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Biomedical subjects

V Hasselblad

Publications and source records attributed to V Hasselblad.

At least 19 recordsLinked to original sources
PubMed1995

Prevalence of heterozygotes for hemochromatosis in the white population of the United States.

In previous studies, the prevalence of HLA-linked hemochromatosis, thought to be the most common genetic illness in whites, has been estimated by identifying homozygotes in the population. Because not all homozygotes express the disease phenotypically, the accuracy of these estimates is uncertain. We analyzed the distribution of transferrin saturation values in the second National Health and Nutrition Examination Survey to estimate the prevalence of hemochromatosis heterozygotes in the US population. After removing values for possible homozygotes, two populations were present (P < .01 for each gender). When weighted to reflect the US adult white male population as a whole, a proportion of 850 per 1,000 (95% confidence interval, 0.81 to 0.89) were included in a population with a lower mean saturation of 29.7% (29.1% to 30.3%), whereas 150 per 1,000 (0.11 to 0.19) comprised a population with a higher mean saturation of 47.0% (45.1% to 49.0%). Similar results were found for the female population. The gene frequencies were estimated to be 0.081 from the male population and 0.070 from the female population corresponding to prevalences of homozygotes of 6.6 and 4.8 per 1,000, respectively. Our results confirm that the gene for hemochromatosis is common.

Adolescent↗
PubMed1995

Noninvasive carotid artery testing. A meta-analytic review.

PURPOSE: To compare the operating characteristics of six noninvasive tests for carotid artery stenosis. DATA SOURCES: A structured search was done using MEDLINE, reference lists from selected articles, and bibliographies from neurology textbooks that focused on the diagnosis of carotid artery stenosis in humans. The search yielded 568 articles. STUDY SELECTION: Articles were selected if the noninvasive test results they presented used carotid angiography as the reference standard for comparison, if carotid artery occlusion was considered as a separate category, and if contingency tables could be constructed. DATA EXTRACTION: At least two physicians reviewed all selected articles. Items abstracted included patient demographics, study design, sites of patient enrollment, whether the interpretation of test outcomes was blinded, and specific results. Sensitivity, specificity, receiver operating characteristic (ROC) curves, and summary measures of effectiveness for each test were calculated. RESULTS: Carotid duplex ultrasonography, carotid Doppler ultrasonography, and magnetic resonance angiography have sensitivities between 0.82 and 0.86, specificities at 0.98, and test-effectiveness measures at or exceeding 3.0 when predicting 100% occlusion. For carotid stenosis of 70% or more, these three tests and supraorbital Doppler ultrasonography all have sensitivities of 0.83 to 0.86, specificities of 0.89 to 0.94, test-effectiveness measures approaching 3.0, and composite ROC areas of 0.91 to 0.92. Limiting analysis to studies that enrolled consecutive patients and those in which interpretation of the noninvasive tests was independent of the angiograms did not substantially change our results. CONCLUSIONS: Carotid duplex ultrasonography, carotid Doppler ultrasonography, and magnetic resonance angiography are all similarly successful at predicting 100% carotid artery occlusion and 70% stenosis. Other factors, such as cost, availability, and local experience may influence the decision to use these tests to screen for carotid artery atherosclerosis that may respond to surgery.

Carotid Stenosis↗
PubMed1995

Meta-analysis of environmental health data.

Most airborne contaminants have minimal health effects at typical ambient levels, and this is especially true for contaminant levels in the Arctic. Among multiple studies of similar health effects, usually only one or two of the studies show statistically significant effects. Only after an attempt to quantitatively combine the data might some consistency emerge or might the sources of inconsistency be explored. Methods for combining the data from such studies are presented. The methods include combining P-values, combining effect measures using fixed effects models, and combining effect measures using random effects models. Examples of the methods applied to several pollutants, including lead and oxides of nitrogen, are given. Some discussion of the implications of these findings to contaminant levels found in the Arctic is presented.

Air Pollutants↗
PubMed1995

Meta-analysis of screening and diagnostic tests.

