Biomedical subjects

G G Koch

Publications and source records attributed to G G Koch.

At least 73 records · Page 4Linked to original sources

PubMed1994

Application of sample survey methods for modelling ratios to incidence densities.

We describe ratio estimation methods for multivariately analysing incidence densities from prospective epidemiologic studies. Commonly used in survey data analysis, these ratio methods require minimal distributional assumptions and take into account the random variability in the at-risk periods. We illustrate their application with data from a study of lower respiratory illness (LRI) in children during the first year of life. One question of interest is whether children with passive exposure to tobacco smoke have a higher rate of LRI, on average, than those with no exposure and in a setting where age of child and season are taken into account. A second question is whether the relationship persists after adjusting for background variables such as family's socioeconomic status, crowding in the home, race, and type of feeding. The basic strategy consists of a two-step process in which we first estimate subgroup-specific incidence densities and their covariance matrix via a first-order Taylor series approximation. These estimates are used to test for differences in marginal rates of LRI between children exposed to tobacco smoke and those not exposed. We then fit a log-linear model to the estimated ratios in order to test for significant covariate effects. The ability to produce direct estimates of adjusted incidence density ratios for risk factors of interest is an important advantage of this approach. For comparison purposes and to address the limitations of the ratio method with respect to the number of covariates that can be controlled simultaneously, we consider survey logistic regression methods for the example data as well as logistic and Poisson regression models fitted via generalized estimating equation methods. Although the analysis strategy is illustrated with illness data from an epidemiologic study, the context of application is broader and includes, for example, data on adverse events from a clinical trial.

Bronchitis↗

PubMed1994

Correlates of blood pressure in Seventh-Day Adventist (SDA) and non-SDA adolescents.

OBJECTIVE: This comparative study was designed to discover early determinants of systolic (S) and diastolic (D) blood pressure (BP) elevations in 138 Seventh-Day Adventist (SDA) and 89 non-SDA male and female adolescents (median age, 17 years) living at three residential secondary schools in North Carolina. METHODS: Measurements were made of blood pressure, body weight, and height, and information was collected on lifestyle factors, dietary intake, and other behaviors, including exercise, religiosity, Type A behavior, and anger, by questionnaire. Multiple stepwise regression analyses were performed with BP, either SBP or DBP, as the independent variable. RESULTS: A significant direct association was found only between body weight and BP, but weak associations were shown between BP and other variables, including exercise, diet, religiosity, Type A behavior, and anger. Male and female SDA students showed significantly higher SBPs and DBPs than did non-SDA adolescents though the differences were small (approximately 5 mm for each sex). CONCLUSIONS: These findings suggest that the higher BP values of SDA adolescents, who were all practicing lacto-ovo-vegetarians, compared to similarly aged health-conscious non-SDAs, are determined more by eating behaviors that contribute to gains in body weight than by any other lifestyle variable. Furthermore, these data support the notion that the BP-protective effects of the vegetarian diet may not emerge in these SDA youth until early adulthood.

Adolescent↗

PubMed1994

Statistical evaluation of relationships between analgesic dose and ordered ratings of pain relief over an eight-hour period.

Statistical considerations are discussed for the application of alternative methods to a clinical trial involving repeated ordinal ratings and multiple dosage levels of active drugs. Analyses included summary measures traditionally employed in studies of acute pain: sum of pain intensity differences from baseline, total pain relief, and total pain half gone. Estimators and confidence intervals of relative potency are developed for univariate and multivariate situations, using weighted least squares analysis with mean response and variances from Taylor series linearizations. The estimates from these methods are compared to those from traditional methods, such as ordinary least squares regression and Fieller's method for confidence intervals, as well as those from more recent developments, such as generalized estimating equations and sample survey data regression. A double-blind, two-center, randomized clinical trial of acute pain relief comparing placebo with two analgesics, each at two dosage levels, over an 8-hr period serves as an illustrative example for these techniques and comparisons.

