1 Comunicación y Gerencia 18/4/2011Dr Salwa Tayel (Screening) بسم الله الرحمن...

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Comunicación y Gerencia

18/4/2011 Dr Salwa Tayel (Screening)

بسم الله الرحمن الرحيم

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Screening

Associate Professor Associate Professor

Family and Community Medicine DepartmentFamily and Community Medicine DepartmentKing Saud UniversityKing Saud University

18/4/2011

Dr. Salwa Tayel Dr. Salwa Tayel

Dr Salwa Tayel (Screening)

Learning Objectives:Learning Objectives:

1.1. Define screening and mention its purpose.Define screening and mention its purpose.

2.2. List WHO criteria for effective screening List WHO criteria for effective screening

3.3. Calculate and interpret measures of the validity of a Calculate and interpret measures of the validity of a

screening test:screening test:

------ SensitivitySensitivity

------ SpecificitySpecificity

4.4. Calculate and interpret measures of the performance (yield) Calculate and interpret measures of the performance (yield)

of a screening test:of a screening test:

------ Predictive value positive (PV+)Predictive value positive (PV+)

--- Predictive value negative (PV-)--- Predictive value negative (PV-)


ScreeningScreening

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.


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Screening Vs. diagnosisScreening Vs. diagnosis TestsTests The purpose of screening is to segregate those The purpose of screening is to segregate those

who may have the condition. It is an initial step, as who may have the condition. It is an initial step, as it needs further confirmation. it needs further confirmation.

On the other hand, diagnosis is a procedure to On the other hand, diagnosis is a procedure to confirm or disprove the existence of a disease or confirm or disprove the existence of a disease or abnormality. abnormality.

Both screening and diagnosis can be achieved by Both screening and diagnosis can be achieved by obtaining medical history, performing physical obtaining medical history, performing physical examination and the application of laboratory or examination and the application of laboratory or non-laboratory test.non-laboratory test.

18/4/201118/4/2011 Dr Salwa Tayel (Screening)Dr Salwa Tayel (Screening)

Comparison between screening and Comparison between screening and diagnostic testsdiagnostic tests

Screening testsScreening tests Diagnostic testsDiagnostic testsDone to those who are apparently healthy or asymptomatic

Done to those with suggestive signs or symptoms

Applied to a group of individuals

Applied to a single person

Results are based on one criterion

Results are based on the evaluation of a number of symptoms, signs and investigations

Results are not conclusive

Results are conclusive and final

Less accurate More accurate

Less expensive More expensive

Not a basis for treatment

Basis for treatment 618/4/2011 Dr Salwa Tayel (Screening)

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Types of screening programsTypes of screening programs Mass screeningMass screening

Application of the screening program to the whole Application of the screening program to the whole population or population subgroups as adults, population or population subgroups as adults, school children, industrial workers.school children, industrial workers.

High risk or selective screeningHigh risk or selective screening

The screening program will be applied to a The screening program will be applied to a selective group of population who are at a high selective group of population who are at a high risk e.g. breast cancer screening to females risk e.g. breast cancer screening to females above 40 years.above 40 years.


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Early Intervention in the Natural History of Disease

HEALTH OUTCOMES

CureControl

DisabilityDeath

Disease Onset

Symptoms Diagnosis TherapyCare Seeking

Good Health

Early detection through Screening


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Flow diagram for a screening program

Population

Test -ve Test +ve

Unaffected Affected

Intervention

Diagnostic procedures

Screening test

Re-screen Re-screen 18/4/2011 Dr Salwa Tayel (Screening)

Screening for Disease Screening for Disease ControlControl

Screening Objective:

To lower morbidity and mortality of the

disease in a population (control, rather

than elimination of disease).


WHO criteria for effective screening:WHO criteria for effective screening:

1)1) The disease should be important public The disease should be important public

health problem (relates to cost effectiveness, health problem (relates to cost effectiveness,

and prognosis).and prognosis).

2)2) There should be an effective and acceptable There should be an effective and acceptable

treatment for the condition if identified in an treatment for the condition if identified in an

early stage.early stage.

3)3) Facilities for the confirmation of the diagnosis Facilities for the confirmation of the diagnosis

and treatment should be available.and treatment should be available.


WHO criteria for effective screening:WHO criteria for effective screening:

4)4) There should be a latent stage of the disease There should be a latent stage of the disease

(long and detectable pre-symptomatic stage).(long and detectable pre-symptomatic stage).

