Biomedical subjects

David T Redden

Publications and source records attributed to David T Redden.

At least 19 recordsLinked to original sources

PubMed2006

Marker genotypes and population admixture and their association with body weight, height and relative body mass in United States federal bison herds.

Elucidating genetic influences on bison growth and body composition is of interest, not only because bison are important for historical, cultural, and agricultural reasons, but also because their unusual population history makes them valuable models for finding influential loci in both domestic cattle and humans. We tested for trait loci associated with body weight, height, and bison mass index (BMI) while controlling for estimated ancestry to reduce potential confounding effects due to population admixture in 1316 bison sampled from four U.S. herds. We used 60 microsatellite markers to model each phenotype as a function of herd, sex, age, marker genotypes, and individual ancestry estimates. Statistical significance for genotype and its interaction with ancestry was evaluated using the adaptive false discovery rate. Of the four herds, two appeared to be admixed and two were nonadmixed. Although none of the main effects of the loci were significant, estimated ancestry and its interaction with marker loci were significantly associated with the phenotypes, illustrating the importance of including ancestry in the models and the dependence of genotype-phenotype associations on background ancestry. Individual loci contributed approximately 2.0% of variation in weight, height, and BMI, which confirms the utility and potential importance of adjusting for population stratification.

Animals↗

PubMed2006

Regional admixture mapping and structured association testing: conceptual unification and an extensible general linear model.

Individual genetic admixture estimates, determined both across the genome and at specific genomic regions, have been proposed for use in identifying specific genomic regions harboring loci influencing phenotypes in regional admixture mapping (RAM). Estimates of individual ancestry can be used in structured association tests (SAT) to reduce confounding induced by various forms of population substructure. Although presented as two distinct approaches, we provide a conceptual framework in which both RAM and SAT are special cases of a more general linear model. We clarify which variables are sufficient to condition upon in order to prevent spurious associations and also provide a simple closed form "semiparametric" method of evaluating the reliability of individual admixture estimates. An estimate of the reliability of individual admixture estimates is required to make an inherent errors-in-variables problem tractable. Casting RAM and SAT methods as a general linear model offers enormous flexibility enabling application to a rich set of phenotypes, populations, covariates, and situations, including interaction terms and multilocus models. This approach should allow far wider use of RAM and SAT, often using standard software, in addressing admixture as either a confounder of association studies or a tool for finding loci influencing complex phenotypes in species as diverse as plants, humans, and nonhuman animals.

Computer Simulation↗

PubMed2006

Glycemic index and glycemic load of popular weight-loss diets.

CONTEXT: Carbohydrate-restricted diets have been popular in recent years. The theoretical glycemic impact of these diets, compared with other popular weight-loss diets, has not been reported. OBJECTIVE: To assess the glycemic index (GI) and glycemic load (GL) of 2 popular, carbohydrate-restricted diets (South Beach and Sugar Busters!) and compare them with a low-fat, high-carbohydrate diet (Ornish) and a moderate-fat, moderate-carbohydrate diet (EatRight). DESIGN: All available sample menus provided in the book for each diet were extracted and included in the analyses. GI values for all carbohydrate-containing foods were assigned based on a published list. GL values were determined based on these GI values and recommended serving sizes. MAIN OUTCOME MEASURES: Median daily GI and GL values were calculated for each diet and for each phase of South Beach and each meal pattern of EatRight. RESULTS: The median daily GLs of South Beach and Sugar Busters! (34 and 48, respectively) were less than one half those of Ornish and EatRight (113 and 104, respectively). Adjusting the diets to 1500 kcal attenuated the differences slightly. The median daily GIs of the diets were in a very narrow range (46-53); however, South Beach (46) was significantly lower than Ornish (53). CONCLUSIONS: The GLs of 2 carbohydrate-restricted diets were significantly lower than those of a low-fat, high-carbohydrate diet and a moderate-fat, moderate-carbohydrate diet. The differences were due primarily to lower carbohydrate content rather than to differences in overall GIs of the diets.

