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

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

SCREENING Suitable Screening Test: Validity:(Sensitivity, Specificity) 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 PS (FP) (TP) NS (TN) (FN) GT TH TD Validity of Screening Test: Screening test compared to gold standard Total Gold standard Screening test Negative Positive PS (FP) (TP) NS (TN) (FN) GT TH TD a b c d TP: True positive; FP: False positive; FN: False negative; TN: True negative; PS: positive screening; NS: negative screening TD: Total disease; TH: Total healthy; GT: Grand total

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

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

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 D No D 90 5 95 10 105 100 200 a b c 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%

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 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: Calculate: Disease Test D No D 19 99 118 1 1881 1882 20 1980 2000 a b c d Calculate: PV+ =19/118=16% PV-= 1881/1882=99.95%

SCREENING Yield(Performance) of Screening Test: Calculate: Disease Test D No D 57 2 59 3 38 41 60 40 100 a b c d Calculate: PV+ =57/59=96.6% PV-= 38/41=93%

SCREENING Factors affecting Yield: Prevalence (%) Sensitivity Specificity PV+ 1.0 90% 95% 15.4% 2.0 90% 95% 26.7% 9.0 90% 95% 64.5% 50.0 90% 95% 94.8%

SCREENING SCREENING Validity &Reliability of Screening Test:

SCREENING SCREENING Validity &Reliability of Screening Test: