Tiêu đề 9. CASE – CONTROL STUDIES.pdf ✅

PHARMD GURU Page 2  Because of their efficiency, they may also be ideal for preliminary investigation of a suspected risk factor for a common condition; conclusions may be used to justify a more costly and time-consuming longitudinal study later. CASES:  Consider a situation in which a large number of cases of post-operative endophthalmitis have occurred in a few weeks.  The case group would consist of all those patients at the hospital who developed post-operative endophthalmitis during a pre-defined period.  The definition of a case needs to be very specific: 1) Within what period of time after operation will the development of endophthalmitis qualify as a case – one day, one week, or one month? 2) Will endophthalmitis have to be proven microbiologically, or will a clinical diagnosis be acceptable? 3) Clinical criteria must be identified in great detail. If microbiologic facilities are available, how will patients who have negative cultures are classified? 4) How will sterile inflammation be differentiated from endophthalmitis?  There are not necessarily any ‘right ‘answers to these questions but they must be answered before the study begins. At the end of the study, the conclusions will be valid only for patients who have the same sort of ‘endophthalmitis’ as in the case definition. CONTROLS:  Controls should be chosen who are similar in many ways to the cases.  The factors (e.g., age, sex, time of hospitalization) chosen to define how controls are to be similar to the cases are the ‘matching criteria’.  The selected control group must be at similar risk of developing the outcome; it would not be appropriate to compare group of controls who had traumatic corneal lacerations with cases who under-went elective intraocular surgery.  In our example, controls could be defined as patients who underwent elective intraocular surgery during the same period of time.
PHARMD GURU Page 3 MATCHING CASES AND CONTROLS:  Although controls must be like the cases in many ways, it is possible to over- match. Over-matching can make it difficult to find enough controls. Also, once a matching variable has been selected, it is not possible to analyze it as a risk factor.  Matching for type of intraocular surgery (e.g., secondary IOL implantation) would mean including the same percentage of controls as cases that had surgery to implant a secondary IOL; if this were done, it would not be possible to analyze secondary IOL implantation as a potential risk factor for endophthalmitis.  An important technique for adding power to a study is to enroll more than one control for every case. For statistical reasons, however, there is little gained by including more than two controls per case. COLLECTING DATA:  After clearly defining cases and controls, decide on data to be collected; the same data must be collected in the same way from both groups.  Care must be taken to be objective in the search for past risk factors, especially since the outcome is already known, or the study may suffer from researcher bias.  Although it may not always be possible, it is important to try to mask the outcome from the person who is collecting risk factor information or inter-viewing patients.  Sometimes it will be necessary to interview patients about potential factors (such as history of smoking, diet, use of traditional eye medicines, etc.) in their past.  It may be difficult for some people to recall all these details accurately. Furthermore, patients who have the out-come (cases) are likely to scrutinize the past, remembering details of negative exposures more clearly than controls. This is known as recall bias. Anything the researcher can do to minimize this type of bias will strengthen the study.