Evidence-based Medicine Toolkit

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Appraising prognosis studiesIs the studyvalid?1 Is the sample representative?• Were they recruited at a commonpoint in their illness?• Did the study account for other importantfactors?• Is the setting representative?2 Was follow up long enough for the clinicaloutcome?3 Was follow up complete?4 Were outcomes measured ‘blind’?Are the resultsimportant?What is the risk of the outcome over time?How precise are the estimates?95% confidence intervals are +/–1.96times the standard error (SE) of a measure.The standard error for a proportion (p) is:SE =(p (1 – p)√ nwhere p is the proportion and n is thenumber of subjects.Can the resultshelp you?See p. 71.46
Appraising prognosis studies 47Is the study valid?In asking questions about a patient’s likely prognosis over time,the best individual study type to look for would be longitudinalcohort study.1 Is the sample representative?Does the study clearly define the group of patients, and is it similarto your patients? Were there clear inclusion and exclusion criteria?Were they recruited at a common point in their illness?The methodology should include a clear description of the stageand timing of the illness being studied. To avoid missing outcomes,study patients should ideally be recruited at an early stage in thedisease. In any case, they should all be recruited at a consistentstage in the disease; if not, this will bias the results.Did the study account for other important factors?The study groups will have different important variables such assex, age, weight and co-morbidity which could affect their outcome.The investigators should adjust their analysis to take accountof these known factors in different sub-groups of patients.You should use your clinical judgement to assess whether anyimportant factors were left out of this analysis and whether theadjustments were appropriate. This information will also help youin deciding how this evidence applies to your patient.Is the setting representative?Patients who are referred to specialist centres often have more illnessesand are higher risk than those cared for in the community.This is sometimes called ‘referral bias’.2 Was follow up long enough for the clinical outcome?You have to be sure that the study followed the patients for longenough for the outcomes to manifest themselves. Longer followup may be necessary in chronic diseases.3 Was follow up complete?Most studies will lose some patients to follow up; the question youhave to answer is whether so many were lost that the information
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