CHP400: Community Health Program-lI Mohamed M. B. Alnoor Muna M H Diab SCREENING

The presumptive identification of unrecognized diseases or defects by the application of tests, examinations or other procedures which can be applied rapidly to sort out apparently well persons who probably have a disease from those who probably do not. DEFINITION

SCREENING Screening for Disease Control Screening: The application of a disease- detection test in asymptomatic apparently healthy individuals. Purpose: To classify individuals with respect to their likelihood of having a particular disease. Screening procedure itself does NOT formally diagnose illness.

SCREENING Screening for Disease Control Stage of Susceptibility Stage of Recovery, Disability, or Death Exposure Pathological Changes Onset of Symptoms Usual Time of Diagnosis Stage of Subclinical Disease Early detection “SCREENING” Secondary Prevention Primordial Prevention Primary Prevention Stage of Clinical Disease Tertiary Prevention Lead time

SCREENING Screening for Disease Control

SCREENING Screening for Disease Control Unlikely to have a disease  Examination of asymptomatic people  Classification as Likely to have a disease

SCREENING Objective Screening for Disease Control To lower morbidity and mortality of the disease in a population (control of disease).

SCREENING WHO criteria for screening: 1)The disease should be important public health problem (relates to cost effectiveness, and prognosis). 2)Facilities for the confirmation of the diagnosis and treatment should be available 3)There should be an effective and acceptable treatment for the condition if identified in an early stage.

SCREENING WHO criteria for screening: 4) There should be a latent stage of the disease (long and detectable pre-symptomatic stage). 5)Natural history of disease should be adequately understood. 6) There should be a suitable screening test or examination that can detect the condition 7) The test should be acceptable to the population.

SCREENING WHO criteria for screening: 8) There should be an agreed upon policy on whom to treat. 9) The total cost of finding a case should be economically balanced in relation to medical expenditure as a whole. 10) Case finding should be a continuous process, not just a “once and for all” project.

SCREENING Suitable Screening Test : Acceptable Safe Rapid Inexpensive Easy to do Valid Reliable Valid Reliable

SCREENING Suitable Screening Test : Acceptable Safe Rapid Inexpensive Easy to do Validity: (Sensitivity, Specificity) Reliability: (repeatability/precision) Yield (performance): Predictive values of the test.

SCREENING Validity of Screening Test : How good is the screening test compared with the confirmatory diagnostic test (Gold Standard test)? The test will correctly classify a diseased person as likely to have the condition (“sensitivity”). The test will correctly classify a non- diseased person as unlikely to have the condition (“specificity”).

SCREENING Validity of Screening Test : Total Gold standard Screening test NegativePositive PS(FP)(TP) Positive NS(TN)(FN) Negative GTTHTD Total Screening test compared to gold standard a b c d

SCREENING Validity of Screening Test : a d c b True Disease Status + - Results of Screening Test + - Sensitivity: The probability of testing positive if the disease is truly present Sensitivity = a / (a + c)

SCREENING Validity of Screening Test : a d c b True Disease Status Specificity: The probability of screening negative if the disease is truly absent Specificity = d / (b + d) Results of Screening Test

SCREENING Adverse effects of screening: Stress and anxiety caused by a false positive screening results. Unnecessary investigation and treatment of false positive results Prolonging knowledge of an illness if nothing can be done about it. A false sense of security caused by false negatives, which may delay final diagnosis. Overuse/waste of medical resources.

SCREENING Validity of Screening Test : Disease Test DNo D Sensitivity: a / (a + c) = 90/100 =90% Specificity: d / (b + d) = 95/100 =95% Prevalence of disease =(a+c)/(a+b+c+d) =100/200 =50% a b c d

SCREENING Reliability of Screening Test : RELIABILITY (Reproducibility) Precision: The extent to which the screening test will produce the same (or very similar) results each time it is administered (repeated). --- A test must be reliable before it can be valid.

SCREENING Yield(Performance) of Screening Test: Yield is the amount of previously unrecognized disease that is diagnosed and brought to treatment as a result of screening. It is measured by: Predictive Value Positive (PV+) Predictive Value Positive (PV+) Predictive Value Negative (PV-) Predictive Value Negative (PV-)

SCREENING Yield(Performance) of Screening Test: a d c b True Disease Status + - Results of Screening Test + - Predictive value positive (PV+): The probability that a person actually has the disease given that he or she tests positive. i.e. The ability to predict the presence of disease from test results. PV+ = a / (a + b)

SCREENING Yield(Performance) of Screening Test: a d c b True Disease Status + - Results of Screening Test + - Predictive value negative (PV-): The probability that a person is truly disease free given that he or she tests negative. i.e. The ability to predict the absence of disease from test results. PV- = d / (c + d)

SCREENING Yield(Performance) of Screening Test: Disease Test DNo D a b c d Calculate: PV+ =19/118=16% PV-= 1881/1882=99.95%

SCREENING Yield(Performance) of Screening Test: Disease Test DNo D a b c d Calculate: PV+ =57/59=96.6% PV-= 38/41=93%

SCREENING Factors affecting Yield : Prevalence (%) Sensitivity Specificity PV % 95% 15.4% 2.090% 95% 26.7% 9.090% 95% 64.5% % 95% 94.8%

SCREENING Validity &Reliability of Screening Test :

SCREENING Validity &Reliability of Screening Test :

SCREENING Bias in Screening : 1. (volunteer bias) Those who choose to participate are likely to be different from those who don’t. Volunteers tend to have: Better health Lower mortality Likely to adhere to prescribed medical regimens On the other hand…. The “worried well” (who have higher risk) may be more likely to participate.

SCREENING Bias in Screening : 2. Lead Time Bias Survival will appear to be prolonged in screened people simply because survival is measured from an earlier point in the disease’s evolution.

SCREENING Lead time bias : Lead time: interval between the diagnosis of a disease at screening and the usual time of diagnosis (by symptoms) Diagnosis by screening Diagnosis via symptoms Lead Time

SCREENING Lead time bias : The apparently better survival for screened persons is because diagnosis is being made at an earlier point in the natural history of the disease. There is no additional life is added but there may be added anxiety from knowing the disease earlier. Diagnosis by screening in 1994 Death in 2008 Survival = 14 years

SCREENING Lead time bias : Diagnosis by screening in 1994 Usual time of diagnosis via symptoms in 1998 Lead Time 4 years Death in 2008 True Survival = 10 years Survival = 14 years

SCREENING 3. Length bias Screening selectively identifies those with a long preclinical and clinical phase (i.e., those who would have a better prognosis regardless of the screening program) If disease is slowly progressive at one stage, it is likely to be slowly progressive at others and hence, to have a better overall prognosis regardless of any effects of early treatment. Bias in Screening :