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In a limited number of preliminary-study patients buy 150mg zyban mastercard anxiety depression symptoms, they noted that conventional doses of 7 order zyban online definition depression in elderly. The explanation for the lower antibiotic concentrations in the conventional dosing regimen was found in the larger Vd and short T1/2 that were seen in the trauma patients compared to normal controls buy zyban 150 mg depression symptoms graves disease. In a study of eight patients that averaged 37 years of age and had normal creatinine generic 150mg zyban visa depression in the elderly, each received between 6. Subsequent studies of an additional 28 trauma patients confirmed the impact of the increased Vd and the increased elimination rates of the drug in adversely affecting preventive antibiotic concentrations (3). A prospective study examined the wound and intra-abdominal infection rates of penetrating abdominal trauma patients who received different doses of amikacin (2). Significantly, higher doses of amikacin resulted in statistically reduced infection rates in all patients studied. Subgroup analysis indicated that lower infection rates were identified in patients with high-volume blood loss and in patients with injury severity scores >20. No improvement in rates infections was seen in patients when colon injury was present, indicating that high inocula of surgical site contamination cannot likely be overcome by preventive antibiotics. This observed uncertainty about antibiotic pharmacoki- netics in the setting of blood loss and injury has led to some experimental investigation in the use of continuous infusion of antibiotics as a means to overcome the problem. Another strategy has been to simply not use potentially toxic agents like the aminoglycosides, but rather choose Table 2 Differences in Clinical Outcomes of Infection when 7. Antibiotic Kinetics in the Multiple-System Trauma Patient 527 b-lactam alternatives where toxicity concerns are minimized and larger doses can be safely utilized. The data that evaluate other antibiotics in preventive indications in trauma patients is very limited. They identified lower antibiotic concentrations in selected patients in the recovery room, and found that lower postoperative antibiotic concentrations were predictive of postoperative infections. They identified blood loss, extensive intraoperative resuscitation, and expanded Vd as likely causes for reduced postoperative antibiotic concentrations and recommended consideration for increased preop- erative dose of preventive antibiotics. They recommended re-dosing or continuous infusion as a requirement for effective use of preventive antibiotics in this population. Aminoglycosides The aminoglycosides more than any antibiotic group have been studied most extensively in the setting of critical illness. Nephro- and ototoxicity have been the driving issues that have stimulated pharmacokinetic studies of the aminoglycosides. However, the data indicate that perhaps more patients have been underdosed than have received toxic levels of these antibiotics. Given that gentamicin and the other aminoglycosides have been demonstrated to have highly variable pharmacokinetics even with patients that appear to have normal kidney function (6), it is not surprising that physiologic changes of trauma and clinical fever will further compound an already difficult situation. All study patients received at least one aminoglycoside with the majority receiving gentamicin or tobramycin. The Vd increased approximately 50% greater than normal for this population with one patient demonstrating a threefold increase. Using individual patient pharmacokinetic parameters, adjustments in gentamicin doses ranged from 1. In this latter study, drug elimination rates were strongly influenced by the patient’s serum creatinine as a marker of clinical renal function. Despite larger doses that were required, doses of the aminoglycosides were given less frequently with patients having a creatinine above 1 mg/dL. They identified 59% of patients that had blood concentration of the antibiotic that was significantly below expected concentrations. The expanded Vd was considered to be responsible for the low blood concentrations. Dasta and Armstrong (10) studied aminoglycoside pharmacokinetics in 181 critically ill patients in a surgical intensive care unit. Additional studies have validated that the observations of increased Vd and highly variable T1/2 are applicable to all of the aminoglycosides in trauma (11) and intensive care unit patients (12). Understanding these changes of aminoglycosides under circumstances of trauma, fever, and critical illness should lead to pharmacokinetic dosing and changes in the management of these patients. Once-daily dosing of aminoglycosides has become very common at present, but again the pharmacokinetic observations have demonstrated that conventional doses will be inadequate, especially for the younger trauma patient with normal renal function. Vancomycin Like the aminoglycosides, the pharmacokinetics of vancomycin is highly variable among patients with normal renal function (14). They assumed and documented that the Vd of vancomycin was essentially that of total body water, or 0. In selected cases, the Vd was so high that it actually exceed the theoretical maximum of 1. Pharmacokinetic dosing required a 20% increase in the predicted dose of vancomycin, but a 50% increase in the interval between