Blood Glucose↗

PubMed2006

The effect of assortative mating upon genetic association studies: spurious associations and population substructure in the absence of admixture.

Spurious associations due to confounding factors are an often cited and intensely debated concern for genetic association studies. Great attention has been focused upon the specific threat of confounding due to population stratification. This emphasis has spurred the development of many statistical genetic methods to detect and correct for the potentially confounding effects of admixture. Unfortunately, this emphasis on admixture has led some authors to suggest that if ethnically homogenous populations are used, spurious associations are unlikely to occur. We show that under small and realistic degrees of assortative mating over time, spurious associations arise even in ethnically homogeneous populations. We demonstrate that structured association and genomic control tests can, under certain conditions, correct for these spurious associations. We conclude that investigators should not assume spurious associations will not occur in association studies using ethnically homogenous populations and recommend the use of genomic control methods and/or family-based association tests within genetic association studies.

Chromosome Mapping↗

PubMed2006

Outcomes with porcine graft placement in the anterior vaginal compartment in patients who undergo high vaginal uterosacral suspension and cystocele repair.

OBJECTIVE: The purpose of this study was to prospectively assess the cases of patients who had undergone a high uterosacral suspension and anterior repair with anterior compartment placement of porcine dermis graft. STUDY DESIGN: Thirty-six patients who underwent transvaginal high uterosacral suspension and cystocele repair with graft augmentation from June 2001 to July 2004 were identified from the University of Alabama at Birmingham Genitourinary Disorders database. Analysis included the pre- and postoperative Pelvic Organ Prolapse Quantification examinations and incontinence impact questionnaire-7/urogenital distress inventory-6. RESULTS: Mean Pelvic Organ Prolapse Quantification Ba improved from +3.3 +/- 2.2 cm to -0.6 +/- 1.7 cm (P < .01). Postoperative Ba was prolapse stage II or greater in 50% of subjects. Mean incontinence impact questionnaire-7 scores improved from 36.2 +/- 31.9 to 15.6 +/- 26.2 (P < .01), as did mean urogenital distress inventory-6 scores from 58.2 +/- 26.8 to 23.8 +/- 22.6 (P < .01). CONCLUSION: Significant improvements in Pelvic Organ Prolapse Quantification measures, urinary symptoms, and the impact of incontinence were seen after the operation. However, a significant proportion of patients had Pelvic Organ Prolapse Quantification stage II prolapse or greater, which made it unclear whether graft use confers a significant advantage.

Aged↗

PubMed2006

Preoperative biofeedback assisted behavioral training to decrease post-prostatectomy incontinence: a randomized, controlled trial.

PURPOSE: We tested the effectiveness of preoperative biofeedback assisted behavioral training for decreasing the duration and severity of incontinence, and improving quality of life in the 6 months following radical prostatectomy. MATERIALS AND METHODS: We performed a prospective, randomized, controlled trial comparing preoperative behavioral training to usual care. The volunteer sample included 125 men 53 to 68 years old who elected radical prostatectomy for prostate cancer. Patients were stratified according to age and tumor differentiation, and randomized to 1 preoperative session of biofeedback assisted behavioral training plus daily home exercise or a usual care control condition, consisting of simple postoperative instructions to interrupt the urinary stream. The main outcome measurements were duration of incontinence (time to continence), as derived from bladder diaries, incontinence severity (the proportion with severe/continual leakage), pad use, Incontinence Impact Questionnaire, psychological distress (Hopkins Symptom Checklist) and health related quality of life (Medical Outcomes Study Short Form Health Survey). RESULTS: Preoperative behavioral training significantly decreased time to continence (p = 0.03) and the proportion of patients with severe/continual leakage at the 6-month end point (5.9% vs 19.6%, p = 0.04). There were also significant differences between the groups for self-reported urine loss with coughing (22.0% vs 51.1%, p = 0.003), sneezing (26.0% vs 48.9%, p = 0.02) and getting up from lying down (14.0% vs 31.9%, p = 0.04). No differences were found on return to work and usual activities or quality of life measures. CONCLUSIONS: Preoperative behavioral training can hasten the recovery of urine control and decrease the severity of incontinence following radical prostatectomy.

