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" >radioactivity. Symbol t_½ or T_½.

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.

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

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

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-time2. 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)

单词	half-note
释义	half-note half-note n (Music, other) US and Canadian a note having the time value of half a semibreve. Also called: minim EncyclopediaSeeminim half-life 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" >radioactivity. Symbol t_½ or T_½. 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. 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 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 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-time2. 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-time2. Time required for the serum concentration of a drug to decline by 50%. ThesaurusSeehalf note
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