doses reflected a longer T1/2 than expected. Vancomycin clearance was 143 mL/min in the burn patient which was more than twice as great as that seen in control patients (68 mL/min). Vancomycin patients required larger and more frequent doses of the drug to achieve satisfactory peaks and troughs during therapy. The hyperdynamic circulation of the burn patient with normal kidney function was thought to be the basis for accelerated drug clearance. Garrelts and Peterie (17) made similar observations with respect to a reduced T1/2 in burn patients receiving vancomycin. Van Dalen and Vree (18) studied Vd and T1/2 in critically ill patients after the administration of ceftriaxone, the most commonly employed third-generation cephalosporin. They identified that the pharmacokinetics patterns were very similar to the aminoglycosides with an expanded Vd and wide inter-patient variability with T1/2. They concluded that unique nomograms needed to be developed to permit dosing of ceftriaxone that was consistent with each patient’s unique severity of disease profile. Yet another study demonstrated similar findings with a 90% increase in Vd and that drug clearance was increased in patients with normal renal function (19). Patients with diminished renal function demonstrated a very prolonged T1/2 and posed a serious problem of potential drug accumulation. They suggested that the expanded Vd could serve as a reservoir for the drug and result in slow return to the circulation, which would explain the reduced clearance. They concluded that continuous infusion would prove to use less total drug and would ensure reliable therapeutic drug concentrations. Cefepime is a commonly used antibiotic especially later in the trauma patient’s course when fever and nosocomial infection are significant issues. The pharmacokinetics of aztreonam were studied in 28 critically ill, mostly trauma patients, with gram-negative infections (27). The patients were a relatively young group (age ¼ 35 years) and received 2 g of aztreonam every six hours.
The major drivers Sources of Payment for Dental Care of dental expenditures are the general wealth of the population discount 150 mg zyban visa depression test accurate, employer and public contributions to Four basic sources of funds to pay for dental care dental prepayment premiums order 150 mg zyban otc anxiety 4th herefords, the perceived need are employer-based prepayment plans zyban 150mg amex depression zinc, direct patient for and value of dental services order zyban with amex depression pathophysiology, and oral health payment, public prepayment, and free from the status. Through the 1970s and the An important factor related to the demand for 1980s, employer-based private prepayment grew dental services, and thus access, is the availability, rapidly. By the early 1990s more than 40% of all extent and character of third party financing for Americans were covered by some form of private services (Tuominen, 1994). Direct patient payments, as a dental services are willing, under certain conditions, proportion of total financing, has declined. Prepayment encourages people to receive routine Patients pay approximately 47% of total dental preventive care and early detection of dental prob- expenditures, or $25. Enrollment in employer-based dental prepay- cludes cost sharing and contribution to premiums ment is about 105 million people and has increased for patients with dental prepayment and the full slowly in the past 10 years with the expanding econ- out-of-pocket payments for those without prepay- omy. To date, capitated managed care has income individuals that meet state program criteria. Few states cover indigent adults for Public financing of services for economically dis- basic dental services. The total value of charitable care is equivalent ditures and improved program administration. Stephen Eklund and Mark Mallat, Personal In addition, dentist volunteers have been provid- Communication, September 1, 2000). According to the National although some medical managed care plans offer Foundation of Dentistry for the Handicapped, limited dental benefits to attract elderly enrollees. This compares favorably to the dental prepayment were enrolled in indemnity estimated $2. In absolute terms, the decline in the number expenditures, from public sources for 1987. In order to maintain consistency in terms of what is being compared over time, the results presented in this section are based on the total charges for dental services. Others felt the 40 way to resolve this imbalance was 30 to limit the type and/or amount of treatment provided. Still other den- 20 tists offered services, procedures or materials that were not covered 10 benefits, requiring the patients to 0 assume greater financial burden 1994 1995 1996 1997 1998 1999 than they had previously expected. At a minimum, an individual would want Dissatisfied capitation plan dentists were most likely also to have a catastrophic health insurance policy (54. Thirty-two state In addition to employer-based prepayment prod- Medicaid programs do not cover adult dental care, ucts, defined contribution plans are emerging. Most Medicaid pro- Under a defined contribution plan, the employer grams do provide coverage for indigent children and provides an agreed amount to a discretionary health parents, mainly single mothers, enrolled in the Aid services fund for each employee. In addition, states are required by Federal law to Defined contribution plans are a potentially im- provide