Evidence-based Medicine Toolkit

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48 Evidence-based Medicine Toolkitis of no use to you. You should look carefully in the paper for anaccount of why patients were lost and consider whether this introducesbias into the result.• If follow up is less than 80% the study’s validity is seriouslyundermined.You can ask ‘what if’ all those patients who were lost to follow uphad the outcome you were interested in, and compare this withthe study to see if loss to follow up had a significant effect. Withlow incidence conditions, loss to follow up is more problematic.4 Were outcomes measured ‘blind’?How did the study investigators tell whether or not the patients actuallyhad the outcome? The investigators should have defined theoutcome/s of interest in advance and have clear criteria which theyused to determine whether the outcome had occurred. Ideally,these should be objective, but often some degree of interpretationand clinical judgement will be required.To eliminate potential bias in these situations, judgementsshould have been applied without knowing the patient’s clinicalcharacteristics and prognostic factors.Are the results important?What is the risk of the outcome over time?Three ways in which outcomes might be presented are:• as a percentage of survival at a particular point in time;• as a median survival (the length of time by which 50% of studypatients have had the outcome);• as a survival curve that depicts, at each point in time, the proportion(expressed as a percentage) of the original study samplewho have not yet had a specified outcome.Survival curves provide the advantage that you can see how thepatient’s risk might develop over time.How precise are the estimates?Any study looks at a sample of the population, so we would expectsome variation between the sample and ‘truth’. Prognosticestimates should be accompanied by Confidence Intervals to representthis (see p. 55). You should take account of this range whenextracting estimates for your patient. If it is very wide, you would
Appraising prognosis studies 49question whether the study had enough patients to provide usefulinformation.The standard error for a proportion (p) is:(p (1 – p)SE =√ nwhere p is the proportion and n is the number of subjects.Assuming a normal distribution, the 95% confidence interval is1.96 times this value on either side of the estimate.Further readingLaupacis A, Wells G, Richardson WS, Tugwell P. Users’ guides to the medicalliterature. V. How to use an article about prognosis. J Am Med Assoc1994;272:234–7.Sackett DL, Straus SE, Richardson WS, Rosenberg WMC, Haynes RB. Evidence-BasedMedicine: How to practice and teach EBM, 2nd Edn. NewYork: Churchill Livingstone, 2000.
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