Screening and diagnostic tests are common in the fields of psychology, medicine, and education. Often there are several competing tests, and decisions must be made about the relative accuracy of those tests. This article describes a general measure that can be used for both continuous and dichotomous outcome measures. It is the standardized distance between the means of the 2 populations. For continuous measures, it is the effect size measure. For dichotomous measures, it is proportional to the logarithm of the odds of the sensitivity plus the logarithm of the odds of the specificity. The measure is easily computed for both kinds of outcomes. Properties of this measure are discussed, and examples are given. Ths use of this measure to compare the average performance of different tests is described.

Diagnostic Tests, Routine↗
PubMed1995

Comparison and meta-analysis of randomized trials of endarterectomy for symptomatic carotid artery stenosis.

OBJECTIVE: Comparison and meta-analysis of randomized trials of carotid endarterectomy for symptomatic stenosis of the extracranial carotid artery. BACKGROUND: Randomized trials (North American Symptomatic Carotid Endarterectomy Trial [NASCET], the European Carotid Surgery Trial [ECST], and the VA Cooperative Study [VACS]) each show that carotid endarterectomy improves outcomes in selected symptomatic patients with high-grade extracranial carotid artery stenosis. Direct comparisons among the studies have not been possible because of differing methodologies, endpoints, and formats of data reporting. DESIGN/METHODS: Data for specified endpoints and corresponding person-years at risk were obtained for each trial. The rates of nonfatal stroke, nonfatal myocardial infarction, or death for surgically or medically treated patients in the perioperative period (30 days) and thereafter were compared (both including and excluding perioperative events) and then combined using meta-analytic techniques. Data from NASCET and ECST were also analyzed for differences in outcomes by sex. RESULTS: Event rate estimates (with 95% confidence intervals [95% CI]) for the first 30 days (events per person-year, primarily nonfatal stroke) for medically treated patients were 0.44 (0.22 to 0.76) for NASCET, 0.15 (0.04 to 0.38) for ECST, and 0.27 (0.03 to 0.96) for VACS. For surgically treated patients, the corresponding rates (per person-year) were 0.78 (0.49 to 1.19), 0.63 (0.41 to 0.94), and 1.27 (0.58 to 2.43). Event rates per year after 30 days were higher for medically treated patients (0.20 [0.16 to 0.25] versus 0.08 [0.05 to 0.11] for NASCET; 0.12 [0.10 to 0.15] versus 0.07 [0.06 to 0.09] for ECST; and 0.15 [0.07 to 0.25] versus 0.07 [0.03 to 0.16] for VACS). There were no significant differences among the trials, with an overall benefit for surgical therapy (risk ratio estimate, RR = 0.67, 95% CI = 0.54 to 0.83). There were no significant sex-based differences between NASCET and ECST and the overall benefit was not significantly different for men and women (RR = 0.58, 95% CI = 0.45 to 0.74 for men; RR = 0.84, 95% CI = 0.57 to 1.25 for women). CONCLUSIONS: Adjusting for primary endpoints and duration of follow-up, carotid endarterectomy has a similar benefit for symptomatic patients across trials and a similar benefit for men and women.

Aged↗
PubMed1995

A survey of current problems in meta-analysis. Discussion from the Agency for Health Care Policy and Research inter-PORT Work Group on Literature Review/Meta-Analysis.

The United States Agency for Health Care Policy and Research brought together representatives from each of the Patient Outcomes Research Teams (PORTs) as the Inter-PORT Meta-Analysis Work Group. The purpose of the meeting was to discuss a wide range of problems the panel had encountered in the area of meta-analysis. Several actual problems were presented, and tentative solutions to those problems were given. The PORTs had to consider issues such as quality assessment, survival analysis, rare events, sensitivity and specificity, and random effects models. The solutions presented represent the methods used by several researchers who are active in the area of meta-analysis.

Evaluation Studies as Topic↗
PubMed1992

Synthesis of environmental evidence: nitrogen dioxide epidemiology studies.

The use of meta-analysis is becoming more common in the medical literature, but it is not common in the environmental literature. Although meta-analysis cannot combine a group of poorly executed, conflicting studies to get an unequivocal answer, there are certain situations where it can be helpful. The inability of studies to produce similar results may be a function of the power of the studies rather than a reflection of their quality. The literature on the effects of nitrogen dioxide on the odds of respiratory illness in children is such an example. Three quantitative methods for the synthesis of this evidence are presented. Although the methods produce slightly different results, the conclusion from all three methods is that the increase in the odds of respiratory illness in children exposed to a long-term increase of 30 micrograms/m3 (comparable to the increase resulting from exposure to a gas stove) is about 20 percent. This estimated increase is not sensitive to the method of analysis.