Adolescent↗

PubMed1994

Statistical considerations for design and analysis of a multiperiod crossover study to compare two treatments for migraine headache.

The purpose of this paper is to discuss a long-term, multiperiod crossover study to compare two treatments for migraine headache. Principal attention is given to the analysis of an example in which patients randomly received a treatment sequence with test drug for three migraine headaches and placebo for one. An issue that requires attention for this example is the influence of a carryover effect for test drug that was greater when placebo was the subsequent treatment than when test drug was the subsequent treatment. A way to address this issue without excessive loss of power is to consider tests of "total treatment effects," which are weighted averages across headaches of differences between average response to test drug and average response to placebo. Since these weighted averages are linear combinations of treatment effects and carryover effects, their use requires an argument that carryover effects are at least partly a further form of treatment effects. For the example in this paper, this argument is realistic because the test drug could have provided much better relief to migraine headache than other treatment patients might have previously used. A second purpose of the paper is to present some alternative designs for situations like that represented by the example. The structure of variances for alternative specifications for treatment comparison is provided for models of interest for these designs.

Cross-Over Studies↗

PubMed1994

Evaluation of alternative statistical models for crossover studies to demonstrate caffeine adjuvancy in the treatment of tension headache.

This paper discusses alternative statistical models for the analysis of six crossover studies to determine whether better relief of tension headache occurs from treatment with an analgesic plus caffeine (C) than with the analgesic alone (A) or with placebo (P). Each patient in these crossover studies randomly received a pair of distinct medications in such a way as to treat the first two of four headaches with the initial medication in the pair and to treat the third and fourth headaches with the last medication in the pair. In order to have greater power for the C versus A comparison, three times as many patients were randomly assigned to the A:C and C:A sequence groups as to the A:P, C:P, P:A, and P:C sequence groups. An issue of statistical interest for these crossover studies is the extent to which the possibility of unequal carryover effects of the three medications influences the roles of alternative models for data analysis and the interpretation of results. When carryover effects for all three medications are equal, univariate analysis of variance for the difference scores between the average response for the first two headaches and the average response for the third and fourth headaches for each patient provides nearly the same power for pairwise treatment comparisons as more comprehensive multivariate methods for all four headaches. However, for comparisons concerning carryover effects and for treatment comparisons with adjustment for carryover effects, multivariate methods encompassing all four headaches jointly can provide greater power than univariate analysis for difference scores, particularly when there is low intraclass correlation for responses within the same patient. Another noteworthy role for multivariate methods in situations with potentially unequal carryover effects is their capacity to clarify whether multiple types of carryover effects occur across the second, third, and fourth headaches in the respective sequence groups. Multivariate models with alternative specifications of carryover effects are fit to the data from the six crossover studies to compare C, A, and P by weighted least squares. The role of potential variation among centers is addressed in these analyses by the use of stratified proportional means over centers, means of center means, and means ignoring centers. The primary focus of attention in the respective analyses is the evaluation of treatment comparisons with and without adjustment for potential differences among carryover effects of the treatments.(ABSTRACT TRUNCATED AT 400 WORDS)

Caffeine↗

PubMed1994

The MDS Cognition Scale: a valid instrument for identifying and staging nursing home residents with dementia using the minimum data set.