Aged↗

PubMed2006

No evidence for a major role of polymorphisms during bupropion treatment.

This study evaluated the ability of polymorphisms in five candidate genes to predict weight gain among patients taking bupropion or placebo in a smoking cessation trial. Five hundred fifty-three smokers were enrolled into a randomized double-blind, placebo-controlled trial and followed for 12 months. Five candidate genes [DRD2 Taq1 (rs1800497), DRD2-141 (rs1799732), C957T (rs6277), COMT (rs4818), and SLC6A3] were genotyped. Weights at baseline, at end of treatment, and after 6 and 12 months of follow-up were self-reported. Smoking abstinence at each endpoint was self-reported and confirmed biochemically. A self-reported average weight gain after 12 months of 1.1 +/- 6.0 kg (mean +/- standard deviation) in the bupropion group and 1.8 +/- 4.8 kg in the placebo group was noted. For subjects with biochemically confirmed abstinence from smoking, the HL genotype (alleles coding Val at codon 108 are denoted as H, and those coding Met are denoted as L) at the COMT locus and A1A1 genotype at the DRD2 Taq1 locus were associated with less weight gain at the end of treatment. The TC genotype at the C957T locus was associated with increased weight gain at 6 months of follow-up. However, no polymorphisms or their interactions with bupropion consistently and significantly predicted baseline BMI or weight change during treatment for all study subjects. Overall, our results do not support a major role for these five candidate genes in weight gain after smoking cessation.

Adult↗

PubMed2005

Treatment of intestinal helminths does not reduce plasma concentrations of HIV-1 RNA in coinfected Zambian adults.

BACKGROUND: Infection with intestinal helminths may stimulate dysfunctional immune responses in human immunodeficiency virus (HIV)-infected persons. Studies have yielded conflicting results regarding the impact of antihelminthic treatment on plasma concentrations of HIV-1 RNA.Methods. We conducted a prospective study of 54 HIV-1- and helminth-coinfected and 57 HIV-1-infected, helminth-uninfected asymptomatic adults living in Lusaka, Zambia, to assess the impact of antihelminthic treatment on plasma concentrations of HIV-1 RNA. RESULTS: Median baseline viral load was 0.33 log(10) copies/mL lower in the helminth-infected group than in the uninfected group. Mean viral load between pretreatment and posttreatment visits increased in the helminth-infected (mean, 4.23 vs. 4.29 log(10) copies/mL; P=.6) and helminth-uninfected (mean, 4.39 vs. 4.52 log(10) copies/mL; P=.2) groups. Helminth-infected participants with high pretreatment viral loads had a mean 0.25-log(10) copies/mL decrease after treatment (P=.3), and helminth-uninfected participants had a mean 0.02-log(10) copies/mL decrease (P=.8). CONCLUSIONS: We did not find an overall association between treatment of intestinal helminth infections and reduction in viral load in coinfected adults. Future studies may need to focus on adults with intense helminth infections who live in rural areas or on adults or children who harbor higher helminth burdens and plasma concentrations of HIV-1 RNA.

Adult↗

PubMed2005

The association of child condition severity with family functioning and relationship with health care providers among children and youth with special health care needs in Alabama.

OBJECTIVE: The purpose of the present study is to assess how the severity of a child's condition affects family functioning and the relationship with health care providers among children with special health care needs in Alabama. METHODS: Using the data from the National Survey of Children with Special Health Care Needs (CSHCN), three variables were used as measures of condition severity: responses to the CSHCN screener questions, whether condition affected the ability to do things for children and youth with special health care needs (CYSHCN), and the level of severity of CYSHCN's condition. The dependent variables included family functioning and provider relationship. RESULTS: CYSHCN who only take prescription medicine for their chronic condition (MO) had lower condition severity from those who have other needs (NMO). In NMO CYSHCN, higher condition severity was associated with increased strain on family functioning outcomes and higher unmet needs in provider relationship outcomes, adjusted for demographic and insurance variables. Families of NMO CYSHCN with a more severe condition spent more temporal and financial resources and had a higher need for professional care coordination, and were less likely to have sensitive providers. CONCLUSIONS: Severity of condition is an important factor increasing strain on family resources and relationship with the provider. Our results indicate the need for professional care coordination and family support, particularly among those families in which there is a NMO CYSHCN with a more severe condition. This finding supports the mandate that all CYSHCN should have their health care coordinated and provided in the context of a medical home.

