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PubMed2001

Intention-to-treat: methods for dealing with missing values in clinical trials of progressively deteriorating diseases.

Since it came up in the 1960s, the principle of intention-to-treat (ITT) has become widely accepted for the analysis of controlled clinical trials. In this context the question of how to perform such an analysis in the presence of missing information about the main endpoint is of major importance. Uncritical use of several ad hoc strategies for dealing with missing values is common in the practice of clinical trials. On the other hand, little is known about possible dangers and problems of applying these strategies. We therefore performed a detailed investigation of different methods for dealing with missing values in order to develop recommendations for their practical use. A simulation study was performed investigating possible consequences on type I error and power of applying different methods for dealing with missing values. The simulations were based on a clinical trial of osteoporosis, a progressively deteriorating disease. The strategies examined can be roughly classified into numerical imputation strategies (last observation carried forward, mean and regression based methods) and non-parametric strategies (rank and dichotomization based methods). Different drop-out mechanisms and different types of progression of disease are considered. The type I error increases drastically for the different strategies, especially if the courses of disease vary between treatment groups. The loss in power can be substantial. There is no strategy which is adequate for all different combinations of drop-out mechanisms, drop-out rates and courses of disease over time. For drop-out rates less than 20 per cent and similar courses of disease in the treatment groups, missing values might be replaced by the mean of the other group, or counted as treatment failures after dichotomization of the endpoint. For larger drop-out rates or less similar courses of disease, no adequate recommendations can be given. Because of the drastic consequences of increasing drop-out rates, it has to be a primary goal in clinical trials to keep missing values to a minimum. Unobserved information cannot be reliably regained by any methodological resources. As there are no strategies for universal use, reasons for the choice of a certain method have to be provided when designing and analysing clinical trials.

Computer Simulation↗

PubMed2001

Prognostic value of Doppler echocardiographic mitral inflow patterns: implications for risk stratification in patients with chronic congestive heart failure.

OBJECTIVES: This prospective study tested whether transmitral flow patterns add incremental value to peak oxygen consumption (VO2) in determining the prognosis of patients with chronic congestive heart failure (CHF) and systolic dysfunction. BACKGROUND: Peak VO2 is an objective marker of functional capacity and is routinely used as a criterion to identify heart transplant candidates. Diastolic dysfunction limits functional capacity, but its prognostic importance relative to that of peak VO2 is unknown. METHODS: Peak VO2 and mitral inflow velocities were prospectively measured in 311 consecutive patients (mean age 54 years, 84% male) with impaired left ventricular function (ejection fraction <40%; 88 patients with ischemic and 223 with dilated cardiomyopathy) who were evaluated for heart transplant candidacy. RESULTS: During a mean follow-up period of 512 +/- 314 days, 65 patients died and 43 patients underwent heart transplantation. Diastolic filling patterns, peak VO2 and left ventricular end-diastolic diameters were independent predictors of cardiac mortality. In patients with peak VO2 < or = 14 ml/min per kg body weight, the outcome was markedly poorer in the presence of restrictive filling patterns as compared with their absence (two-year survival rate 52% vs. 80%). Similarly, despite peak VO2 levels >14 ml/min per kg, the outcome was less favorable in the presence of restrictive filling patterns (two-year survival rate 80% vs. 94%). A risk-stratification model based on the identified independent noninvasive predictors separated groups into those with high (93%), intermediate (65%) and low (39%) two-year survival rates. CONCLUSIONS: Transmitral flow patterns add incremental value to peak VO2 in determining the prognosis of patients with CHF and impaired systolic function.

Blood Flow Velocity↗

PubMed2000

Nonconvulsive status epilepticus--a possible cause of mental retardation in patients with Lennox-Gastaut syndrome.