OBJECTIVE: We report the development and validation of an MDS-based cognitive index, the MDS Cognition Scale (MDS-COGS), by evaluating it against two popular dementia rating scales, the Global Deterioration Scale (GDS) and the Mini-Mental State Examination (MMSE). DESIGN: A Cross-sectional study. PARTICIPANTS: Two hundred nursing home residents. MEASUREMENTS: Each study participant was assessed on the GDS and the MMSE by trained medical students. At the same time, but independent of the medical students, a geriatric nurse completed the Minimum Data Set (MDS) instrument on each participant. MAIN RESULTS: The Cognitive Performance Scale (CPS), a categorical measure of cognition that uses MDS items, was compared with the GDS in 133 subjects, using a split-sample technique. The GDS was found to be more appropriate as a 4-stage than a 7-stage scale, with GDS stages 1-4 acting as a single stage. The CPS showed very poor percent agreement with GDS stages 5 and 7 (50% or less) and, therefore, was revised by adding other MDS predictors. The new instrument, designated MDS-COGS, is a 0-10 point scale generated from eight MDS cognitive items. The MDS-COGS was then validated against the 4-stage GDS and MMSE in the remaining 67 nursing home residents. Chance-corrected agreement (kappa) between the MDS-COGS and GDS in the 67 subjects was 0.80 (95% CI = 0.70, 0.88), and percent agreement with GDS stages 5 and 7 was 60% and 85%, respectively. The MDS-COGS was also demonstrated to be a valid measure of cognitive impairment as defined by the MMSE, with sensitivity, specificity, chance-corrected agreement (kappa), and area under the ROC curve, all above 0.80. CONCLUSIONS: The MDS Cognition Scale, the MDS-COGS, provides a valid measure of the presence and severity of cognitive impairment in nursing home residents using items from the Minimum Data Set.

Aged↗

PubMed1994

Characteristics of older adults experiencing periodontal attachment loss as gingival recession or probing depth.

While attachment loss currently is the best field examination indicator that periodontal disease has progressed, this measure is not disaggregated enough to differentiate between attachment loss that results in deeper pockets and attachment loss that results primarily in recession of the gingiva. If these two conditions have different etiologies, then efforts at predicting who will experience the condition from a single, general risk assessment model will be difficult. The purpose of this paper is to determine whether the characteristics of people who experience attachment loss primarily as increased probing depths are different from those people who experience attachment loss primarily as gingival recession. Baseline and 18-month follow-up periodontal measures were gathered on a representative sample of community-dwelling adults aged 65+. Based on measurement errors obtained one week apart on a reliability subsample, a threshold level of 3+mm was set to define attachment loss. Attachment loss was further characterized as resulting from mostly increased probing depth or from mostly increased gingival recession. In mesio-buccal sites, 58% of the people had attachment loss mostly due to increased probing depth with 42% having attachment loss mostly due to gingival recession. For buccal sites, the figures were 24% and 76% respectively. The logistic regression models for both sites indicated that people who had their attachment loss in the form of increased probing depths had different characteristics than people who had most of their attachment loss in the form of gingival recession.(ABSTRACT TRUNCATED AT 250 WORDS)

Black or African American↗

PubMed1994

Eighteen-month coronal caries incidence in North Carolina older adults.

In this longitudinal study of a random sample of North Carolinians over the age of 65 and living in their homes, 325 blacks and 280 whites were examined and interviewed 18 months after baseline examinations. Coronal caries incidence was greater among whites than blacks. The increment due to teeth becoming root fragments were similar for both races; however, there were more newly crowned teeth among whites. Newly crowned surfaces were not used as part of the caries increment in logistic regression models to investigate potential risk predictors. For blacks, caries development over the 18-month period was associated with a higher lactobacillus score and more coronal caries at baseline, more previously filled coronal surfaces, and lack of active membership in clubs or other groups. For whites, having no self-reported tooth sensitivity, having a lower socioeconomic index score, taking antihistamine medications at baseline, and having the perception of more problems after the age of 40 than before were all associated with the development of coronal caries.

Black or African American↗

PubMed1994

The prevalence of dental sealants in North Carolina schoolchildren.

