3 SCREENING Suitable Screening Test: Validity:(Sensitivity, Specificity) AcceptableSafeRapidInexpensiveEasy to doValidity:(Sensitivity, Specificity)Reliability:(repeatability/precision)Yield (performance): Predictive values of the test.
4 SCREENING Validity of Screening Test: How good is the screening test comparedwith 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”).
5 SCREENING PS (FP) (TP) NS (TN) (FN) GT TH TD Validity of Screening Test:Screening test compared to gold standardTotalGold standardScreening testNegativePositivePS(FP)(TP)NS(TN)(FN)GTTHTDa bc dTP: 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
6 SCREENING Validity of Screening Test: - + a + b - c d True Disease Status+-ResultsofScreeningTest+ab-cdSensitivity: The probability of testing positive if the disease is truly presentSensitivity = a / (a + c)
7 SCREENING Validity of Screening Test: - + a + b - c d True Disease Status+-ResultsofScreeningTest+ab-cdSpecificity: The probability of screening negative if the disease is truly absentSpecificity = d / (b + d)
8 SCREENING Adverse effects of screening: Stress and anxiety caused by a false positive screening results.Unnecessary investigation and treatment of false positive resultsProlonging 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.
9 SCREENING Validity of Screening Test: Disease Test D No D 90 5 95 10 105100200a bc dSensitivity: 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%
10 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.
11 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-)
12 SCREENING Yield(Performance) of Screening Test: a d c b True Disease Status+-Results ofScreeningTestPredictive value positive (PV+): The probability that aperson actually has the disease given that he or shetests positive. i.e. The ability to predict the presence of disease from test results.PV+ = a / (a + b)
13 SCREENING Yield(Performance) of Screening Test: a d c b True Disease Status+-Results ofScreeningTestPredictive value negative (PV-): The probability that aperson is truly disease free given that he or shetests negative. i.e. The ability to predict the absenceof disease from test results.PV = d / (c + d)
14 SCREENING Yield(Performance) of Screening Test: Calculate: DiseaseTestDNo D19991181188118822019802000a bc dCalculate:PV+ =19/118=16%PV-= 1881/1882=99.95%
15 SCREENING Yield(Performance) of Screening Test: Calculate: DiseaseTestDNo D57259338416040100a bc dCalculate:PV+ =57/59=96.6%PV-= 38/41=93%