5) There should be a suitable screening test or 5) There should be a suitable screening test or

examination that can detect the condition examination that can detect the condition

6) The test should be acceptable to the 6) The test should be acceptable to the

populationpopulation..


7) Natural history of disease should be 7) Natural history of disease should be

adequately understood.adequately understood.

8) There should be an agreed upon policy 8) There should be an agreed upon policy

on whom to treat.on whom to treat.


9) The total cost of finding a case should be 9) The total cost of finding a case should be

economically balanced in relation to economically balanced in relation to

medical expenditure as a whole. medical expenditure as a whole.

10) Case finding should be a continuous 10) Case finding should be a continuous

process, not just a “once and for all” process, not just a “once and for all”

project.project.


Diseases for which screening Diseases for which screening has been recommendedhas been recommended

Diseases for which screening Diseases for which screening has been recommendedhas been recommended

Cervical cancer

Breast cancer

Ovarian cancer

Colorectal cancer

Skin cancer

Diabetes

Hypertension


Characteristics of a screening test:

• Validity (Sensitivity, Specificity)

• Reliability (repeatability/precision)

• Yield (performance): Predictive values of

the test.


Validity of Screening TestsValidity of Screening Tests

How good is the screening test comparedwith the confirmatory diagnostic test (Gold Standard test)?

Gold Standard: Gold Standard: A method, procedure, or A method, procedure, or

measurement that is widely accepted as being the best measurement that is widely accepted as being the best

available. Often used to compare with new methods of available. Often used to compare with new methods of

unknown effectivenessunknown effectiveness. . (Dictionary of Epidemiology)(Dictionary of Epidemiology)


1919

Results of screening test compared to gold Results of screening test compared to gold

standardstandard

Screening testScreening test

Gold standardGold standard

TotalTotalPositivePositive NegativeNegative

PositivePositive (TP)(TP) (FP)(FP) PSPSNegativeNegative (FN)(FN) (TN)(TN) NSNS

TotalTotal TDTD THTH GTGT

18/4/201118/4/2011


a

dc

b

True Disease Status

+ - Results ofScreening Test

Positive test +

a = true positive (Agreement)b = false positive (Disagreement)c = false negative (Disagreement)d = true negative (Agreement)


Negative test -


a

dc

b

True Disease Status


+

-

Sensitivity: The probability of testing positive if the disease is truly present

Sensitivity = a / (a + c)212118/4/201118/4/2011 Dr Salwa Tayel (Screening)Dr Salwa Tayel (Screening)


a

dc

b

True Disease Status


+

-

Specificity: The probability of screening negative if the disease is truly absent

Specificity = d / (b + d)222218/4/201118/4/2011 Dr Salwa Tayel (Screening)Dr Salwa Tayel (Screening)


100

1000

0

True Disease Status


Positive test +

a = true positive (Agreement)b = false positive (Disagreement)c = false negative (Disagreement)d = true negative (Agreement)


Negative test -

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Results of screening 100 men for prostate cancer Results of screening 100 men for prostate cancer

using (PSA)using (PSA)

Screening test Screening test

(PSA)(PSA)


(Prostatic biopsy)(Prostatic biopsy) TotalTotal

CancerCancer No cancerNo cancer

PositivePositive 3 3 (TP)(TP) 7 7 (FP)(FP) 1010NegativeNegative 2 2 (FN)(FN) 88 88 (TN)(TN) 9090

TotalTotal 55 9595 10010018/4/201118/4/2011

Sensitivity: a / (a + c)Sensitivity = 90%

Specificity: d / (b + d)Specificity = 95%

Prevalence of disease =(a+c)/(a+b+c+d)=100/200=50%


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• Number of false positives=bNumber of false positives=b

• False Positive Rate=b/b+dFalse Positive Rate=b/b+d

• Number of false negatives=cNumber of false negatives=c

• False Negative Rate=c/a+cFalse Negative Rate=c/a+c


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Sensitivity: a / (a + c) = 90%

Specificity: d / (b + d) 95%

False Positive Rate=b/b+d=5/100=5%

False negative Rate=c/a+c=10/100=10%

Prevalence of disease = (a+c)/ (a+b+c+d) 100/200= 50%18/4/2011 Dr Salwa Tayel (Screening)

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Adverse effects of screeningAdverse effects of screening

Stress and anxiety caused by a Stress and anxiety caused by a false positive false positive

screening results.screening results.

Unnecessary investigation and treatment of false Unnecessary investigation and treatment of false

positive resultspositive results

Prolonging knowledge of an illness if nothing can Prolonging knowledge of an illness if nothing can

be done about it.be done about it.