OBJECTIVES: This study reports the prevalence of dental sealants derived from a survey of schoolchildren in North Carolina, and its variation according to several sociodemographic characteristics. METHODS: A single-stage, stratified probability sample of grade K-12 classrooms in the state yielded a sample of 8,026 students. Clinical examinations were performed on 83 percent of the sample. Estimates for sealant prevalence and their variation according to seven sociodemographic variables were determined. RESULTS: About 117,000 children in the state, or 12 percent of those 6-17 years of age, have sealants. The prevalence of sealants varied according to geographic region of residence and the socioeconomic status of whites. Prevalence was affected little by age, sex, degree of urbanism, or socioeconomic status of those with races other than white. CONCLUSIONS: While underutilized, there are indications in these data that sealant use is increasing, thus having the potential to contribute to further declines in the prevalence of dental caries in the state. Those factors included in this study and associated with the prevalence of sealants suggest that major gains in sealant use will need to address not only those specific barriers preventing their use by the profession or public, but broader issues preventing the utilization of dental services in general.

Adolescent↗

PubMed1994

Review of nonparametric methods for the analysis of crossover studies.

This paper reviews nonparametric methods for the analysis of crossover studies. Primary attention is given to crossover studies to compare two treatments for a response variable that has a metric measurement level. For this situation, one can often test hypotheses or obtain confidence intervals for parameters of interest by applying well known univariate nonparametric rank methods (e.g., the Wilcoxon rank sum test, or the Wilcoxon signed rank test) to appropriately specified functions of the data. Related extensions are also available, to some degree, for crossover studies to compare more than two treatments or those for which the measurement level of the response variable is ordinal or has a censored time-to-event nature. Methods for several specific situations along these lines are discussed in terms of principles with potentially broader applicability. Several examples are provided to illustrate the performance of some of the methods.

Confidence Intervals↗

PubMed1994

Attachment loss trends over 3 years in community-dwelling older adults.

Most of our knowledge regarding the natural history of periodontitis in adults has come from clinical studies of dental patients. However, it is known that dental patients usually are not representative of what actually happens in the population. For this type of information, epidemiologic studies are required. The purpose of this paper is to present the trends in attachment loss over a 3-year period in a population of community-dwelling elderly blacks and whites. Specifically, this paper presents attachment loss trends both at the person and site level to address the following issues: 1) whether attachment loss during one period in time makes a person at higher risk for attachment loss at a subsequent period; 2) whether sites with deeper periodontal pockets at baseline are more likely to experience future attachment loss; and 3) whether teeth that experience attachment loss during one time period are more likely to be lost at the next time period. In 1988, the University of North Carolina School of Dentistry initiated the Piedmont 65+ Dental Study, which is a subsample of the parent study, the Piedmont 65+ Study of the Elderly under the direction of investigators from Duke University. The dental study subsample was designed to elicit 800 dentate respondents in the 5-county area who were examined again at 18 and 36 months. Our findings indicated that, for the two 18-month periods investigated, attachment loss during the first period was related positively to the incidence of attachment loss at a subsequent period at the person level, but not at the site level.(ABSTRACT TRUNCATED AT 250 WORDS)

Black or African American↗

PubMed1994

Estimating activity limitation in the noninstitutionalized population: a method for small areas.

OBJECTIVES: Although reliable direct state and local estimates of the activity-limited population are frequently unavailable, regression-adjusted synthetic estimates can be made. Such estimates use multivariate methods to model activity limitation at the national level and then apply model-predicted probabilities to corresponding community-specific demographic data. METHODS: Using the 1989 National Health Interview Survey and the 1991 Area Resource File System, this study produced log-linear regression models that included person-level demographic and county-level contextual variables as predictors of activity limitation. Model-predicted rates were then multiplied by corresponding intercensal population data to generate state and local synthetic estimates of activity limitation. RESULTS: Rates of activity limitation generally were found to increase with age and as the socioeconomic conditions of the county in which an individual resided worsened. Race and sex also tended to be statistically significant predictors of activity limitation. CONCLUSIONS: Activity limitation can be effectively modeled by age, sex, race, and community socioeconomic status. Synthetic estimates such as these are relatively simple to generate and can be useful for small-area planning in the absence of direct local estimates.

