Sample size calculations for studies with correlated observations

Correlated data occur frequently in biomedical research. Examples include longitudinal studies, family studies, and ophthalmologic studies. In this paper, we present a method to compute sample sizes and statistical powers for studies involving correlated observations. This is a multivariate extension of the work by Self and Mauritsen (1988, Biometrics 44, 79-86), who derived a sample size and power formula for generalized linear models based on the score statistic. For correlated data, we appeal to a statistic based on the generalized estimating equation method (Liang and Zeger, 1986, Biometrika 73, 13-22). We highlight the additional assumptions needed to deal with correlated data. Some special cases that are commonly seen in practice are discussed, followed by simulation studies.     

Hypertension as a risk factor for diabetic neuropathy: a prospective study

The pathogeneses of diabetic neuropathy is still unclear. This study prospectively investigated the risk factors for distal symmetrical polyneuropathy (DSP) in a cohort of childhood-onset IDDM patients. Subjects from the Epidemiology of Diabetes Complications (EDC) Study were clinically examined at baseline and then biennially. DSP was diagnosed by a combination of clinical criteria, symptoms and signs (Diabetes Control and Complications Trial [DCCT] exam), and quantitative sensory threshold (QST). Among the 463 (70.4%) subjects who were free of DSP at baseline, 453 (97.8%) participated in at least one biennial reexamination during the first 6 years of follow-up and were included in the current analysis. A total of 68 (15.0%) subjects developed DSP in 6 years, giving a cumulative probability of 0.29. The Cox proportional hazards model shows that longer IDDM duration, hypertension, poor glycemic control, height, and smoking were all independent predictors of the incidence of DSP (all P < 0.0001, except for smoking for which P = 0.03). Hypertension showed the greatest impact on the development of DSP for individuals with either short or long IDDM duration. This study confirms some risk factors for DSP found in cross-sectional studies and suggests a strong relationship between hypertension and DSP. The results indicate that in addition to good glycemic control, avoidance of smoking and good blood pressure control may be helpful in preventing or delaying the onset of DSP in IDDM patients.     

Racial differences in Helicobacter pylori seroprevalence in Singapore: correlation with differences in peptic ulcer frequency

The aim of this study was to determine, first, whether racial differences exist in the seroprevalence of Helicobacter pylori infection in Singapore, and second, whether these differences correlate with racial differences in peptic ulcer frequency. A commercial serological test for immunoglobulin (Ig)G antibody to H. pylori which was 90% sensitive and 83% specific in our population was used to screen 403 adult blood donors of Chinese, Malay and Indian origin, aged between 15-60 years. Serum specimens from 84 paediatric patients admitted to the Paediatrics Department, National University of Singapore, with non-gastroenterological illnesses were also tested. In all three races, seroprevalence of H. pylori increased with age. Indians have the highest prevalence of infection followed by Chinese and Malays. Peptic ulcer prevalences are known to be highest in Chinese, followed by Indians and Malays. The Malays have the lowest prevalence of H. pylori and peptic ulcer among the three races in Singapore. Indians have a higher prevalence of H. pylori antibodies but a lower frequency of peptic ulcer than the Chinese. Racial differences in peptic ulcer frequency between Chinese and Indians are not explained by the prevalence of H. pylori infection; other environmental or genetic factors may be involved.     

Genomic analysis reveals chromosomal variation in natural populations of the uncultured psychrophilic archaeon Cenarchaeum symbiosum

Molecular phylogenetic surveys have recently revealed an ecologically widespread crenarchaeal group that inhabits cold and temperate terrestrial and marine environments. To date these organisms have resisted isolation in pure culture, and so their phenotypic and genotypic characteristics remain largely unknown. To characterize these archaea, and to extend methodological approaches for characterizing uncultivated microorganisms, we initiated genomic analyses of the nonthermophilic crenarchaeote Cenarchaeum symbiosum found living in association with a marine sponge, Axinella mexicana. Complex DNA libraries derived from the host-symbiont population yielded several large clones containing the ribosomal operon from C. symbiosum. Unexpectedly, cloning and sequence analysis revealed the presence of two closely related variants that were consistently found in the majority of host individuals analyzed. Homologous regions from the two variants were sequenced and compared in detail. The variants exhibit >99.2% sequence identity in both small- and large-subunit rRNA genes and they contain homologous protein-encoding genes in identical order and orientation over a 28-kbp overlapping region. Our study not only indicates the potential for characterizing uncultivated prokaryotes by genome sequencing but also identifies the primary complication inherent in the approach: the widespread genomic microheterogeneity in naturally occurring prokaryotic populations.     

