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Biomedical subjects

N Thomas

Publications and source records attributed to N Thomas.

At least 109 records · Page 6Linked to original sources
PubMed1996

The role of pharmacoeconomics in disease management. A pharmaceutical benefit management company perspective.

In the US, managed-care organisations (MCOs) have turned to pharmacy benefit management (PBM) companies to contain costs and provide total disease management packages. The demand for pharmacoeconomic research and information by purchasers of healthcare has contributed to the growth of PBM companies and to recent acquisitions by pharmaceutical manufacturers. PBMs influence prescribing via formulary management and drug utilisation review (DUR). Formularies are generally open, and pharmacoeconomic data contribute to formulary management decisions. Prospective and concurrent DURs used in hospitals can soon be integrated into community pharmacy, as PBMs obtain integrated online information about prescribing. PBM prescription databases are used as as educational tools for physicians, providing drug utilisation data, formulary compliance, generic prescribing rates, and guidelines for pharmacotherapy for particular conditions. They can also evaluate medication regimens for patients with particular problems such as drug-drug interactions. However, PBM databases typically lack patient diagnostic, outcome and medical claim data. PBMs will be better placed to move from medication management to disease management when they can obtain these data and link them appropriately to their own prescribing and physician databases.

Drug Utilization Review↗
PubMed1996

Matching using estimated propensity scores: relating theory to practice.

Matched sampling is a standard technique in the evaluation of treatments in observational studies. Matching on estimated propensity scores comprises an important class of procedures when there are numerous matching variables. Recent theoretical work (Rubin, D. B. and Thomas, N., 1992, The Annals of Statistics 20, 1079-1093) on affinely invariant matching methods with ellipsoidal distributions provides a general framework for evaluating the operating characteristics of such methods. Moreover, Rubin and Thomas (1992, Biometrika 79, 797-809) uses this framework to derive several analytic approximations under normality for the distribution of the first two moments of the matching variables in samples obtained by matching on estimated linear propensity scores. Here we provide a bridge between these theoretical approximations and actual practice. First, we complete and refine the nomal-based analytic approximations, thereby making it possible to apply these results to practice. Second, we perform Monte Carlo evaluations of the analytic results under normal and nonnormal ellipsoidal distributions, which confirm the accuracy of the analytic approximations, and demonstrate the predictable ways in which the approximations deviate from simulation results when normal assumptions are violated within the ellipsoidal family. Third, we apply the analytic approximations to real data with clearly nonellipsoidal distributions, and show that the theoretical expressions, although derived under artificial distributional conditions, produce useful guidance for practice. Our results delineate the wide range of settings in which matching on estimated linear propensity scores performs well, thereby providing useful information for the design of matching studies. When matching with a particular data set, our theoretical approximations provide benchmarks for expected performance under favorable conditions, thereby identifying matching variables requiring special treatment. After matching is complete and data analysis is at hand, our results provide the variances required to compute valid standard errors for common estimators.

Analysis of Variance↗
PubMed1995

A theory of tension fluctuations due to muscle cross-bridges.

We present an analytical theory for the spectrum of tension fluctuations due to muscle cross-bridges. The theory is based upon the Langevin theory of brownian motion, and is illustrated using a simplified three-state model for the cross-bridge cycle, which is intended to model cross-bridges in fibrillar insect flight muscle. Langevin white-noise sources, representing fluctuations in the net transition rates for each step in the cycle, are introduced into the rate equations, and their strengths are adjusted to give the correct mean-square fluctuations in the occupation probabilities. The Langevin theory shows that the noise is closely related to the elastic properties of cross-bridges, and it also shows in detail how each step in the cross-bridge cycle contributes differently to the noise spectrum. We find that the total noise increases with filament displacement. For small filament displacements, the noise is dominated by the power stroke and by dissociation at the end of the cycle. These contributions increase in the region of stretch activation, whilst at larger displacements, where the cross-bridge becomes locked in the strong-binding state, the noise is much larger and is dominated by attachment and detachment at the beginning of the cycle. The cross-bridge properties in this regime are strongly affected by free inorganic phosphate. Finally, we show how the noise spectrum is modified by the inclusion of a series compliance representing a practical force transducer.

Animals↗
PubMed1995

HLA-DQ alpha allele and genotype frequencies in a native Kuwaiti population.

We report the allele and genotype frequencies in a sample of an unrelated native Kuwaiti population determined by the use of polymerase chain reaction (PCR) and reverse dot-blot analysis. This technique, involving the use of commercially available AmpliType HLA-DQ alpha forensic DNA amplification and typing kit, has permitted the definition of six alleles and 21 genotypes in a sample of 220 people. The allelic frequencies are in the range 5.7-27.5%. This locus demonstrated a heterozygosity of 0.80 with an allelic diversity of 0.81 and the power of discrimination (PD) was 0.93. The distribution of observed genotypes conformed to Hardy-Weinberg equilibrium thus indicating genetic equilibrium of the different variants. This population data should permit the use of HLA-DQ alpha marker for individual identification in forensic casework.

Alleles↗
PubMed1995

Response of the Io plasma torus to comet Shoemaker-Levy 9.

