EQUINE INTERNAL MEDICINE: FROM ASCARIDS TO ZEBRAS


Table 1. Questions to Ask Before Beginning to Critically Appraise an Article


1. Is the article from a peer reviewed journal? 2. Are the study subjects similar to my patients? 3. Is the study sponsored by an organization that might influence the study design or results?


4. If the results are true, will there be a direct impact on the health of my patients?


5. Is the problem addressed one that is common in my practice?


6. Is the test or intervention feasible? Is it available to me? 7. If it is true, will the information require me to change my practice?


mation about the design of the study. In this sec- tion, the reader should answer the question, “Is the study valid?” The method of appraisal varies based on the design and objectives of the study. Epidemiologic research involves the study of the relationship of one event or characteristic, termed an exposure, to a specific disease or disease state (outcome). There are two main types of study de- signs in veterinary literature: experimental stud- ies (randomized control trials) and observational studies (cross-sectional, case-control, cohort, and case reports/case series). Not all studies are cre- ated equal. The evidenced-based medicine pyra- mid of evidence illustrates that the quality of evidence improves as you work up the pyramid (Fig. 1).5 Each design has potential limitations that can be minimized through careful planning by the investigator. In equine veterinary medicine, it is rare to en- counter meta-analysis or systematic reviews. Ex- perimental epidemiologic studies are also known as a randomized controlled trial (RCT). In a RCT an intervention is applied and variables are manipu- lated. Animals are randomly assigned into an ex- perimental group or a control group and are followed over time for the variable or outcome of interest. In an observational study, no intervention is made and no variables are manipulated; the animal is


studied in its natural environment and results are directly relevant to the natural disease process. The most common observational study designs in the equine literature are cross-sectional, case- control, and cohort studies. Cross-sectional studies provide a “snapshot” of all parameters (exposures and outcomes) which are assessed at the same time. In a cohort study, the patients are classified by ex- posure status at the onset of the study and are followed forward in time to compare the rate of disease occurrence in the exposed and unexposed groups. These studies can be performed prospec- tively (followed forward in time) or retrospectively (information gathered from the past). In a case- control study, investigators first identify animals that have the outcome of interest (cases) and pa- tients without the same outcome (controls), and then they collect historic exposure information. Finally, case reports and case-series provide a summary of either one or several patients with an outcome of interest, with no control group. In patient-based research, the clinical question usually fits into one of four categories: 1) diagnosis and screening, 2) cau- sation, 3) therapy, and 4) prognosis. The type of study employed depends on the clinical activity in question.1,6


Diagnostic Tests


Studies that evaluate a diagnostic test are usually cross-sectional in design. The accuracy of a diag- nostic test is best determined by comparing it to the “truth.” Readers must assure that an appropriate reference or “gold standard” has been applied to every patient, along with the test under investiga- tion. If the “gold standard” is acceptable, the next question is whether the test results and the stan- dard were assessed independently of each other. Clinical experience demonstrates why this is impor- tant. For instance, once we have identified degen- erative joint disease on radiographs, a previously unidentified lameness may become more apparent. Finally, a common mistake is to apply the new di- agnostic to only patients with severe forms of dis- ease and patients without disease. Rather, the test should be applied to a broad spectrum of disease severity (mild to severe) to reflect a more practical clinical application.7 To interpret the results, a solid understanding of general epidemiologic principles, including sensitiv- ity, specificity, predictive values, and likelihood ra- tios, is essential.8


Causation


Fig. 1. Evidenced-based medicine pyramid of evidence: the best quality evidence is at the top of the pyramid.


514 2019  Vol. 65  AAEP PROCEEDINGS


To determine risk factors for development of dis- ease, either cohort or case-control studies should be used to evaluate the strength of association between the putative cause and the adverse outcome. It is essential in these studies to evaluate the outcomes and exposures in the same way in the groups being compared and the duration of follow up should be sufficiently long (for cohort studies).


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