Influence of reported study design characteristics on intervention effect estimates from randomised controlled trials: combined analysis of meta-epidemiological studies.

BACKGROUND: The design of randomised controlled trials (RCTs) should incorporate characteristics (such as concealment of randomised allocation and blinding of participants and personnel) that avoid biases resulting from lack of comparability of the intervention and control groups. Empirical evidence suggests that the absence of such characteristics leads to biased intervention effect estimates, but the findings of different studies are not consistent. OBJECTIVES: To examine the influence of unclear or inadequate random sequence generation and allocation concealment, and unclear or absent double blinding, on intervention effect estimates and between-trial heterogeneity, and whether or not these influences vary with type of clinical area, intervention, comparison and outcome measure. DATA SOURCES AND METHODS: Data were combined from seven contributing meta-epidemiological studies (collections of meta-analyses in which trial characteristics are assessed and results recorded). The resulting database was used to identify and remove overlapping meta-analyses. Outcomes were coded such that odds ratios < 1 correspond to beneficial intervention effects. Outcome measures were classified as mortality, other objective or subjective. We examined agreement between assessments of trial characteristics in trials assessed in more than one contributing study. We used hierarchical Bayesian bias models to estimate the effect of trial characteristics on average bias [quantified as ratios of odds ratios (RORs) with 95% credible intervals (CrIs) comparing trials with and without a characteristic] and in increasing between-trial heterogeneity. RESULTS: The analysis data set contained 1973 trials included in 234 meta-analyses. Median kappa statistics for agreement between assessments of trial characteristics were: sequence generation 0.60, allocation concealment 0.58 and blinding 0.87. Intervention effect estimates were exaggerated by an average 11% in trials with inadequate or unclear (compared with adequate) sequence generation (ROR 0.89, 95% CrI 0.82 to 0.96); between-trial heterogeneity was higher among such trials. Bias associated with inadequate or unclear sequence generation was greatest for subjective outcomes (ROR 0.83, 95% CrI 0.74 to 0.94) and the increase in heterogeneity was greatest for such outcomes [standard deviation (SD) 0.20, 95% CrI 0.03 to 0.32]. The effect of inadequate or unclear (compared with adequate) allocation concealment was greatest among meta-analyses with a subjectively assessed outcome intervention effect (ROR 0.85, 95% CrI 0.75 to 0.95), and the increase in between-trial heterogeneity was also greatest for such outcomes (SD 0.20, 95% CrI 0.02 to 0.33). Lack of, or unclear, double blinding (compared with double blinding) was associated with an average 13% exaggeration of intervention effects (ROR 0.87, 95% CrI 0.79 to 0.96), and between-trial heterogeneity was increased for such studies (SD 0.14, 95% CrI 0.02 to 0.30). Average bias (ROR 0.78, 95% CrI 0.65 to 0.92) and between-trial heterogeneity (SD 0.37, 95% CrI 0.19 to 0.53) were greatest for meta-analyses assessing subjective outcomes. Among meta-analyses with subjectively assessed outcomes, the effect of lack of blinding appeared greater than the effect of inadequate or unclear sequence generation or allocation concealment. CONCLUSIONS: Bias associated with specific reported study design characteristics leads to exaggeration of beneficial intervention effect estimates and increases in between-trial heterogeneity. For each of the three characteristics assessed, these effects were greatest for subjectively assessed outcomes. Assessments of the risk of bias in RCTs should account for these findings. Further research is needed to understand the effects of attrition bias, as well as the relative importance of blinding of patients, care-givers and outcome assessors, and thus separate the effects of performance and detection bias. FUNDING: National Institute for Health Research Health Technology Assessment programme.

Impact of allocation concealment on conclusions drawn from meta-analyses of randomized trials.

BACKGROUND: Randomized trials without reported adequate allocation concealment have been shown to overestimate the benefit of experimental interventions. We investigated the robustness of conclusions drawn from meta-analyses to exclusion of such trials. MATERIAL: Random sample of 38 reviews from The Cochrane Library 2003, issue 2 and 32 other reviews from PubMed accessed in 2002. Eligible reviews presented a binary effect estimate from a meta-analysis of randomized controlled trials as the first statistically significant result that supported a conclusion in favour of one of the interventions. METHODS: We assessed the methods sections of the trials in each included meta-analysis for adequacy of allocation concealment. We replicated each meta-analysis using the authors' methods but included only trials that had adequate allocation concealment. Conclusions were defined as not supported if our result was not statistically significant. RESULTS: Thirty-four of the 70 meta-analyses contained a mixture of trials with unclear or inadequate concealment as well as trials with adequate allocation concealment. Four meta-analyses only contained trials with adequate concealment, and 32, only trials with unclear or inadequate concealment. When only trials with adequate concealment were included, 48 of 70 conclusions (69%; 95% confidence interval: 56-79%) lost support. The loss of support mainly reflected loss of power (the total number of patients was reduced by 49%) but also a shift in the point estimate towards a less beneficial effect. CONCLUSION: Two-thirds of conclusions in favour of one of the interventions were no longer supported if only trials with adequate allocation concealment were included.