Adolescent↗

PubMed2005

Regret, satisfaction, and symptom improvement: analysis of the impact of partial colpocleisis for the management of severe pelvic organ prolapse.

OBJECTIVE: The purpose of this study was to assess a cohort of patients who underwent a colpocleisis procedure more than 1 year post operation to determine: 1) the proportion of patients who regretted having the procedure, 2) patient satisfaction with the procedure, and 3) changes in symptom severity after surgery. STUDY DESIGN: Using the University of Alabama at Birmingham (UAB) Genitourinary Disorders Center database, a prospective analysis was performed on 54 patients who underwent colpocleisis between August 1996 and April 2003. From August to October of 2004, participants were contacted by an investigator not involved with the surgery and were asked 1) "do you regret having your surgery, and, if so, why?," 2) "how satisfied are you with your progress (completely, somewhat, or not)?," and 3) to repeat the short form Incontinence Impact Questionnaire/Urogenital Distress Inventory (IIQ-7/UDI-6). RESULTS: Fifty-nine percent (32/54) of potential candidates participated in the study. Nine percent (3/32) of patients regretted having colpocleisis performed. Fifty-seven percent (16/28) were completely satisfied, 29% (8/28) somewhat satisfied, and 14% (4/28) not satisfied. Mean IIQ score improved significantly from 40.9 (+/-31.7) at baseline to 14.1 (+/-26.7) at last interview (P = .003). Mean UDI score improved significantly from 63.1 (+/-24.3) at baseline to 24.2 (+/-26.7) at last interview (P = .001). There was a negative correlation between change in UDI scores with time since procedure (r = -.397, P = .055) and age (r = -.435, P = .034). CONCLUSION: Few patients reported regret after partial colpocleisis. There was a high rate of patient satisfaction and significant symptom improvement more than a year post surgery. Stable but smaller improvements were reported with longer time from surgery and increasing age.

Aged↗

PubMed2005

Pelvic floor surgery in the older woman: enhanced compared with usual preoperative assessment.

OBJECTIVE: To examine whether knowledge of deficits obtained in a preoperative geriatric assessment may benefit postoperative health outcomes in older women undergoing pelvic surgery. METHODS: This study employed a pre-post intervention cohort design. Primary outcome was difference in scores of the Physical Component Summary and Mental Component Summary of the Medical Outcomes Study Short Form 36 Health Survey in 62 older women who had undergone "usual" compared with an "enhanced" preoperative assessment consisting of Activities of Daily Living, Instrumental Activities of Daily Living, Get Up and Go Test, Draw a Clock Test, Mini Nutritional Assessment, Geriatric Depression Scale, and Social Support Scale. The assessment results were placed on the participant's hospital chart. Repeated measures analysis was used. RESULTS: There were no significant differences in Mental Component Summary scores between the usual and enhanced assessment cohorts preoperatively (mean +/- standard deviation; 49.14 +/- 10.61 compared with 53.2 +/- 9.33), at 6 weeks (53.69 +/- 8.61 compared with 55.47 +/- 9.46), or at 6 months postoperatively (53.85 +/- 10.77 compared with 56.25 +/- 7.25); P = .120 for group effect and P = .798 for group by time interaction. Significant time effect was noted (P = .036). There was no significant difference in Physical Component Summary scores between the usual and enhanced assessment cohorts with respect to group effect (P = .986); there was a significant time effect (P = < .001) and a significant group by time interaction (P = .026). Satisfaction with treatment was high in both cohorts at 6 weeks and 6 months. CONCLUSION: A preoperative geriatric assessment did not seem to have differential benefit in healthy older women undergoing elective pelvic floor surgery. LEVEL OF EVIDENCE: II-2.