basic dental care to all Medicaid eligible portant alternative to defined benefit plans. Two of these plan much smaller, percentage receives comprehensive types will be discussed here (direct reimbursement preventive and curative care. As such, the gener- Bureau of Primary Health Care, Health Resources and al public and state legislatures expect schools to Services Administration function as safety net providers. Another source of financial support for dental Some schools have dental clinics that provide care services is the Bureau of Primary Health Care, to low-income children who are covered by the Health Resources and Services Administration. Bureau gives grants to private non-profit Migrant These clinics are usually partially subsidized by and Community Health Centers to provide compre- state, county and municipal funds even though they hensive health care, including dental services, to the bill Medicaid where possible. Approximately 56% of the 671 hensive national data available on school-based Migrant and Community Health Centers offer den- dental clinics. Measured as the number of visits per year or Alaska natives in dental facilities located in pre- the number of individuals with at least one visit dur- dominantly Native American communities and ing the previous year, these studies have provided reservations. Expenditures are a dollar meas- ure of the extent of utilization of resources once The United States Department of Veterans entry into the dental delivery system occurs. The available data on use of dental care is char- acterized by variability from one source to another Dental Schools (Brown and Lazar, 1999). One survey suggests that Dental schools are an important source of care for 75% of the adult population have an annual dental the poor and individuals with disabilities. This is more in the use of dental care among subgroups of the than twice the rate of general inflation. Between population even though the true rate of annual 1993 and 1998, dental care expenditures were rising dental visits in the United States is currently faster than hospital and physician services expendi- uncertain. This represents a historic shift in the compara- According to data from the National Health tive rates of growth between the two sectors. This is only slightly greater than the rate of increase in the United States population, which is 1. As mentioned 64 previously, the percent of the pop- 62 ulation who visited a dentist with- 60 in a year increased during this same period (see Figure 4. In order to maintain consistency in terms of what is being compared over time, the results presented in this section are based on the total charges for dental services. The increase in young chil- role of edentulism in utilization of dental care is dren may be due to an increasing realization of the demonstrated clearly in the literature (Meskin and importance of oral health in the young, and the Brown, 1988). As fewer additional American adults increased utilization in the elderly is partly due to the become edentulous, more adults will seek regular den- reduced incidence of total edentulism in adults. By 1998, pre- a higher percentage of the White population visited the school children living below the poverty level exhib- dentist compared to the African American population. Among adults, the increase in the likelihood of a Non-Hispanics are more likely to visit a dentist visit was smaller than that for children (see Table than Hispanics. Across all age groups, persons living below the ference has not decreased over time. As for dental ex- national poverty level were less likely to see a den- penditures, non-Hispanics spend more than Hispanics tist than those above the poverty level. However, there is approximately Below Poverty At or Above Poverty Age a 20 percentage point difference Group 1989 1998 1989 1998 in the likelihood of a visit between 2-4 8 8 these two groups. For children 5- 17 years old below the poverty level, the likelihood of Unlike the experience of children, however, among visiting a dentist increased by 15. However, dif- years old, living above the poverty level, the increase ferences between individuals at the highest and lowest among those who visited the dentist was 6. Among children 2-4 sented earlier clearly indicates that middle class children years old, the increase was from $82 to $187. Among children 5-17 years old, the increase was Untreated caries in this age group is largely concentrat- from $235 to $431. Thus, one explanation of decreased among 5-17-year-old children from fami- these data may be that middle class children did not lies living at the 100-200% of the poverty level. In contrast, lower income Expenditure Data for Children 2 to 17 Years Old, 9 children may have had a larger need by Age Group and Poverty Level for restorative care, and somehow 1987 Charges 1996 Charges P-Value found the funds to receive it. Healthcare is a sector of the economy where mar- Nevertheless, as shown in Table 4.
