Chapter 11 Screening Presumptive identification of unrecognized disease or defects by the application of tests examinations or other procedures that can be applied rapidly Multiphasic Screening o Cost effective approach is to screen for more than one disease o The use of 2 or more screening tests together among large groups of people o Administered as a Preemployment physical Passing examination as a condition of employment Repetitive screening on an annual basis o Assess risk factor status o Individual or family history of illness o Collect physiological and health measurements o Population screening refers to screening of total population groups on large scale o Targeted screening applied to subsets of the population at high risk for disease or Mass screening regardless of risk status Selective screening certain conditions as a result of Family history Age Mass health examinations Environmental exposures o Population or epidemiologic surveys purpose is to gain knowledge regarding the distribution of diseases in selected populations o No benefit to participant is implied o Epidemiologic surveillance aims at the protection of community health through case detection and intervention Continuous observation of the trends and distribution of disease incidence in a community Sources of data for surveillance include morbidity and mortality report by the CDC Surveillance programs are used for detection and control of conditions ranging from infectious diseases to injuries to chronic diseases o Case finding opportunistic screening the utilization of screening tests for detection of conditions unrelated to the patient s chief complaint Appropriate situations o Social The health problem should be important for the individual and the community Diagnostic follow up and intervention should be available to all who require them Favorable cost benefit ration Public acceptance must be high o Scientific o Ethical Natural history of the condition should be adequately understood This knowledge permits identification of early stages of disease and appropriate biologic markers of progression The program can alter the natural history of the condition in a significant proportion of those screened Suitable acceptable tests for screening and diagnosis of the condition as well as acceptable effective methods of prevention are available Characteristics of a good Screening Test o Simple easy to learn and perform o Rapid test and results do not take long o Inexpensive good cost benefit ratio o Safe no harm to participants o Acceptable to target group Evaluation of screening tests o The purpose of screening is to classify individuals as to whether they are likely to have disease or be disease free o To do this measuring instruments or a combination of instruments is required o Examples of measuring instruments are Clinical laboratory test A fever thermometer Weighing scales and Standardized questionnaire o Reliability o Validity Ability of a measuring instrument to give consistent results of repeated trials Repeated measurement reliability degree of consistency among repeated measurements of the same individual on more than one occasion Ability of measuring instrument to give a true measure Can be evaluated only if an accepted and independent method for confirming the test measurement exists o Interrelationships Possible for a measure to be highly reliable but invalid Not possible to be valid but unreliable Measurement bias constant errors that are introduced by a faulty measuring device and tend to reduce the reliability of measurements Ex miscalibrated blood pressure manometer Halo Effect bias that effects the validity of questionnaire measurements Example All items of a checklist evaluation of an employee may be filled out in the same general direction based on the supervisor s opinion of the individual Social desirability effects Respondent answers questions in a manner that agrees with desirable social norms have the disease a a c o Sensitivity o Specificity the disease d b d o Predictive value the disease a a b o Predictive value disease d c d value rises o Sensitivity and Specificity The ability of the test to identify correctly all screened individuals who actually The ability of the test to identify only non diseased individuals who do not have The proportion of individuals screened positive by the test who actually have The proportion of individuals screened negative by the test who do not have the o Accuracy of a screening test determined by the following formula a d a b c d o Prevalence determined by the formula a c a b c d o When the prevalence of a disease falls the predictive value falls and the predictive The improve sensitivity the cut point is used to classify individuals as diseased should be moved farther To improve specificity the cut point should be moved farther in the range typically associated with the disease To improve specificity the cut point should be moved farther in the range typically associated with the disease o Improving sensitivity and specificity Retrain screeners Recalibrate screening instrument Utilize a different test Utilize more than one test Sources of Bias o Lead time bias Perception that the screen detected case has longer survival because the disease was identified early Particularly relevant to cancer screening Tumors identified by screening are slower growing and have a better prognosis o Length bias o Selection bias Motivated participants have a different probability of disease than do those who refuse to participate