Activities of Daily Living↗

PubMed2005

Urinary and anal incontinence in morbidly obese women considering weight loss surgery.

OBJECTIVE: To estimate prevalence and correlates of urinary and anal incontinence in morbidly obese women undergoing evaluation for laparoscopic weight loss surgery. METHODS: From October 2003 to February 2005, 180 women with body mass index (BMI) of 40 or greater underwent evaluation for laparoscopic weight loss surgery. Using an established Web site, questionnaires were completed to assess symptoms of urinary incontinence, including the Medical, Epidemiological, and Social Aspects of Aging Questionnaire (MESA). Anal incontinence was assessed by asking, "Do you have any uncontrolled anal leakage?" A number of clinical and demographic variables were examined as potential risk factors for urinary incontinence and anal incontinence. RESULTS: Mean age was 39.8 years (range 16-55). Body mass index ranged from 40 to 81 (mean 49.5). Prevalence of urinary incontinence was 66.9% and anal incontinence was 32.0% (45.6% loss of gas only, 21.1% liquid stool only, 24.6% gas and liquid stool only, 8.8% solid stool). In simple logistic regression, presence of urinary incontinence was associated with age (odds ratio [OR] 1.05, 95% confidence interval [CI] 1.01-1.09), number of children (OR 1.54, 95% CI 1.15-2.07), anal incontinence (OR 6.34, 95% CI 2.52-15.93), arthritis (OR 6.04, 95% CI 1.76-20.78), and sleep apnea (OR 2.30, 95% CI 1.21-4.37). Multivariable logistic regression identified 3 factors independently associated with urinary incontinence: number of children (OR 1.55, 95% CI 1.12-2.12), arthritis (OR 5.46, 95% CI 1.51-19.73), and anal incontinence (OR 6.27, 95% CI 2.42-16.26). Presence of anal incontinence was associated only with the presence of urinary incontinence (OR 6.34, 95% CI 2.52-15.93). CONCLUSION: Prevalence of urinary and anal incontinence is high in this group of morbidly obese women as compared with the general population. Studies are needed to determine the effect of weight loss on urinary and anal incontinence symptoms in the morbidly obese woman.

Adolescent↗

PubMed2005

Prevalence and correlates of fecal incontinence in community-dwelling older adults.