A false sense of security caused by A false sense of security caused by false negativesfalse negatives, ,

which may even delay final diagnosis.which may even delay final diagnosis.

Overuse/waste of medical resources.Overuse/waste of medical resources.


Reliability of Screening TestsReliability of Screening Tests

RELIABILITY (ReproducibilityReproducibility) Precision:

The extent to which the screening test will produce the same or very similar results each time it is administered (repeated).

•It can be assessed by repeating the test using the same or different observers.

--- A test must be reliable before it can be valid.


Reliability of Screening TestsReliability of Screening Tests

Sources of variability that can affect the reproducibility of results of a screening test:

1. Biological variation (e.g. blood pressure)

2. Reliability of the instrument itself

3. Intra-observer variability (differences in repeated measurement by the same screener)

4. Inter-observer variability (inconsistency in the way different screeners apply or

interpret test results)3018/4/2011 Dr Salwa Tayel (Screening)

Yield (Performance)Yield (Performance)

It is measured by:It is measured by:

Predictive Value Positive (PV+)Predictive Value Positive (PV+)

Predictive Value Negative (PV-)Predictive Value Negative (PV-)


YieldYield

a

dc

b

True Disease Status

+ -

Results ofScreening

Test

+

-

Predictive 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)


YieldYield

a

dc

b

True Disease Status

+ -

Results ofScreening

Test

+

-

Predictive value negative (PV-): The probability that aperson is truly disease free given that he or shetests negative. i.e.The ability to predict the absence of disease from test results.

PV- = d / (c + d)3318/4/2011 Dr Salwa Tayel (Screening)

Calculate:• PV+ =19/118=16%• PV-= 1881/1882=99.99 9


Calculate:• PV+=57/59=96.6%• PV-=38/41=93% useful test


Sensitivity: a / (a + c)= 19/20 Sensitivity = 95%

Specificity: d / (b + d)= 1881/1980 Specificity =95%

Prevalence=20/2000*100 Prevalence=1%3618/4/2011 Dr Salwa Tayel (Screening)

Sensitivity: a / (a + c)= 57/60 Sensitivity = 95%

Specificity: d / (b + d)= 38/40 Specificity =95%

Prevalence= 60/100*100 Prevalence=60%3718/4/2011 Dr Salwa Tayel (Screening)

Prevalence (%) Sensitivity Specificity PV+

0.1 90% 95% 1.8%

1.0 90% 95% 15.4%

5.0 90% 95% 48.6%

50.0 90% 95% 94.7%

Factors affecting the yield of a screening test


Sensitivity:Sensitivity:Specificity:Specificity:Prevalence:Prevalence:

• The higher the prevalence of preclinical disease in the screened The higher the prevalence of preclinical disease in the screened population, the higher the PV+.population, the higher the PV+.• PV+ is maximized when used in “high risk” populations since the PV+ is maximized when used in “high risk” populations since the prevalence of pre-clinical disease is higher than in the general prevalence of pre-clinical disease is higher than in the general population.population.

• Screening a total population for a relatively infrequent disease Screening a total population for a relatively infrequent disease can be very wasteful of resources and may yield few previously can be very wasteful of resources and may yield few previously undetected cases.undetected cases.

Factors affecting the yield of a screening test


Characteristics of a suitable screening test:

• Validity – the extent to which the test distinguishes between persons with and without the disease: High validity requires:

• High Sensitivity

• High Specificity

• Reliability (High)

• Performance (Yield)

• Low cost, invasiveness, and discomfort

• Costs.

1. Costs of applying the test itself.

2. Costs of performing additional tests on people with false positives, in order to correct the test’s mistakes


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Calculate:

• Sensitivity:• Specificity:• PV+:• PV-:• False Positive rate• False Negative rate• Disease prevalence


Comparison of mammography results with findings

from surgical excisional biopsies in women without

palpable breast masses

Screening test Screening test (Mammography)(Mammography)


))Surgical biopsySurgical biopsy(( TotalTotal

CancerCancer No cancerNo cancer

PositivePositive 1818 4949 6767NegativeNegative 22 931931 933933

TotalTotal 2020 980980 10001000

18/4/2011 42Dr Salwa Tayel (Screening)

4343

Thank Thank youyou

End of End of the the

lecturelecture

Bibliotheca Alexandrina

18/4/201118/4/2011

Website http://faculty.ksu.edu.sa/73234/default.aspx

[email protected]

1 Comunicación y Gerencia 18/4/2011Dr Salwa Tayel (Screening) بسم الله الرحمن...

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