The problem has been suggested to be even more pro- of the disorder because samples from higher generations are nounced among the elderly purchase zyban without prescription depression symptoms quiz test, who often use more medication generally not available order 150mg zyban with visa anxiety 8 letters. Other major classes of drugs known to cause permanent murine models of age-related hearing loss generic zyban 150 mg otc karst depression definition. Inner ear function is hearing loss are the platinum-based chemotherapeutic agents similar between mice and humans generic zyban 150mg with mastercard depression natural cures, which suggests that the such as Cisplatin, used in the treatment of cancer. Aminoglycosides have also been reported include those that may increase the susceptibility to noise, to intensify the ototoxic effects of noise exposure and vice versa ototoxicity, or ageing. The involvement of environmental factors is Cardiovascular disease and its risk factors have been shown implied, for example, by the fact that hearing levels are gener- to affect hearing to some extent (153). Stroke, myocardial ally poorer in industrialised than in isolated or agrarian societies infarction, claudication, hypertension, hyperlipidaemia, and (137). Apart from family history, the most commonly studied diabetes mellitus have all been previously associated with risk factors of age-related hearing loss include noise-induced excessive hearing loss (154–156). In some studies, long-term damage, otological, and other disorders as well as exposure to smoking (157) and excessive alcohol intake (158) have been ototoxic agents. A variety of work- cell function can thus be mediated via a variety of different place chemicals are known as potentially ototoxic if exposure mechanisms, including impaired mitochondrial protein synthe- exceeds a certain level (159), and there is accumulating evi- sis, accumulation, and defective turnover of abnormal transla- dence that many of these toxins may be able to potentiate the tion products, bioenergy insufﬁciency, oxidative stress, calcium ototoxicity of noise through oxidative stress mechanisms (1). Overall, the future namely the cochlear hair cells, the cells of the stria vascularis, development of efﬁcient treatment strategies will clearly require or the spiral ganglion neurons. Based on animal studies, treatment with antioxidant gen transfer by a chemiosmotic type of mechanism. Intl Rev Cytol mice also develop a progressive impairment of hearing with 1992; 141:233–357. Sequence and organi- levels does indeed result in progressive degeneration of the zation of the human mitochondrial genome. Nature 1981; auditory system and leads to age-related hearing loss in the 290:357–465. How rapidly does the human by cyberscreening of the expressed sequence tags database. A novel mitochondrial point mitochondrial genome indicates signiﬁcant differences between mutation in a maternal pedigree with sensorineural deafness. Hum patients with Alzheimer disease and controls in a French-Canadian Mutat 1994; 3:243–247. Hearing impairment and inherited variants are associated with successful aging and neurological dysfunction associated with a mutation in the mito- longevity in humans. Clustering of Caucasian Leber lactic acidosis, and stroke-like episodes: prevalence of the muta- hereditary optic neuropathy patients containing the 11778 or tion in an adult population. Maternally ground plays a role in the expression of Leber hereditary optic inherited non-syndromal hearing impairment in a Spanish family neuropathy by increasing the penetrance of the primary mutations with the 7510T C mutation in the mitochondrial 11778 and 14484. Am J Hum Genet 2003; side-induced and nonsyndromic deafness is associated with the 72:1005–1012. Mater- lymphoblastoid cell-line homoplasmic for the np 7445 deafness- nally inherited diabetes and deafness is a distinct subtype of diabetes associated mitochondrial mutation. Hum Mol Genet 1997; and associates with a single point mutation in the mitochondrial 6:443–449. Quantitative allele-spe- for the inherited susceptibility to aminoglycoside ototoxicity. Nucl Acids Res 2000; A1555G and A7445G mutations among children with prelingual 28:4350–4355. Aging: a theory based on free radical and radiation control region for replication. Mitochondrial between cardiovascular disease and cochlear function in older enzyme-deﬁcient hippocampal neurons and choroidal cells in adults. Hearing threshold in patients mutations cause aging phenotypes without affecting reactive oxy- with diabetes mellitus. Correlations between pres- in secondary processes and spread to primary processes of strial byacusis and extrinsic noxious factors. Acta Otolaryngol Suppl 2004; bone analysis of patients with presbycusis reveals high frequency 552:16–24. Thus parents of a deaf hearing difﬁculties, now widely regarded as being responsible child with a clearly dominant family history may insist that the for at least 50% of permanent hearing loss both in young chil- child was deafened as a result of a pertussis infection. In certain isolated communi- year-old patients have reported that their parents’ hearing loss ties, a particular genetic cause of prelingual hearing impairment was due to “old age” even though it began at the age of 60 and may achieve a high prevalence and result in a different set of their own hearing loss dated back to such an age or younger. The present author is particularly indebted to Probably the best known example of a high prevalence of the contributions in this respect of Sylviane Chéry-Croze, congenital deafness affecting societal attitudes was the case of Lionel Collet, Berth Danermark, Lesley Jones, Sophia Martha’s Vineyard, an island off the coast of Massachusetts, Kramer, Kerstin Möller, Wanda Neary, and Hung Thai Van. The population, in that case, had a discussions, was Anna Middleton, author of the next chapter in high prevalence of a nonsyndromal recessive condition, which the present book. The aim of the