OBJECTIVES: To determine prevalence and correlates of fecal incontinence in older community-dwelling adults. DESIGN: A cross-sectional, population-based survey. SETTING: Participants interviewed at home in three rural and two urban counties in Alabama from 1999 to 2001. PARTICIPANTS: The University of Alabama at Birmingham Study of Aging enlisted 1,000 participants from the state Medicare beneficiary lists. The sample was selected to include 25% black men, 25% white men, 25% black women, and 25% white women. MEASUREMENTS: The survey included sociodemographic information, medical conditions, health behaviors, life-space assessment (mobility), and self-reported health status. Fecal incontinence was defined as an affirmative response to the question "In the past year, have you had any loss of control of your bowels, even a small amount that stained the underwear?" Severity was classified as mild if reported less than once a month and moderate to severe if reported once a month or greater. RESULTS: The prevalence of fecal incontinence in the sample was 12.0% (12.4% in men, 11.6% in women; P=.33). Mean age+/-standard deviation was 75.3+/-6.7 and ranged from 65 to 106. In a forward stepwise logistic regression analysis, the following factors were significantly associated with the presence of fecal incontinence in women: chronic diarrhea (odds ratio (OR)=4.55, 95% confidence interval (CI)=2.03-10.20), urinary incontinence (OR=2.65, 95% CI=1.34-5.25), hysterectomy with ovary removal (OR=1.93, 95% CI=1.06-3.54), poor self-perceived health status (OR=1.88, 95% CI=1.01-3.50), and higher Charlson comorbidity score (OR=1.29, 95% CI=1.07-1.55). The following factors were significantly associated with fecal incontinence in men: chronic diarrhea (OR=6.08, 95% CI=2.29-16.16), swelling in the feet and legs (OR=3.49, 95% CI=1.80-6.76), transient ischemic attack/ministroke (OR=3.11, 95% CI=1.30-7.41), Geriatric Depression Scale score greater than 5 (OR=2.83, 95% CI=1.27-6.28), living alone (OR=2.38, 95% CI=1.23-4.62), prostate disease (OR=2.29, 95% CI=1.04-5.02), and poor self-perceived health (OR=2.18, 95% CI=1.13-4.20). The following were found to be associated with increased frequency of fecal incontinence in women: chronic diarrhea (OR=6.39, 95% CI=2.25-18.14), poor self-perceived health (OR=5.37, 95% CI=1.75-16.55), and urinary incontinence (OR=4.96, 95% CI=1.41-17.43). In men, chronic diarrhea (OR=5.38, 95% CI=1.77-16.30), poor self-perceived health (OR=3.91, 95% CI=1.39-11.02), lower extremity swelling (OR=2.86, 95% CI=1.20-6.81), and decreased assisted life-space mobility (OR=0.73, 95% CI=0.49-0.80) were associated with more frequent fecal incontinence. CONCLUSION: In community-dwelling older adults, fecal incontinence is a common condition associated with chronic diarrhea, multiple health problems, and poor self-perceived health. Fecal incontinence should be included in the review of systems for older patients.

Black or African American↗

PubMed2005

Effects of behavioral and drug therapy on nocturia in older incontinent women.

OBJECTIVES: To examine changes in nocturia from a multicomponent behavioral training program or drug therapy in older women with urge or mixed (urge-predominant) urinary incontinence. DESIGN: A secondary analysis of data from a prospective, randomized clinical trial. SETTING: Parent trial was an outpatient research protocol in Alabama. PARTICIPANTS: One hundred ninety-seven women (aged 55-92) with incontinence and urodynamic evidence of bladder dysfunction. At baseline, 131 women (66% of participants) had nocturia. INTERVENTION: In the parent study, participants received behavioral training, including four sessions of biofeedback-assisted pelvic floor muscle exercises, drug treatment (oxybutynin IR titrated from 2.5 mg per day to 5.0 mg three times a day), or placebo. MEASUREMENTS: Participant-completed bladder diaries were used to calculate changes in nocturia. RESULTS: Behavioral training reduced nocturia by a median 0.50 episodes per night and was significantly more effective than drug treatment (median reduction=0.30 episodes; P=.02) and placebo (median reduction=0.00 episodes; P<.001). Also, drug treatment was more effective than control (P=.007). CONCLUSION: Both behavioral training and drug treatment reduced nocturia more than placebo, but behavioral training was the most effective.

Aged↗

PubMed2005

Catechol-O-methyl-transferase functional polymorphism and nicotine dependence: an evaluation of nonreplicated results.

Review articles have focused attention on and cited possible reasons for the nonreplication of genetic association studies. Herein, we illustrate how one might work through these possible reasons to make a judgment about the most plausible reason(s) when faced with two or more studies which yield seemingly inconsistent results. In the first study, 342 treatment-seeking smokers were genotyped for the Val108Met polymorphism in the functional catechol-O-methyl-transferase (COMT) locus. Alleles coding Val at codon 108 are denoted as H and those coding Met are denoted as L. An association between presence of the "H" (high activity) allele and pretreatment level of nicotine dependence level using the Fagerstrom Test for Nicotine Dependence was detected (P = 0.0072), after controlling for baseline body mass index (BMI, kg/m2), depression symptoms, and age. To validate this initial finding, 443 treatment-seeking smokers from an independent smoking cessation clinical trial were genotyped for the COMT polymorphism. Within the second study, no association between presence of the "H" allele and nicotine dependence was detected (P = 0.6418) after controlling for baseline BMI, depression symptoms, and age. We critically reviewed both studies with regard to often cited reasons for nonreplication, including type I error, population stratification, low statistical power, and imprecise measures of phenotype. Although in our opinion the failure to replicate the initial association in the second study is likely either the result of low statistical power to detect a small effect or effect heterogeneity, thorough analyses failed to definitively identify the reason for nonreplication.

