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Prospective Cohort Study

Posted by Surgery on Sep 6, 2008

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In a prospective cohort design , two or more study groups with similar baseline characteristics (defined as the cohort:) are followed prospectively from an exposure to a predefined outcome. For the purpose of studying surgical outcomes, the operative intervention acts as a surrogate for the exposure. For example, a center may want to compare laparoscopic versus open splenectomy in children. Patients who meet selection criteria based on a predetermined cohort definition (e.g., children ages 4 to 10 with a diagnosis of hereditary spherocytosis) are then approached for enrollment. They then undergo one of the procedures being studied based on the decision made by parents along with their surgeon. Patients from both study groups are then followed prospectively and observed for relevant predetermined outcome measures such as analgesia requirement and length of hospital stay.

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The strength of the prospective cohort study lies in its prospective design and predetermined set of criteria for assessing outcome measures. In the hierarchy of clinical evidence, these studies provide the best evidence of causality outside a clinical trial. Cohort studies are powerful tools for characterizing the natural history of a disease, and for establishing incidence rates and measures of relative risk based on defined outcomes. The basic framework of the cohort design can be used to develop powerful multiinstitutional prospective databases for the study of rare pediatric surgical conditions. Such studies have the ability to study multiple risk factors and outcome measures simultaneously.:

The relative scientific rigor of a cohort study lies in the validity of its cohort definition and its ability to address potential biases in the selection of patients into treatment groups. The validity of results are a function of the similarity of the groups outside the treatment intervention; therefore, great care must be taken in defining these critical baseline characteristics at the onset of the study. Many statistical methods have been developed to take inequalities in these factors into account during the analysis of data. However, even the most rigorous of cohort designs cannot address the unknown but potentially significant sources of bias that surround the selection of patients into treatment groups. This is a consequence of nonrandomization in the assignment of treatment and is the primary reason that these studies are inferior to randomized clinical trials.:
Another limitation of this design is that investigator bias may sway the outcome. For example, if the outcome of the splenectomy study is length of hospital stay, it is possible that the investigators would be highly motivated to prove that laparoscopic splenectomy has a shorter length of stay and to discharge these patients based on different criteria than the open splenectomy group. This bias can occur consciously or subconsciously. Other disadvantages of prospective cohort design are similar to the RCT. They can be very expensive to conduct and require a multiinstitutional framework to study rare conditions. For exceedingly rare conditions (and outcomes), both cohort studies and RCTs are inappropriate. In these situations, the case-control design offers the most effective investigative approach. : As with RCTs, overly stringent admission criteria may compromise the study’s generalizability to the target population.
In spite of its limitations, the prospective cohort study is of value to the pediatric surgeon, and its results are far more useful than the study designs listed as follows. To optimize the value of this design, it is important for the investigators to match covariates likely to affect outcome between the control and experimental groups. This process, of course, requires that we know which covariates are likely to affect outcome and to what degree. Typically, we do not.
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