Air Pollution↗
PubMed1990

A Bayesian method for synthesizing evidence. The Confidence Profile Method.

This article describes a collection of meta-analysis techniques based on Bayesian statistics for interpreting, adjusting, and combining evidence to estimate parameters and outcomes important to the assessment of health technologies. The result of an analysis by the Confidence Profile Method is a joint posterior probability distribution for the parameters of interest, from which marginal distributions for any particular parameter can be calculated. The method can be used to analyze problems involving a variety of types of outcomes, a variety of measures of effect, and a variety of experimental designs. This article presents the elements necessary for analysis, including prior distributions, likelihood functions, and specific models for experimental designs that include adjustment for biases.

Bayes Theorem↗
PubMed1988

The value of mammography screening in women under age 50 years.

Two quantitative methods, Confidence Profiles and CAN*TROL, are used to analyze evidence and estimate the health and economic consequences of adding annual mammography to annual breast physical examinations in asymptomatic women aged 40 to 49 years who are at average risk for breast cancer. Such women have about a 128 in 10,000 chance of having breast cancer in the next ten years and about an 82 in 10,000 chance of dying of such a cancer. Adding annual mammograms to annual breast physical examinations each year during that age decade would reduce the probability of death to about 60 in 10,000, a reduction of about 26%. Screening would increase the expected lifetime of a woman destined to get breast cancer between ages 40 and 49 years by about 3.5 years. Ten years of screening with mammography in that age decade carries a risk of radiation-induced cancer of about one in 25,000 and a risk of a surgery recommendation for a lesion that is not cancer of about one in ten. If 25% of the women in this age group in the United States were screened every year, breast cancer mortality in the year 2000 would be decreased by about 373 deaths. In 1984 dollars, the cost of screening, workups, and continuing care in the year 2000 would be about $408 million. Treatment costs would be decreased by about $6 million, leaving a net increase in costs in the year 2000 of approximately $402 million (1984 dollars).

Adult↗
PubMed1987

Statistical and graphical evaluation of erythrocyte volume distributions.

A protocol for the statistical and graphical evaluation of erythrocyte volume distributions has been developed using expectation-maximization methods for distribution parameter estimation. In a reference sample group of 50 healthy individuals, none of the red cell volume distributions determined by an aperture impedance method differed significantly from a log normal model. Measurements in 211 patients with anemia provided evidence that similar statistical methods could be used for the characterization of abnormal red cell volume distributions. Graphical displays accurately summarize the quantitative information derived from the analysis of volume distributions and show a reference distribution fitted to the observed distribution against the range of distributions found in healthy individuals. This protocol provides a specific, statistically valid implementation of the general principles for the analysis of cell volume distributions proposed by the International Committee for Standardization in Hematology.

Anemia↗
PubMed1986

Analysis of the volume of red blood cells: application of the expectation-maximization algorithm to grouped data from the doubly-truncated lognormal distribution.

In accordance with general principles recommended by the International Committee for Standardization in Haematology (1982, Journal of Clinical Pathology 35, 1320-1322), we have developed statistical methods for the analysis of red cell volume distributions. To select an appropriate reference distribution for goodness-of-fit testing, we derived a mathematical model of erythropoiesis that predicted a lognormal form for the distribution of erythrocyte volumes. Model predictions were then tested using samples obtained from 50 healthy individuals. Each grouped red cell volume distribution was doubly-truncated to eliminate artifactual frequency counts. Distribution parameter estimates were computed using the expectation-maximization algorithm, a missing information technique. Results of the one-sample chi-square goodness-of-fit test showed a fairly even distribution of P-values over the interval. Examples of the application of these statistical procedures to distributions from patients with anemia are given. Our results suggest that, for the analysis of red blood cell volumes, (i) parameter estimation should be made with the expectation-maximization method, and (ii) the truncated lognormal distribution should be used as a reference distribution for goodness-of-fit testing. This method could be applied to any set of grouped doubly-truncated data which, after transformation, follows the normal model.

Anemia, Hypochromic↗
PubMed1985

Prevalence of persistent cough and phlegm in young adults in relation to long-term ambient sulfur oxide exposure.

