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,Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,#,Overview,Introduction,TN Biases,Defintions,Problems with observational studies,Volunteer bias,Lead time bias,Length bias,Stage migration bias,Pseudodisease,Screening tests:TN Biases,“When your only tool is a hammer,you tend to see every problem as a nail.”,Clinical care accounts for 95%of spending but only 20%of determinants of health*,Biggest threats are public health threats,Interventions aimed at individuals are overemphasized because they are more profitable and we know how to do/sell them,*Teutsch SM,Fielding JE.Comparative effectiveness:looking under the lamppost.JAMA 2011;305:2225-6,Cultural characteristics,We live in a wasteful,technology driven,individualistic and death-denying culture.,-George Annas,New Engl J Med,1995,What is screening?,Common definition:testing to detect asymptomatic disease,Better definition*:application of a test to detect a,potential disease or condition,in people with no,known,signs or symptoms of that disease or condition.,Disease vs.condition,Asymptomatic vs.no known signs or symptoms,*,Common screening tests,.,David M.Eddy,editor.Philadelphia,PA:American College of Physicians,1991,Screening tests may be history questions,Screening Spectrum,Risk factor,Recognized symptomatic disease,Presymp-tomatic disease,Unrecognized symptomatic disease,Decreasing numbers labeled and treated,Decreasing difficulty demonstrating benefit,Examples and overlap,Unrecognized symptomatic disease,:vision and hearing problems in young children;iron deficiency anemia,depression,Presymptomatic disease,:neonatal hypothyroidism,syphilis,HIV,Risk factor:,hypercholesterolemia,hypertension,Somewhere between:,prostate cancer,ductal carcinoma in situ of the breast,more severe hypertension,Screened,Not screened,Mortality after Randomization,R,D+,D-,D-,D+,Mortaltiy after Randomization,Evaluating Studies of Screening,Ideal Study:,Randomize patients to be screened or not,Compare outcomes in ENTIRE screened group to ENTIRE unscreened group,Observational studies:Patients are not randomized,Compare outcomes in screened vs.unscreened patients,Or among patients,with disease,:,Compare outcomes in those diagnosed by,screening,vs.those diagnosed by,symptoms,Compare stage-specific survival with and without screening,KEY DIFFERENCE:Mortality vs.Survival,Mortality:denominator is a,population,most of whom never get the disease,Survival:denominator is,patients with the disease,Beware,of any studies evaluating screening tests using,survival,Possible Biases in Observational Studies of Screening Tests,Volunteer bias,Lead time bias,Length time bias,Stage migration bias,Pseudodisease,Volunteer Bias,People who volunteer for screening differ from those who do not,Examples,HIP Mammography study:,Women who volunteered for mammography had lower,heart disease,death rates,Multicenter Aneurysm Screening Study(MASS;Problem 6.3),Men aged 65-74 were randomized to either receive an invitation for an abdominal ultrasound scan or not.,MASS Within Groups Result in Invited Group,Avoiding Volunteer Bias,Randomize patients to screened and unscreened,Otherwise,try to control for factors(confounders)associated with both screening and outcome,Examples:family history,level of health concern,other health behaviors,baseline health/illnesses,Lead Time Bias(zero-time bias),Screening identifies disease during a,latent period,before it becomes symptomatic,If survival is measured from,time of diagnosis,screening will always improve survival,even if treatment is ineffective,Lead time bias,Source:EDITORIAL:Finding and Redefining Disease.,Effective Clinical Practice,March/April 1999.Available at:ACP-Online,http:/www.acponline.org/journals/ecp/marapr99/primer.htm,accessed 8/30/02,Avoiding Lead Time Bias,Only occurs when survival from diagnosis is compared between diseased persons,Screened vs.not screened,Diagnosed by screening vs.by symptoms,Avoiding lead time bias,Measure mortality,not survival,Count from date of randomization,Follow patients for a long time(20 years?)and use total,not e.g.5-year survival,Length Bias(Different natural history bias),Screening picks up prevalent disease,Prevalence=incidence x duration,Slowly growing tumors have greater duration in presymptomatic phase,therefore greater prevalence,Therefore,cases picked up by screening will be disproportionately those that are slow growing,Length bias,Source:EDITORIAL:Finding and Redefining Disease.,Effective Clinical Practice,March/April 1999.Available at:ACP-Online,http:/www.acponline.org/journals/ecp/marapr99/primer.htm,Length Bias,Early detection,Higher cure rate,Slower growing tumor with better prognosis,?,Avoiding Length Bias,Only present when,survival from diagnosis,is compared,AND,disease is heterogeneous,Lead time bias usually present as well,Avoiding length bias:,Compare mortality in the ENTIRE screened group to the ENTIRE unscreened group,Study disease subgroups with
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