Activities of Daily Living↗

PubMed1994

Pooling of long term clinical wear data for posterior composites.

Clinical studies to evaluate the wear of posterior composite restorations are complicated by the large number of factors which influence the findings. A multi-factorial equation has been developed which successfully normalizes the effects of these factors within studies. This equation is not capable of normalizing these effects for other investigations reported in the literature because study characteristics essential to the analysis are rarely reported. Quantitative estimates of wear rate often differ dramatically between studies and between groups of investigators. The objective of this study was to investigate a potential solution to this problem by ranking materials within studies, using common materials to relate rankings across studies, and achieving an overall pooled ranking for products. An intensive literature search disclosed 78 articles and 46 abstracts on clinical posterior composite wear. All studies were analyzed which involved: (1) more than five restorations per material, (2) Class I or II restorations in posterior adult teeth, (3) characterized commercial products, (4) 2 or 3-year wear data, and (5) information on more than one composite material per study. There were 10 2-year studies involving 25 materials, and 10 3-year studies with 26 materials. Within each study the materials were ranked by wear, and the rankings were converted to centered modified ridits. A meta-analysis combining the data across studies was conducted using ANOVA. Although caution is needed for interpreting significance levels because of the small numbers of products evaluated per study, a high level of agreement in rank correlation of 28 products across qualifying studies was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1993

The roles of von Willebrand factor and factor VIII in arterial thrombosis: studies in canine von Willebrand disease and hemophilia A.

We have studied the roles of von Willebrand factor (vWF) and factor VIII in arterial thrombosis in four canine phenotypes: normal (n = 6), hemophilia A (n = 11), von Willebrand disease (vWD) (n = 9), and hemophilia A/vWD (n = 1). vWF activity was determined by botrocetin-induced agglutination of fixed human platelets and vWF antigen (vWF:Ag) by Laurell electroimmunoassay and crossed immunoelectrophoresis. Plasma from normal dogs and those with hemophilia A had vWF activity, vWF:Ag, and a full range of vWF:Ag multimers on gel electrophoresis equivalent to normal canine plasma pool. Platelet cytosol contents were isolated by freezing and thawing, triton X-100 solubilization, or sonication of washed platelets with and without protease inhibitors and inhibitors of platelet activation. Washed platelets were also stimulated with calcium ionophore and MgCl2. There was no measurable vWF activity or vWF:Ag in platelet lysates or releasates in any dog regardless of phenotype. All dogs were studied using a standard arterial stenosis and injury procedure to induce arterial thrombosis. Thromboses were detected by cyclic reductions in Doppler blood flow velocity. Vessels were examined by light and scanning electron microscopy. Thrombosis developed in the arteries of normal (9 of 10) and hemophilia A dogs (16 of 16) but in none of the vWD dogs (0 of 10). Infusion of canine vWF cryoprecipitate into vWD dogs markedly shortened bleeding time but did not support thrombosis as seen in dogs with vWF in the plasma and subendothelium. Thrombosis, then, fails to occur when vWF is absent from the plasma and subendothelial compartments or present only in the plasma compartment. These data are consistent with the hypothesis that vWF in the plasma and subendothelium supports thrombosis. Neither plasma FVIII nor platelet vWF is essential for thrombosis in this model.

Animals↗

PubMed1993

Beta-endorphin response to exercise and mental stress in patients with ischemic heart disease.

UNLABELLED: We compared symptomatic, hemodynamic and opioid responses of heart disease patients to exercise testing and a stressful public speaking task. Plasma beta-endorphins were measured at rest and immediately post stress. Nineteen of 50 patients had angina during exercise; 31 had asymptomatic ischemia. No patient had angina during the speech, but two had ECG changes and 39% had radionuclide changes indicating ischemia. Patients with asymptomatic ischemia on exercise had a significantly greater beta-endorphin response than those with angina. Public speaking elicited a significantly larger beta-endorphin increase relative to change in double product (an index of stress) than did exercise. CONCLUSIONS: (1) Patients with silent vs painful ischemia experience a greater beta-endorphin response to exercise. (2) beta-endorphin response to a speech stressor is greater than to exercise when controlled for an index of stress. (3) Increased beta-endorphin response to a speech stressor may partially explain the predominance of silent ischemia during psychological stress.