In early 1976, a survey of persistent cough and phlegm (PCP) prevalence was conducted in 5,623 young adults in 4 Utah communities. Over the previous 5 years, community-specific mean sulfur dioxide levels had been 11, 18, 36, and 115 micrograms/m3. Corresponding mean suspended sulfate levels had been 5, 7, 8, and 14 micrograms/m3. No intercommunity exposure gradient of total suspended particulates or suspended nitrates was observed. In nonsmoking mothers, PCP prevalence was 4.2% in the high-exposure community and approximately 2.0% in all other communities. In smoking mothers, PCP prevalence was 21.8% in the high-exposure community and approximately 15.0% elsewhere. In nonsmoking fathers, PCP prevalence was 8.0% in the high-exposure community and averaged 3.0% elsewhere. In smoking fathers, PCP prevalence was less strongly associated with ambient sulfur oxide exposure. Intercommunity prevalence differences in smoking and nonsmoking mothers, and in nonsmoking fathers, were significant at alpha = 0.05. A categorical logistic regression model, testing simultaneously for effects of community and several covariates on PCP prevalence, yielded similar results. The results of this survey were similar to those of a similar survey conducted in Utah in 1970 and to those of other surveys. These results disclose an association of PCP prevalence with ambient sulfur oxide exposure, stronger in mothers than in fathers, stronger in nonsmokers and ex-smokers than in smokers, and stronger in 1970 than 1976.

Adult↗
PubMed1981

Modeling the Ames test.

Despite the value and widespread use of the Ames test, little attention has been focused on standardizing quantitative methods of analyzing these data. In this paper, a realistic and statistically tractable model is developed for the evaluation of Ames-type data. The model assumes revertant colony formation at any dose follows a Poisson process, while the mean number of revertants per plate is a nonlinear function of up to 4 parameters. An exponential decay term can be included in the model to adjust for toxicity. The resultant system of nonlinear equations is solved using a modified Gauss-Newton iterative scheme to obtain maximum likelihood estimates of the model parameters. Significance of the key parameters is tested by fitting reduced models and using likelihood ratio tests. The model's performance is demonstrated on data from organic extracts of various environmental contaminants. Among the advantages of the proposed model are (1) no data is discarded in the parameter estimation process, (2) no arbitrary constants need to be added to zero counts or doses, and (3) no mathematical transformation of the data is required.

Mathematics↗
PubMed1981

Indoor environmental determinants of lung function in children.

Using pulmonary function and family respiratory questionnaire data for 16,689 white children 6 to 13 yr of age from 7 geographic areas, the investigators examined the effect of several environmental and other factors on performance, in a standard test of breathing. As expected, FEV0.75 was correlated most strongly with age, height, and sex. A dose-response relationship was observed with maternal smoking habits and explained 0.1% of the variance. No effect caused by the father's smoking habits was observed. A decrease (p = 0.0524) in FEV among older girls was associated with the presence of a gas cooking stove in the home. Although the statistical significance of the decreases was largely attributable to the size of the sample, the decreases in FEV, even though small, were thought to be biologically significant.

Adolescent↗
PubMed1978

Spirometric changes in normal children with upper respiratory infections.

Recent evidence that certain uncomplicated upper respiratory infections induce pulmonary function abnormalities in adults prompted a prospective study in children, in whom such infections occur more frequently. In a longitudinal study, 55 children 2.5 to 11 years of age were observed for a mean duration of 2 years. Spirometry and lung volume studies were obtained routinely every 3 months, during each upper respiratory infection, and 4 weeks after illnes, providing data for 617 "well" and 237 "illness" observations. After grouping of data by sex and age (less than 84 of greater than 84 months), each spirometric parameter was analyzed using linear regression with individual identification, height, and clinical status (normal versus upper respiratory illness) as independent variables. Adjusted mean values of forced vital capacity, 1-sec forced expiratory volume, peak expiratory flow, maximal mid-expiratory flow, and expiratory flow at 50 per cent of the forced vital capacity all decreased during upper respiratory illness. The data suggest that lower respiratory tract involvement without signs or symptoms of lower airway or alveolar disease occurs with upper respiratory illnesses of varied etiologic origin in childhood.

Body Height↗