Readers guide to critical appraisal of cohort studies: 3. Analytical strategies to reduce confounding

New natural history of interferon-beta-treated relapsing multiple sclerosis

OBJECTIVE

To investigate the impact of interferon-beta (IFNbeta) on disease progression in relapsing-remitting multiple sclerosis patients.

METHODS

A cohort of 1,504 relapsing-remitting multiple sclerosis (1,103 IFNbeta-treated and 401 untreated) patients was followed for up to 7 years. Cox proportional hazards regression adjusted for propensity score inverse weighting was used to assess the differences between the two groups for three different clinical end points: secondary progression (SP) and irreversible Expanded Disability Status Scale (EDSS) scores 4 and 6. Times from first visit and from date of birth were used as survival time variables.

RESULTS

The IFNbeta-treated group showed a highly significant reduction in the incidence of SP (hazard ratio [HR], 0.38, 95% confidence interval [CI], 0.24-0.58 for time from 1st visit; HR, 0.36, 95% CI, 0.23-0.56 for time from date of birth; p < 0.0001), EDSS score of 4 (HR, 0.70, 95% CI, 0.53-0.94 for time from first visit; HR, 0.69, 95% CI, 0.52-0.93 for time from date of birth; p < 0.02), and EDSS score of 6 (HR, 0.60, 95% CI, 0.38-0.95 for time from first visit; HR, 0.54, 95% CI, 0.34-0.86 for time from date of birth; p < or = 0.03) when compared with untreated patients. SP and EDSS scores of 4 and 6 were reached with significant delays estimated by times from first visit (3.8, 1.7, and 2.2 years) and from date of birth (8.7, 4.6, and 11.7 years) in favor of treated patients. Sensitivity analysis confirmed findings.

INTERPRETATION

IFN-beta slows progression in relapsing-remitting multiple sclerosis patients.

Red blood cell transfusion in critically ill children is independently associated with increased mortality

OBJECTIVE

To test the hypothesis that RBC transfusion in critically ill children is independently associated with increased mortality and morbidity.

DESIGN

Retrospective, descriptive epidemiologic cohort study.

SETTING

Single-center experience of a nine-bed pediatric intensive care unit (PICU) facility.

PATIENTS

Critically ill children without ongoing active blood loss aged 0[Symbol: see text]months to 18[Symbol: see text]years, excluding prematurely born infants or patients after cardiothoracic surgery, and patients with chronic anemia.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Data of 295 consecutive patients was studied. Of these patients, 13.4% had a Hb concentration less than 9.6 g/dl. Sixty-seven (22.7%) of all patients were transfused, 39 only once. Transfused patients had a higher mortality (16.4 vs. 2.6%, p < 0.001). Mortality seemed related to the number of transfusion (p = 0.002) rather than the pre-transfusion Hb concentration (p = 0.10). Transfused patients required prolonged ventilatory support (11.1 +/- 1.8 vs. +/- 0.3 days, p < 0.001), infusion of vaso-active agents (8.2 +/- 1.8 vs. 2.8 +/- 0.6 days, p < 0.001) and PICU stay (13.0 +/- 1.8 vs. 3.2 +/- 0.2 days, p < 0.001). After multivariate analysis adjusting for age, PIM probability of death, mean TISS-28 score during the first 48 h, post-operative admission, diagnosis of sepsis or trauma or malignancy, pre-transfusion Hb concentration, and RBC transfusion remained independently associated with mortality and morbidity.

CONCLUSIONS

RBC transfusion in critically ill children is independently associated with increased mortality and prolonged duration of mechanical ventilation, prolonged infusion of vaso-active agents and prolonged PICU stay.

Readers should systematically assess methods used to identify, measure and analyze confounding in observational cohort studies

OBJECTIVE

To describe techniques used to address confounding in published observational studies.

STUDY DESIGN AND SETTING

A systematic literature review identified studies using administrative or registry data to investigate health effects of drug therapies. Studies published from January 2001 to December 2005 came from BMJ, New England Journal of Medicine, Lancet, Annals of Internal Medicine, and JAMA. A structured abstraction form was used to collect information about confounding.