Spectroscopic and imaging observations of the Io plasma torus were made in June and July 1994 in conjunction with the encounter of periodic comet Shoemaker-Levy 9 with Jupiter. Characteristic emissions from sulfur and oxygen ions showed a decline of about 30 percent in the extreme ultraviolet and an increase of about 40 percent in the far ultraviolet relative to preimpact observations. Changes in the extreme ultraviolet may be indicative of small changes in the torus electron temperature as a result of quenching of electrons by dust associated with the comet passage. However, no new emission features indicative of fragment dust within the torus were detected. The characteristic torus morphology seen in ground-based imaging was typical of that observed in the past.

Atmosphere↗
PubMed1995

Prediction of ovarian cycle outcome by follicular characteristics, stage 1.

Both dominant and subdominant follicles have been identified and individually monitored over the follicular phases of 54 natural cycles in 36 women. Population dynamics of all antral follicles, visible by ultrasonography, in the ovary over the entire cycle have also been characterized for the first time. This has been accomplished by developing a new system of mapping and monitoring follicles, including a three-dimensional computer imaging model and correlation program. The different dominant follicle types were characterized by their ultrasonographic properties. Ovulatory follicles were rounded in shape and mid-range echogenic, with a smooth antral edge by late follicular phase. Luteinized unruptured follicles were round, had low echogenicity and a very smooth antral edge. Atretic follicles were irregular in shape and antral edge and had mid-range echogenicity. A further important finding was that follicle success appears to be related to timing of growth and to the subdominant follicle population present in the ovary. An analysis of follicle population dynamics in ovulatory cycles showed a drastic decrease in number at the end of the luteal phase, followed by a sharp increase at the beginning of the follicular phase. This study has demonstrated that characteristics of individual follicles and population dynamics of both dominant and subdominant follicles are strongly associated with cycle outcome. These findings will contribute towards a predictive model of dominant follicle status and cycle outcome. A new hypothesis of follicle competition has also been proposed.

Adult↗
PubMed1995

Implementing standards for radiation sterilization. The challenges.

This article discusses the limitations of the methods offered in the radiation standards for selecting radiation dose. It highlights the difficulty of using these methods for product-specific dose-setting and, in particular, their inability to determine this for products with low bioburdens. Issues relating to the appropriateness of 25 kGy are also discussed.

Equipment and Supplies↗
PubMed1994

Empirical Bayes methods for estimating hospital-specific mortality rates.

We present alternative methods for estimating hospital-level mortality rates to those used by the Health Care Finance Administration for Medicare patients. We use an empirical Bayes model to represent the different sources of variation in observed hospital-specific mortality rates and we use a logistic regression model to adjust for severity differences (in patient mix) across hospitals. In addition to providing a principled derivation of a standard error for the commonly used estimator, our fully model-based formulation produces much more accurate estimates and resolves the severe problem of multiple comparisons that arises when extreme estimates are used to identify exceptional hospitals. We estimate models for each of four disease conditions using the national Medicare mortality data base which does not contain patient severity descriptors, and mortality data from national samples which do include patient severity descriptors. We find substantial between-hospital variation in the unadjusted death rates from the national data base. Mortality rates differ substantially with patient severity in our models, but the sample sizes are too small to yield reliable estimates of the between-hospital variation in adjusted mortality rates.

Bayes Theorem↗
PubMed1994

Treatment with the gonadotrophin releasing hormone-agonist goserelin before hysterectomy for uterine fibroids.

OBJECTIVE: To investigate the effect of the gonadotrophin releasing hormone (GnRH)-agonist goserelin, given by monthly subcutaneous injection for three months prior to total abdominal hysterectomy for uterine leiomyomata, on the pre-operative symptoms, difficulty of operation and operative blood loss. DESIGN: Randomised placebo-controlled study. SETTING: Patients were recruited from the gynaecological outpatient departments from hospitals in Edinburgh, Glasgow and Newcastle. SUBJECTS: Seventy-one premenopausal women with uterine leiomyomata who were on the waiting list for hysterectomy. INTERVENTIONS: After the presence of leiomyomata was confirmed using ultrasonography, the women were randomised to receive either the GnRH-agonist goserelin by monthly subcutaneous injection or a sham injection for three months prior to operation. At the monthly visits, patients were asked about treatment related symptoms, fibroid related symptoms, and their bleeding patterns. Blood was taken for haematological assessment. MAIN OUTCOME MEASURES: Haemoglobin concentrations at recruitment, at operation and post-operatively, pre-operative symptoms, operative difficulty and blood loss and post-operative complications. RESULTS: Treatment with goserelin induced amenorrhoea in over 80% of the women, and this was associated with a significant rise in haemoglobin level. At the time of operation, fibroid related symptoms were less in the goserelin group than in the placebo group. The hysterectomy was technically easier and the median (range) operative blood loss was significantly lower in the goserelin group compared with the placebo group (187 (60-600) ml vs 308 (118-1000) ml respectively; P < 0.05, Wilcoxon signed rank test). There was no difference between the two groups in the duration of hospital stay or the frequency of post-operative complications. The fibroids were smaller at the time of operation in the goserelin group, and more women treated with goserelin were able to have their operations through a transverse incision. CONCLUSIONS: This study demonstrates the benefits of goserelin in women having total abdominal hysterectomy for uterine leiomyomata.

Adult↗