Clinical Trials

❘❙❚■ Chapter 5 | Patient SelectionWhen estimating the likely number of patients to be selected for a trial at aparticular center, the investigator often relies on guesswork based on the numberof previous patients with the condition. A more useful method is to request thatinvestigators complete a screening log, prior to the start of the study, of allpatients who present with the condition within a predetermined timeframe, and torecord whether each patient is eligible for the study.Recruitment will often not be possible at a single center due to its limited capacityand resources; there might also be an insufficient number of patients presentingwith the disease, resulting in the center failing to recruit the required numberswithin the allotted timeframe. It is important to recruit patients for a study in asshort a timeframe as possible in order to maintain enthusiasm for the study.One way of improving recruitment centers is to enroll more centers; however, thisis not the most economical way to conduct a trial, given the resources needed totrain new centers.Eligibility criteriaIn order to reduce bias and variability in a clinical trial, and therefore increase thepower of the study, it is essential to have well-developed eligibility criteria.However, these criteria must not be too restrictive as this will result in a smallerpatient population – there could be difficulties with patient recruitment, andwhilst the patient population would be more homogeneous, the generalizability ofthe findings would be reduced in the non-trial setting.A key aspect of eligibility criteria is to define patients with the disease or conditionunder investigation. The disease state must be established using standardizedcriteria. Although some diseases are more difficult to define, it should not be leftto subjective clinical assessment as this makes it hard to relate the study findingsto other patients with the condition. In these cases, it might be sensible to usemore than one definition for the disease criteria.Eligibility criteria are usually based on:• patient characteristics• diagnostic test results• disease duration• disease severityPatients must meet all of the inclusion criteria and none of the exclusion criteria.A general rule is to have one or two inclusion criteria and several exclusioncriteria. In the CHARM trial, for example, the investigators stated two inclusioncriteria and multiple exclusion criteria (see Table 1) [1]. The aim was to include as50