RESULTS

The search identified 29 studies. Twenty-two studies (76%) had 10,000 or more subjects and 18 (62%) used a mortality outcome. None mentioned use of a literature search to identify confounders, however, 28 (97%) listed confounders included, and 26 (90%) listed confounders not included in the study. Eighteen (62.1%) discussed the validity of confounder data. Most (22, or 76%) studies included a table with the distribution of confounders but none used effect size to assess imbalance between comparison groups. Almost all studies used regression techniques (28, or 97%); fewer used stratification (16, or 55%) or matching (four, or 14%) to address confounding. Eleven (40%) studies discussed sensitivity analyses.

CONCLUSION

Published cohort studies routinely include a list of potential confounders but there is room for improvement in confounder identification, measurement, and analysis.

Effectiveness of preventive interventions inBRCA1/2 gene mutation carriers: A systematic review

A systematic review of the literature was conducted to assess the outcomes of preventive interventions (prophylactic surgery, intensive cancer screening, and chemoprevention) in women who carry mutations in BRCA1/2 genes, in terms of reducing breast and gynaecological cancer incidence and/or mortality. A search for relevant articles published between 1996 and 2005 (inclusive) was run on Medline, Embase and other databases. From the 749 journal articles retrieved from this search strategy, 18 studies were eligible for this review (2 systematic reviews, 10 cohort studies and 6 case-control studies). The critical appraisal of the studies was performed by two independent reviewers with a list of ad hoc selected criteria. The synthesis of results was qualitative. Mastectomy and prophylactic gynaecological surgery (oophorectomy or salpingo-oophorectomy) reduced breast and gynaecological cancer incidence in carriers of BRCA mutations, by comparison to surveillance. However, all the studies presented flaws in internal and external validity, none of these preventive interventions is risk-free, and protection against breast and gynaecological cancer, as well as other cancers linked to BRCA mutations, is incomplete. No studies comparing surveillance programmes of varying intensity were found. Exposure to drugs (tamoxifen, and oral contraceptives) in women carrying BRCA mutations was assessed through a limited number of papers. All of these were case-control studies with prevalent cases and presented major methodological flaws.

Use of antihypertensive and lipid-lowering drugs: the management of cardiovascular risk in clinical practice

The objective of the study was to analyse the treatment of high blood pressure (BP) and hypercholesterolaemia, as well as the effect of individual or combined antihypertensive-hypocholesterolaemic therapy on BP control and on circulating cholesterol. A retrospective study was performed using clinical data recorded in the general practitioner's database. The sample included all patients, aged > or =18 years, with BP reading or low-density lipoprotein (LDL) cholesterol measurement recorded between January 2003 and December 2004. BP and LDL cholesterol targets were defined using cutoffs based on the guidelines of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC7) and the National Cholesterol Education Program (NCEP/ATPIII). The study included 4764 patients (mean age 67.6+/-11.8 years, 43.5% males). Target BP was achieved in a higher number of patients under combined antihypertensive-hypocholesterolaemic therapy than in those treated only with antihypertensives: 57.0 vs 50.0% in patients with history of cardio/cerebrovascular (CV) hospitalization, 27.0 vs 16.9% in patients with diabetes or chronic renal insufficiency (CRI) and 59.7 vs 49.1% in patients with no CV hospitalization nor diabetes and nor CRI. The LDL cholesterol target was achieved in 61.3% of the subjects: it was independent on the therapy (individual or combined), but related to the degree of cardiovascular risk. Analysing the data contained in the general medicine database made it possible to evaluate the treatment of high BP and hypercholesterolaemia in relation to cardiovascular risk in clinical practice and to establish the need to pay greater attention to achieving the objective set by guidelines.

The impact of administration of tranexamic acid in reducing the use of red blood cells and other blood products in cardiac surgery

Background

To study the effect of administration of tranexamic acid on the use of blood and blood products, return to theatre for post-operative bleeding and the length of intensive care stay after primary cardiac surgery, data for 4191 patients, of all priorities, who underwent primary cardiac operation during the period between 30/10/00 and 21/09/04 were analysed.