working group was to provide appeared to have originated in Southeast England. The high an interface between the molecular and clinical geneticists and prevalence of the condition resulted in “deafness” being those people facing the real world problems caused by genetic regarded as a normal state and the hearing population using disorders affecting the auditory system. Hearing disorders (6), and one of the most interesting examples is found in the affecting working age and older adults are studied using both northern part of the island of Bali. Here there is a village called epidemiological approaches and clinic-based studies, and this Bengkala where some 2% to 3% of the population has congen- provides the main focus for the chapter. These are followed by a qualitative analysis of 146 Current management people’s perception of the impact of their family history on are likely to be better adjusted (15,16), to have a more positive themselves. That, in turn, leads to investigations of such an coping framework (17) and less likely to have psychiatric prob- impact on activity limitations and participation restrictions, lems (18). It has been strongly argued that many such differ- motivation for seeking rehabilitative help, and on rehabilita- ences may be attributable to early and effective mother–child tive outcomes. This is followed by a consideration of the inﬂu- communication, leading to the development of a more stable ence of a family history on the impact of tinnitus and ﬁnally by individual (19). Recently a large-scale study on children death may occur and which generally presents with a hearing loss. Overall, in nonsyndromal hearing impairment, it would The results for 338 children whose parents had some hearing seem that a family history with role models available is what difﬁculties were compared with those of 2519 children whose has had the greatest effect on people affected themselves, rather parents had no such difﬁculties. The total impact of that ethnicity, average unaided hearing level, age of onset of hearing from a psychosocial standpoint is also relatively modest com- impairment, additional hearing disabilities, parental occupation, pared with other factors such as the severity of the impairment and cochlear implantation, they examined any effect of family and the age of its onset. This indicates that, while the auditory receptive communi- cation of those children with hearing-impaired parents was poorer, their sign language skills were better. It also supports the Family history inﬂuences earlier ﬁndings of better academic achievement in those children in children These studies date back to the 1940s, but two important inves- Table 10. These, together with a num- parents differed from those with hearing parents ber of related investigations, have been discussed in some detail elsewhere (13), but may be summarised as indicating that it is Communicative skills — the fact of having deaf parents, which is important, rather than a Auditory receptive capabilities Poorer having a speciﬁc genetic disorder. No signiﬁcant difference between the four a Key stage attainments Higher groups in terms of the youngsters’ speech intelligibility was Participation and engagement Better found, but those with deaf parents performed signiﬁcantly bet- a in education ter than the other three groups in terms of their reading age and in a speech comprehension ratio of lipreading.
Detection Efﬁciency The detection efﬁciency of a counter is given by the observed count rate divided by the disintegration rate of a radioactive sample cheap 150mg zyban with amex depression symptoms test nhs. The count rate of a sample differs from the disintegration rate because of several factors order cheapest zyban depression definition emedicine. Radiations from a source are emitted isotropically around 4p steridians buy zyban in india depression definition icd, but only a fraction of all photons emitted strikes the detector buy 150 mg zyban depression symptoms signs, depending on the solid angle subtended by the detector on the source. Only a fraction of all photons striking the detector may interact in the detector and produce pulses. Further- more, the count rate is affected by the abundance of a particular radiation Characteristics of Counting Systems 97 from a radionuclide. Considering these factors, the overall counting efﬁ- ciency of a counter for a radiation is given by the following expression: Efﬁciency = fi × fp × fg × Ni (8. Intrinsic Efﬁciency The fraction of all radiations of a given type and energy impinging on the detector that interacts with it to produce pulses is called the intrinsic efﬁ- ciency, fi, of the detector: No. Intrinsic efﬁciency depends on the type and energy of the radiation and the linear attenuation coefﬁcient (m) and thickness of the detector. The dependence of intrinsic efﬁciency on the photon energy and the detector thickness is illustrated in Figure 8. The intrinsic efﬁciency of gas detectors is almost unity for a- and b-particles but is about 0. Photopeak Efﬁciency or Photofraction The fraction of all detected g-rays that contributes only to the photopeak is called the photopeak efﬁciency, or photofraction (fp). It is given as the total photopeak area divided by the total area under the entire spectrum: All counts under the photopeak fp = (8. Geometric Efﬁciency Radiations from a source are emitted uniformly with equal intensity in all directions. This fraction is determined by the solid angle sub- tended by the detector