Studies of variability in the islet amyloid polypeptide gene in relation to Type 2 diabetes.

AIMS: To explore whether the coding region of the islet amyloid polypeptide (IAPP) gene contains genetic variants associated with Type 2 diabetes and whether a previously reported association of the promoter variant -132g-->a with Type 2 diabetes could be reproduced in Danish Caucasians. METHODS: The coding region was analyzed using single strand conformation polymorphism (SSCP) and heteroduplex analysis in 192 Type 2 diabetic patients. Restriction fragment length polymorphism (RFLP) was employed to screen for the promoter variant in 414 Type 2 diabetic patients and 182 glucose-tolerant control subjects. RESULTS: The SSCP analysis identified an IVS+75a-->g variant in two patients. The frequency of heterozygous carriers of the promoter variant in the case-control study was 4.1% (17/414) and 7.1% (13/182), respectively. Odds ratio of the prevalence of Type 2 diabetes in carriers compared with non-carriers was estimated to be 0.47 (95% confidence interval 0.19, 1.15). CONCLUSION: Neither variability in the coding region of the IAPP gene nor the -132g-->a promoter variant was associated with Type 2 diabetes among the studied Danish Caucasians.

Studies of genetic variability of the glucose transporter 2 promoter in patients with type 2 diabetes mellitus.

This study was performed to test the hypothesis that genetic variation in the promoter of the glucose transporter 2 (GLUT2) might predispose to prediabetic phenotypes or type 2 diabetes. A total of 1611 bp comprising the minimal promoter region of the GLUT2 gene were examined by combined single-strand conformational polymorphism and heteroduplex analysis followed by direct sequencing of identified variants on genomic DNA from 96 randomly recruited Danish type 2 diabetic patients. We identified 4 nucleotide variants, -447g-->a, -149c-->a, -122t-->c, and -44g-->a. None of the variants were positioned in known or presumed transcription factor binding sites, TATA-box, or transcriptional start site. Association studies of the -149c-->a, -122t-->c, and -44g-->a variants revealed that the variants were as prevalent in 320 type 2 diabetic patients [11.0% (95% confidence interval, 8.4-13.6), 9.8% (7.4-12.2), and 29.0% (24.4-33.6), respectively] as in 241 age-matched glucose-tolerant subjects [13.1% (9.8-16.4), 11.2% (8.3-14.1), and 33.4% (28.8-38.0), respectively]. The -447g-->a mutation was only identified in a single diabetic patient and did not show cosegregation with diabetes in the family of the proband. The three common variants showed in a primary genotype-phenotype study comprising 241 glucose-tolerant middle-aged subjects association to increased plasma glucose levels during an oral glucose tolerance test. However, this result could not be replicated in a second sample of 298 60-yr-old glucose-tolerant subjects. In conclusion, we found no evidence supporting the hypothesis that genetic variability in the minimal promoter of the GLUT2 is associated with type 2 diabetes or prediabetic phenotypes in the Danish population.

Somatostatin in the boar reproductive system.

Somatostatin (SRIF) is a widely distributed regulatory peptide. Recently, it was shown that human seminal plasma contains high concentrations of SRIF. In order to find the cellular source of seminal SRIF we have examined the presence of SRIF in porcine urogenital tissues. The concentration of SRIF in male accessory reproductive tissues of 3-month-old pigs (N = 4) ranged from 1 to 17 nmol/kg. In the boar, only the prostate gland contained significant amounts of SRIF (median 7 nmol/kg, range 3-18 nmol/kg, N = 4). Testis and semen contained no SRIF. Gel chromatography of extracts of the male accessory sex glands and epididymis showed both SRIF-28 and SRIF-14, whereas urinary bladder contained mainly SRIF-14. In conclusion, the results show a considerable species, tissue and developmental variation in the expression of SRIF in the genitourinary tract.