegrate.
Mentioned in: Positron Emission Tomography (PET)
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.
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%.
Medical Dictionary for the Dental Professions © Farlex 2012