Methods

Retrospective analysis of data collected prospectively during the study period. The main outcome measures were whether or not patients were transfused with red blood cells, fresh frozen plasma or any blood product, the proportion of patients returned to theatre for investigation for post-operative bleeding and length of stay in the intensive care unit. We performed univariate analysis to identify the factors influencing the outcome measures and multivariate analysis to identify the effect of administration of tranexamic acid on the outcome measures.

Results

Administration of tranexamic acid was an independent factor affecting the transfusion of red blood cells, fresh frozen plasma or any blood product. It was also an independent factor influencing the rate of return to theatre for exploration of bleeding. The odds of receiving a transfusion or returning to theatre for bleeding were significantly lower in patients receiving tranexamic acid. The administration of tranexamic acid also significantly decreased blood loss. We did not find any association between the administration of tranexamic acid and the length of intensive care stay.

Conclusion

Based on the analysis of 4191 patients who underwent a primary cardiac operation, administration of tranexamic acid decreased the number of patients exposed to a transfusion or returned to theatre for bleeding in our institute.

Applying evidence standards to rehabilitation research

OBJECTIVE

To describe evidence grading methods employed in the systematic reviews in this special series of articles. To provide an overview of results of these reviews to critique the quality of rehabilitation research. To identify issues in the application of evidence grading methods to rehabilitation.

DESIGN

Conceptual review of evidence-based practice and evidence grading methods. English-language research studies on rehabilitation of persons with spinal cord injury, traumatic brain injury, and burn for the 5-yr period of 1999-2004 were reviewed using methods of the American Academy of Neurology supplemented by Cochrane criteria and summarized.

RESULTS

Rehabilitation has a shortage of high-level studies. The number of level 1 treatment studies was quite limited (five in spinal cord injury, 15 in traumatic brain injury, 12 in burn rehabilitation), as was the number of level 2 studies (26, 4, and 2, respectively). Despite the large number of correlational studies published, the number of high-level (1 or 2) diagnostic and prognostic studies was surprisingly limited (34, 11, and 5, respectively). The rate of production of high-level studies is rapidly increasing. Problems were encountered in applying standard evidence criteria to complex issues encountered in some studies, suggesting limitations and issues in evidence grading methodology.

CONCLUSIONS

Rehabilitation needs more high-level studies. Some improvements in research methodology are relatively affordable (e.g., improved blinding), whereas others are expensive (e.g., large randomized controlled trials). Lower-level investigations reporting promising results need to be followed by more definitive, higher-level trials.

Issues in the planning and conduct of non-randomised studies

This paper discusses topics related to the planning and implementation of non-randomised clinical studies in orthopaedics. A well-conducted case-series is appropriate to demonstrate the safety of a surgical intervention. The case-series design involves the provision of a defined intervention to a group of patients with the ultimate objective of describing the final outcome, including such occurrences as complications. There is no alternative procedure serving as a control. The key aspects are to ensure enrolment of all eligible patients and to obtain a sufficiently large sample size to allow precise and valid estimation of complication risks. Targeted complications should be clearly defined and fully documented during a pre-defined follow-up period. Loss to follow-up should be minimised. Comparative studies are required to demonstrate treatment effectiveness. If a randomised controlled trial (RCT) is not feasible, an observational design such as a cohort or a case-control study should be considered. In observational designs, the treatment decision is made by the surgeons. In a case-control study, patients are selected based on their outcomes and their treatment or exposure status is recorded retrospectively. In a cohort study, groups of patients are selected based on their treatment and are followed for outcomes. There are numerous variations. Data can be collected prospectively or retrospectively; comparison groups may be concurrent or non-concurrent, or studied at different locations. The optimal design is tailored to clinical questions and research settings, while keeping in mind the respective methodological strengths and weaknesses of available options. The strength of the observational study is its proximity to daily clinical practice. The limitations are the possibility of numerous biases and confounding factors. Despite many challenges to the internal validity of non-randomised studies in orthopaedics surgery, it is possible to use such designs in order to provide reasonably valid answers to clinically important questions.

Reader's guide to critical appraisal of cohort studies: 2. Assessing potential for confounding

Although confounding is an important problem of cohort studies, its effects can be minimised to enable valid comparison