A biphasic developmental pattern of circulating leptin in the male rhesus macaque (Macaca mulatta)

To help elucidate the physiological role of leptin during somatic and sexual maturation, circulating concentrations of leptin were measured in 36 male rhesus monkeys of ages ranging from 0-20 yr. The body weight of these animals showed a steady increase of approximately 1 kg/yr during the first decade of life and reached a plateau at approximately 13 yr. In contrast, serum leptin concentrations showed a biphasic developmental pattern, which was highlighted by a strong negative correlation with body weight (r = -0.74, P < 0.001) before the onset of puberty (at approximately 3.5 yr) and by a strong positive correlation afterward (r = 0.77, P < 0.001). Overall, the developmental changes in serum leptin concentrations closely mimicked the expected developmental changes in serum testosterone concentrations (r = 0.62, P < 0.001), which were highly elevated at birth, fell to basal levels during the juvenile phase of development, and gradually rose again after the initiation of puberty. However, mean serum leptin concentrations during the peripubertal period itself (3-5 yr) were significantly lower (P < 0.01) than those observed during the first year of life or those observed in fully mature adults (i.e. > 7 yr) (3.5 +/- 0.3, 1.4 +/- 0.2, and 3.3 +/- 0.6 ng/ml, respectively). These data demonstrate that the role of leptin in energy homeostasis of primates is more than a simple linear relationship, being highly dependent upon the developmental age. Furthermore, the data do not support the hypothesis that leptin plays a major role in triggering the onset of puberty in primates, although the strong correlation between serum concentrations of leptin and testosterone suggests that the secretion of these two hormones may be causally linked.     

Intestinal tuberculosis: clinicopathologic analysis and diagnosis by endoscopic biopsy

OBJECTIVES: Tuberculosis is still an important cause of granulomatous colitis in developing countries. If we can diagnose tuberculosis using endoscopic biopsy material, clinicians can avoid invasive diagnostic procedures and needless operations. For this purpose, we evaluated clinical manifestations, pathological findings, and diagnostic methods in endoscopically biopsied intestinal tuberculosis patients. METHODS: From January 1991 to December 1996, 42 patients with intestinal tuberculosis were endoscopically examined and tissue culture, immunohistochemical stain, Ziehl-Neelsen stain, and polymerase chain reaction in fresh and fixed tissue were applied. The pathological findings were analyzed and compared with the results of the other diagnostic methods. RESULTS: In tuberculosis patients, transverse ulcers with surrounding hypertrophic mucosa and multiple erosions were usual colonoscopic findings. The granulomas were found in 74% of the cases. The positivity ranged from 30-45%. There were no significant differences in the positivity among those diagnostic methods (p > 0.05). The positivity of Ziehl-Neelsen stain in fixed tissue was higher in the group having granulomas and it was reversed in PCR (p < 0.05). The increasing number of biopsy particles raised the positivity of Ziehl-Neelsen stain and PCR in fixed tissue (p < 0.05). CONCLUSIONS: Transverse ulcers were the most characteristic colonoscopic finding and granulomas were frequent pathological findings in intestinal tuberculosis. Higher positivity and reliable results were found in tissue culture, Ziehl-Neelsen stain, and polymerase chain reaction. To increase the diagnostic rate, the endoscopist should take enough tissue and deep biopsy material from ulcer bases and diseased mucosae.     

Determination of ginsenoside Rf and Rg2 from Panax ginseng using enzyme immunoassay

We have developed an enzyme immunoassay (EIA) to quantify trace amounts of ginsenoside Rf (Rf), one of the glycosides of protopanaxatriol from Panax ginseng. A carrier protein of bovine serum albumin (BSA) was coupled to the carbohydrate component of Rf using the periodate oxidation method. Antibodies were raised in rabbits using Rf-BSA conjugate as the immunogen and competitive indirect EIA was used for the determination of Rf. The working range was 0.01-10 ng per assay. The anti-Rf antiserum cross-reacted with ginsenoside Rg2 (105%), which is also a component of Panax ginseng and has a very similar chemical structure to Rf. These results suggest that the anti-Rf antiserum could also be used for the quantitation of ginsenoside Rg2 as well as ginsenoside Rf. In a comparison of EIA and HPLC the linear regression equation and correlation coefficient for the two methods were y(EIA) = 1.31x (HPLC)-11.48 and 0.98, respectively.     

Species differences in regional patterns of 3H-8-OH-DPAT and 3H-zolpidem binding in the rat and human brain

A series of brief office counseling interventions for the prevention and treatment of violence is reviewed in this article. Primary prevention strategies throughout the pediatric age span cover topics from gun storage to nonviolent handling of a potential street fight. Secondary prevention strategies deal with patients who have been injured by violence or patients who engage in street violence, weapon carrying, or dating violence.     

Effect of clozapine on motor function in schizophrenic patients

It is well established that clozapine is less likely than typical antipsychotic drugs to cause clinically discernible extrapyramidal side-effects. There is a paucity of data, however, on clozapine's motor effects. In this report we compare normal controls to groups of chronic schizophrenic patients treated with either typical antipsychotic drugs or with clozapine. Motor function was measured with a target-matching task, a test relying on submaximal sustained force control. Results indicated that patients on clozapine performed with significantly lower accuracy (greater variability) of force control. Even though the clozapine patients were treatment resistant to typical antipsychotic drugs, and many had a history of tardive dyskinesia, we postulate that the observed deficit is likely due to clozapine treatment rather than to earlier treatments or other factors. The observed force control deficit may be the result of an increase in myoclonus and a generally lower level of overall motor activity.