on the source. The geometric efﬁciency, fg, is equal to the number of radiations striking the detector divided by the total number of radiations emitted by the source. Characteristics of Counting Systems 99 When the source and the detector are in close contact, the fg tends to be about 50% (Fig. In the case of gamma well counters and liquid scin- tillation counters, the fg approaches 100% (Fig. Dead Time Each counting system takes a certain amount of time to process a radiation event, starting from interaction of radiation with the detector all the way up to forming a pulse and ultimately recording it. The counter remains insensitive to a second event for this period of time, that is, if a second radi- ation arrives during this time, the counter cannot process it. When the counter recovers after this period, only then can a second radiation be detected. In scintillation detectors two events may be processed simulta- neously to form a single event of amplitude that is equal to the sum of the amplitudes of both events. Dead time loss at high count rates is a serious problem for any counting system and is more so for scin- tillation cameras due to pulse pileup (see Chapter 11). Illustration of geometric efﬁciency, f , of a detector D with a circular area, pr2, g where r is the radius of the detector. Plot of observed count rates versus true count rates indicating the dead time loss in paralyzable and nonparalyzable systems. Based on how successive pulses are processed owing to the dead time, the counting systems fall into two categories: paralyzable and nonparalyz- able. In paralyzable systems, each event sets its own dead time, even if it arrives within the dead time of the previous event and is not counted. Each event prolongs the dead time induced by the previous event, and thus adds to the total dead time of the system, whereby a paralyzable system can become totally unresponsive to process events if the count rate of the source is very high. On the other hand, in nonparalyzable systems, the instrument remains insensitive to successive events for a period of time equal to the dead time, and these events are lost. When the system recovers after the detection of the ﬁrst event, only then is the second event processed and detected. The paralyzable and nonparalyzable systems can be represented by mathematical relationships among the observed count rate Ro, true count rate Rt, and dead time t. Scalers and pulse-height ana- lyzers are nonparalyzable systems, whereas radiation detectors themselves Gamma Well Counters 101 are paralyzable systems. Scintillation cameras have both paralyzable and nonparalyzable components of dead time. Therefore, either count rates must be lowered or corrections must be made to the observed count rates. An empirical method is to plot the observed count rates as a function of increasing concentrations of known activity. For subsequent measurements of unknown samples, correction is made to compensate for the dead time loss giving true count rates. Another method uses two radioactive sources, which are counted in the counter individually and together. From these three mea- surements, one can calculate dead time using appropriate equations (see Cherry et al. Various techniques, such as use of buffers, in which overlapping events are held off during the dead time, use of pulse pileup rejection circuits, and use of high-speed electronics have been employed to improve the dead time correction. Placing a radioactive sample in the central hole of the detector increases the geometric efﬁciency (almost 99%) and hence the counting efﬁciency of the counter. The NaI(Tl) detectors have dimensions in the range of 5-cm diameter × 5-cm thick to 23-cm diameter × 23-cm thick. Smaller detectors are used for low-energy g -rays (less than 200keV), and larger detectors are used for high-energy g -rays. This calibration is called the high-voltage or energy cali- bration of the well counter. After placing a 137Cs source in the well counter, the lower and upper discriminator levels are set at 640 divisions 102 8. Starting from low values, the high voltage is increased in small increments for a given ampli- ﬁer gain until the observed count rate reaches a maximum. The high voltage at the maximum count rate is kept as the operating voltage for subsequent counting of photons of different energies. The discriminator dials are then said to be energy calibrated; for example, each dial unit corresponds to 1keV at an ampliﬁer gain of 1. Thus, the center of the 140-keV photopeak of 99mTc can be set at 140 divisions of the discriminator setting, with lower and upper values set as desired. After calibration, well counters should be checked regularly for any voltage drift using a long-lived source, such as 137Cs. Counting in Well Counters For relative comparison of count rates between samples, the well counter does not need to be calibrated, provided all samples for comparison have the same volume. In radioimmunoassays, ferrokinetics, blood volume, red cell mass measurements, a standard of the same geometry (volume) and with relatively the same activity is counted along with all samples, and then a comparison is made between each sample and the standard. However, when the absolute activity of a radioactive sample needs to be determined, then the detection efﬁciency of the counter must be measured for the g-ray energy of interest using a standard of the radioactive sample of known activity.
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