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**SCREENING CHP400: Community Health Program-lI Mohamed M. B. Alnoor**

Muna M H Diab

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**SCREENING Suitable Screening Test: Acceptable Safe Rapid Inexpensive**

Easy to do Valid Reliable Valid Reliable

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**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.

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**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”).

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**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

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**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)

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**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)

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**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.

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**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%

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**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.

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**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-)

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**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)

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**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)

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**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%

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**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%

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**SCREENING Factors affecting Yield:**

Prevalence (%) Sensitivity Specificity PV+ % % % % % % % % % % % %

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SCREENING SCREENING Validity &Reliability of Screening Test:

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SCREENING SCREENING Validity &Reliability of Screening Test:

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