Aged↗

PubMed1993

HIV seroprevalence in sexually transmitted disease clients in a low-prevalence southern state. Evidence of endemic sexual transmission.

We studied human immunodeficiency virus (HIV) seroprevalence and risk factors in 3052 clients attending three large public, sexually transmitted disease (STD) clinics in central North Carolina in mid-1988. Anonymous self-administered questionnaires linked to HIV serologies obtained by testing extra blood from syphilis serologies without personal identifiers showed the following characteristics of the respondents: 60% were men, 81% were black, the median age was 24 years, 5% were injecting drug users since 1978, 7% reported a history of syphilis, and 8% of men were homosexual or bisexual. HIV seropositivity was found in 76 subjects (2.5%), including 46% of the homosexual men, 25% of the bisexual men, 1.6% of the heterosexual men, and 0.6% of the women. Elevated HIV seroprevalence rates were found in subjects with a history of or seroreactivity for syphilis (HIV-positive rate of 53% in homosexual or bisexual men, 9% in heterosexual men, 3% in women) and with histories of gonorrhea (HIV-positive rate of 37% in homosexual or bisexual men, 2.6% in heterosexual men, 1% in women), and intercourse (41% in homosexual or bisexual men, 2% in women), prostitute contact (5% in heterosexual men), and sex with casual partners (2% in women). Even a state with a low incidence of acquired immunodeficiency syndrome can include subpopulations with a high HIV seroprevalence, apparently disseminated endemically in association with bacterial STDs.

Adolescent↗

PubMed1993

Statistical methods for a three-period crossover design in which high dose cannot be used first.

Design and analysis methods for the three-period crossover trial defined by the sequences: (D0, D1, D2), (D1, D0, D2), and (D1, D2, D0), where D0 is a placebo, and D1 and D2 are a low dose and a high dose of a drug, respectively, are developed. This design may be used when investigators are unwilling to administer a higher dose of a new drug to a patient before administering a lower dose. In using this design, patients should be randomized to sequences in blocks that are integer multiples of 3. Both parametric and non-parametric analysis methods are based on contrasts that capture intrapatient variability only and provide unbiased estimates and hypothesis tests of pairwise differences between carryover, direct dose, and period effects. The design and methods are illustrated with data reflecting the cognitive component of the Alzheimer's disease assessment scale collected in a large clinical trial of Tacrine at doses of 0, 40, and 80 mg/day.

Alzheimer Disease↗

PubMed1993

Statistical considerations for cross-sectional data relating to tracheal reconstruction over time.

Complementary statistical analyses are performed on data concerning tracheal reconstruction in rats. The principal response criterion is vessel area as defined by the area of a graft onto the trachea covered by blood vessels. Postoperatively, this vessel area changes in a nonlinear fashion over time. Starting at 0% immediately postoperation, the vessel area increases to a peak and then sharply decreases to a certain level at which it remains. A total of 64 independently observed cross-sectional pairs (postop day, vessel area) are distributed among four treated groups. Analysis consists of two-way analysis of variance and piecewise linear regression under the usual assumptions of normality and variance homogeneity. Other analyses that relax these assumptions are also considered; treatment differences are established by nonparametric tests, and a quasi-likelihood analysis of the piecewise linear regression model is applied under the assumption of a binomial-like variance function. The resulting fit of the normal-theory-based piecewise linear model is compared to that of a nonlinear model whose shape is that of a gamma function integrand.

Analysis of Variance↗