Adult↗

PubMed2005

Prevalence and predictors of intestinal helminth infections among human immunodeficiency virus type 1-infected adults in an urban African setting.

Sub-Saharan Africa is disproportionately burdened by intestinal helminth and human immunodeficiency virus (HIV)-1 infection. Recent evidence suggests detrimental immunologic effects from concomitant infection with the two pathogens. Few studies, however, have assessed the prevalence of and predictors for intestinal helminth infection among HIV-1-infected adults in urban African settings where HIV infection rates are highest. We collected and analyzed sociodemographic and parasitologic data from 297 HIV-1-infected adults (mean age = 31.1 years, 69% female) living in Lusaka, Zambia to assess the prevalence and associated predictors of helminth infection. We found at least one type of intestinal helminth in 24.9% of HIV-infected adults. Thirty-nine (52.7%) were infected with Ascaris lumbricoides, and 29 (39.2%) were infected with hookworm. More than 80% were light-intensity infections. A recent visit to a rural area, food shortage, and prior history of helminth infection were significant predictors of current helminth status. The high helminth prevalence and potential for adverse interactions between helminths and HIV suggests that helminth diagnosis and treatment should be part of routine HIV care.

AIDS-Related Opportunistic Infections↗

PubMed2004

A simple significance test for quantile regression.

Where OLS regression seeks to model the mean of a random variable as a function of observed variables, quantile regression seeks to model the quantiles of a random variable as functions of observed variables. Tests for the dependence of the quantiles of a random variable upon observed variables have only been developed through the use of computer resampling or based on asymptotic approximations resting on distributional assumptions. We propose an exceedingly simple but heretofore undocumented likelihood ratio test within a logistic regression framework to test the dependence of a quantile of a random variable upon observed variables. Simulated data sets are used to illustrate the rationale, ease, and utility of the hypothesis test. Simulations have been performed over a variety of situations to estimate the type I error rates and statistical power of the procedure. Results from this procedure are compared to (1) previously proposed asymptotic tests for quantile regression and (2) bootstrap techniques commonly used for quantile regression inference. Results show that this less computationally intense method has appropriate type I error control, which is not true for all competing procedures, comparable power to both previously proposed asymptotic tests and bootstrap techniques, and greater computational ease. We illustrate the approach using data from 779 adolescent boys age 12-18 from the Third National Health and Nutrition Examination Survey (NHANES III) to test hypotheses regarding age, ethnicity, and their interaction upon quantiles of waist circumference.

Adolescent↗

PubMed2004

Waist circumference percentiles in nationally representative samples of African-American, European-American, and Mexican-American children and adolescents.

OBJECTIVE: To describe and provide estimates of the distribution of waist circumference (WC) according to percentiles in African-, European-, and Mexican-American children, and to test for group differences at different percentiles. STUDY DESIGN: Cross-sectional data from the Third National Health and Nutrition Examination Survey (NHANES III) were examined. The sample evaluated included 9713 nonpregnant persons 2 to 18 years of age with measured values of WCs. Age-, sex-, and ethnicity-specific percentiles were estimated via percentile regression. RESULTS: WC measurements increased in a monotonic fashion across ages but at nonconstant rates and in a manner that varied across age and sex. At higher percentiles of the distribution, estimates of WC differ between Mexican-American (MA) and European-American (EA) and between African-American (AA) and European-American (EA), and, in some cases, exceeded the adult cutoff value for obesity-related disease risk at as early as 13 years of age. CONCLUSION: Age-, sex-, and ethnicity-specific WC percentiles are available for US children and adolescents and can be used as an assessment tool that could impact public health recommendations. Results suggest concern with respect to high WC values among certain ethnic groups.

Adolescent↗