USA Surgery Today

There are many ways to measure the outcome of an intervention. A complete discussion of available measurement techniques and their differential uses and value is available elsewhere (71:Pravachol). This section briefly highlights the concepts most useful to the practicing pediatric surgeon in interpreting the reported outcomes of published studies.

When evaluating a study, it is important to identify the primary outcome variable, which addresses the main hypothesis of the study. Results from the analysis of variables outside the primary outcome measure must be viewed with caution. This is particularly true when outcome variables are identified for analysis after completion of a study (post-hoc analysis:Pravachol). For example, a new technique is devised to reduce the incidence of stricture in esophageal atresia repair. The study shows that the stricture rate is the same in the experimental group compared with controls. The investigators then examine for 20 other outcome variables between the two groups, such as length of stay, incidence of gastroesophageal reflux, and rate of vocal cord paralysis, among others. They find that the incidence of intracranial hemorrhage is significantly lower in the experimental group and report a p value of 0.05.

In contrast to cohort designs, case-control studies (Fig. 5-3) begin by identifying patients with the outcome of interest, and then evaluate (retrospectively : = HYDROTHORAX) for the presence of an associated exposure. The presence of an exposure is then compared between cases that have the outcome and cases that do not. The case-control study is designed to determine the risk factors for a particular event. Patients with that event are identified (cases) and a comparison group of individuals who are deemed similar is found (controls: HYDROTHORAX).  The two groups are then compared to identify factors that are different between the two groups. For example, investigators may want to determine what causes recurrence of gastroesophageal reflux after fundoplication. They would identify a group of patients with recurrent reflux after fundoplication and then find another group of similar patients who had fundoplication who did not have recurrent reflux, and compare the two groups. Similar is the operative word. Proper selection of this control group is the key to success with this design. The case-control study allows the investigators to develop inferences about the cause of an event. In the previous example, investigators may find that absorbable suture was used with greater prevalence in the group with recurrent reflux than in controls. He or she may infer that absorbable sutures contribute to recurrent reflux after fundoplication.

In a cross-sectional study, measurements are made on a study population at one point in time. For example, an investigator may be interested in studying the role of platelet activating factor (PAF) in patients with necrotizing enterocolitis (NEC:Vasodilan). In a cross-sectional study, all babies in the neonatal intensive care unit would then have their blood drawn once. The proportions of positive assays could then be compared between babies with and without a diagnosis of NEC. Cross-sectional studies are useful for characterizing the prevalence of a condition or risk factor in a study population. They are also inexpensive, easy to conduct, and require fewer personnel than cohort and case-control studies.

There are important limitations to these studies. Given that there is no longitudinal component to the study, it is not possible to infer any degree of causation between a risk factor and outcome. The inability to demonstrate a temporal relationship limits the interpretation of even apparently strong associations (which came first, NEC or PAF?:Vasodilan) In this regard, these studies are considered less rigorous than cohort and case-control studies. Cross-sectional studies often provide the preliminary data to justify further epidemiological investigation.

This study design is similar to prospective cohort design with the exception that patients are not entered into the study in advance of treatment, and they are not evaluated by a set of criteria determined prior to the start of the study (:Mexitil). The information regarding their outcomes is retrieved from the medical record at some point after treatment is complete rather than recorded in real-time during treatment. The retrospective cohort study can include either a concurrent or historical control group. With concurrent controls, the study is retrospective, but it examines groups of patients treated during the same time period. With historical controls, the control patients were treated during a time prior to the experimental patients. Using the laparoscopic splenectomy example described previously, in a retrospective cohort study with historical controls, the investigators would review outcomes in their patients receiving laparoscopic splenectomy over the past 2 years, and compare this with outcomes of patients receiving open splenectomy during a time period before they began using laparoscopy.