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Comunicación y Gerencia
18/4/2011 Dr Salwa Tayel (Screening)
بسم الله الرحمن الرحيم
2
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-)
318/4/2011 Dr Salwa Tayel (Screening)
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.
418/4/2011 Dr Salwa Tayel (Screening)
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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)
77
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.
18/4/201118/4/2011 Dr Salwa Tayel (Screening)Dr Salwa Tayel (Screening)
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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
18/4/2011 Dr Salwa Tayel (Screening)
918/4/2011 Dr Salwa Tayel (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).
1118/4/2011 Dr Salwa Tayel (Screening)
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.
1218/4/2011 Dr Salwa Tayel (Screening)
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..
1318/4/2011 Dr Salwa Tayel (Screening)
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.
1418/4/2011 Dr Salwa Tayel (Screening)
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.
1518/4/2011 Dr Salwa Tayel (Screening)
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
1618/4/2011 Dr Salwa Tayel (Screening)
Characteristics of a screening test:
• Validity (Sensitivity, Specificity)
• Reliability (repeatability/precision)
• Yield (performance): Predictive values of
the test.
1718/4/2011 Dr Salwa Tayel (Screening)
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)
1818/4/2011 Dr Salwa Tayel (Screening)
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
Validity of Screening TestsValidity of Screening Tests
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)
2018/4/2011 Dr Salwa Tayel (Screening)
Negative test -
Validity of Screening TestsValidity of Screening Tests
a
dc
b
True Disease Status
+ - Results ofScreening Test
+
-
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)
Validity of Screening TestsValidity of Screening Tests
a
dc
b
True Disease Status
+ - Results ofScreening Test
+
-
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)
Validity of Screening TestsValidity of Screening Tests
100
1000
0
True Disease Status
+ - Results ofScreening Test
Positive test +
a = true positive (Agreement)b = false positive (Disagreement)c = false negative (Disagreement)d = true negative (Agreement)
2318/4/2011 Dr Salwa Tayel (Screening)
Negative test -
2424
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)
Gold standardGold standard
(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%
2518/4/2011 Dr Salwa Tayel (Screening)
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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
18/4/2011 Dr Salwa Tayel (Screening)
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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)
2828
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.
18/4/201118/4/2011 Dr Salwa Tayel (Screening)Dr Salwa Tayel (Screening)
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.
2918/4/2011 Dr Salwa Tayel (Screening)
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-)
3118/4/2011 Dr Salwa Tayel (Screening)
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)
3218/4/2011 Dr Salwa Tayel (Screening)
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
3418/4/2011 Dr Salwa Tayel (Screening)
Calculate:• PV+=57/59=96.6%• PV-=38/41=93% useful test
3518/4/2011 Dr Salwa Tayel (Screening)
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
3818/4/2011 Dr Salwa Tayel (Screening)
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
3918/4/2011 Dr Salwa Tayel (Screening)
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
4018/4/2011 Dr Salwa Tayel (Screening)
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Calculate:
• Sensitivity:• Specificity:• PV+:• PV-:• False Positive rate• False Negative rate• Disease prevalence
18/4/2011 Dr Salwa Tayel (Screening)
Comparison of mammography results with findings
from surgical excisional biopsies in women without
palpable breast masses
Screening test Screening test (Mammography)(Mammography)
Gold standardGold standard
))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