Lennox-Gastaut syndrome (LGS) is one of the most severe types of childhood epilepsy. It is usually resistant to treatment and associated with mental retardation. To delineate the risk factors associated with the outcome of LGS, we evaluated, in a retrospective and multicentre study, the course of the disease, EEG tracings, and intellectual function in 101 patients. Inclusion criteria were the presence of tonic seizures as well as slow spike and wave complexes in the EEG. The average documented observation period was 16 years (range 4-31 years). Overall, the intellectual and neurological outcome was poor. At the last follow-up, 38% of the patients could not speak, 21% were unable to walk and only 4% were free of seizures. Four independent risk factors for severe mental retardation were identified by multivariate analysis. These were in a decreasing order of importance: nonconvulsive status epilepticus (NCSE), odds ratio (OR) 25.2, a previous diagnosis of West syndrome (OR 11.6), a symptomatic etiology of epilepsy (OR 9.5), and an early age at onset of epilepsy (OR 4.7). The results highlight the association between NCSE and the severity of mental retardation in patients with LGS; this association appears to be independent of symptomatic etiology. Our data provide an indirect evidence that, at least in some of the patients, NCSE is not only a concomitant feature, but also a cause of severe mental retardation.

Adolescent↗

PubMed2000

[Basic principles of clinical therapeutic studies--what, how and why?].

Treatment evaluation is one of the most important tasks in medical research. Detailed standards have been developed during the last decades. The efficacy/effectiveness of treatments can only be assessed in comparison to control groups. To guarantee the internal validity of these comparisons, the groups have to be comparable at the beginning of the study. This can be achieved by randomized allocation of patients to treatment. Furthermore, as far as possible patient and physician should be blinded to treatment in order to avoid subjective influences on treatment results. Groups should still be comparable when analysing the trial, thus an analysis according to the principle of intention-to-treat ("as randomized") should be performed. These indispensable principles for design, conduct and analysis of clinical trials are widely accepted and contribute to reliable and credible results.

Humans↗

PubMed2000

[Monitoring clinical studies. Development, measures and consequences].

BACKGROUND: Clinical trials play an important role in developing and establishing new therapeutic and diagnostic procedures. In the planning and execution of these trials procedures and measures which allow for continual observation, description and evaluation of the study progress are to be taken into account. This is due to ethical, scientific and economic considerations. Together these procedures and measures are termed "monitoring". Repeated evaluation of the main study question is one particular monitoring measure. Results from this sequential procedure may lead to an early termination of patient recruitment. In the last 3 decades methods for interim analyses were developed which take into account the increased chance of errors when evaluating repeatedly the same question. By adjustment they guarantee a prespecified level of significance in the end result of the trial. Even though statistical significance may be evident in an interim analysis, this has not always to result in early termination of the trial. The decision to end a trial early must include other considerations than the mere evaluation of the main study question. In particular, consequences of the decision such as credibility and transferability of the trial result to subsequent therapeutic routine application are to be discussed.

Clinical Trials as Topic↗

PubMed1999

[Monitoring of clinical trials. Methodology, interim analyses, and end results].

BACKGROUND: Clinical trials play an important role in developing and establishing new therapeutic and diagnostic procedures. In the planning and execution of these trials procedures and measures which allow for continual observation, description and evaluation of the study progress are to be taken into account. This is due to ethical, scientific and economic considerations. Together these procedures and measures are termed "monitoring". Repeated evaluation of the main study question is one particular monitoring measure. Results from this sequential procedure may lead to an early termination of patient recruitment. In the last 3 decades methods for interim analyses were developed which take into account the increased chance of errors when evaluating repeatedly the same question. By adjustment they guarantee a prespecified level of significance in the end result of the trial. Even though statistical significance may be evident in an interim analysis, this has not always to result in early termination of the trial. The decision to end a trial early must include other considerations than the mere evaluation of the main study question. In particular, consequences of the decision such as credibility and transferability of the trial result to subsequent therapeutic routine application are to be discussed.

Clinical Trials as Topic↗

PubMed1999

[Basic principles of clinical trials--what, how, and why?].

Treatment evaluation is one of the most important tasks in medical research. Detailed standards have been developed during the last decades. The efficacy/effectiveness of treatments can only be assessed in comparison to control groups. To guarantee the internal validity of these comparisons, the groups have to be comparable at the beginning of the study. This can be achieved by randomized allocation of patients to treatment. Furthermore, as far as possible patient and physician should be blinded to treatment in order to avoid subjective influences on treatment results. Groups should still be comparable when analysing the trial, thus an analysis according to the principle of intention-to-treat ("as randomized") should be performed. These indispensable principles for design, conduct and analysis of clinical trials are widely accepted and contribute to reliable and credible results.

Clinical Trials as Topic↗