screening lecture for sims lahore and post graduatestudents 2017
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By DR MUHAMMAD TAUSEEF JAVED
MBBS, DPH, DIP-CARD,MSC MA, MPHIL, FCPS.Associate Professor Of Community
Medicine and Family Medicine UmulQurrah University Makkaha
SIMS LAHORE-2015
Screening
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DR Muhammad Tauseef Javed SIMS 2017
What is Screening
• Screening is the testing of apparently healthy
populations to identify previously undiagnosed diseases or people at high risk of developing a disease.
• Screening aims to detect early disease before it becomes symptomatic.
• Screening is an important aspect of prevention, but not all diseases are suitable for screening.
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Definitions
1. Screening program -- comprehensive disease control activity based on the identification and treatment of persons with either unrecognized disease or unrecognized risk factors for disease.
2. Screening test -- specific technology (survey questionnaire, physical observation or measurement, laboratory test, radiological procedure, etc.) used to help identify persons with unrecognized disease or unrecognized risk factors for disease.
Definitions
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Definitions
3. Primary prevention -- disease control approach based on the elimination or reduction of risk factors for disease. Primary prevention aims to prevent the occurrence of disease. Primary prevention may use screening tests to identify persons with risk factors.
4. Secondary prevention -- disease control approach based on the active identification and treatment of persons with unrecognized disease. Secondary prevention aims to prevent the occurrence of adverse outcomes from disease (such as fatal outcomes), without necessarily reducing the occurrence of disease. Secondary prevention must screen to identify persons with unrecognized disease
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Generalities
1. Screening often implies a public health related activity involving asymptomatic or healthy subjects coming from the general population.
2. Case-finding refers to special clinical efforts to recognize disease among persons who consult a health professional.
Generalities
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Issues in Screening
Disease-Disease/disorder should be an important public health problem
High prevalenceSerious outcome
-Early Detection in asymptomatic (pre-clinical) individuals is possible
-Early detection and treatment can affect the course of disease (or affect the public health problem?)
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Screening, Case finding and Diagnostic test
Terminology for testing
Target Persons
Screening Apparently healthy individuals who are not seeking health care
Case-finding To detect disease in individuals seeking health care for other reasons
Diagnostic tests
To confirm or disprove the existence of disease in patients presenting with complaints (Symptoms & signs05/02/2023
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The Principles of Screening
• The choice of disease for which to screen;
• There should be longer latent or early a symptomatic stage
• Facilities for confirmation of diagnosis must be available
• The availability of a treatment for those found to have the disease;
• The relative costs of the screening.
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• The disease must be an important health problem.
• There should be a recognizable latent or early symptomatic
stage.
• The natural history of the disease, including latent to
declared disease, should be adequately understood.
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When to screen?
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• There should be a suitable test or examination.
• The test should be acceptable to the population.
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Examples of screening• Screening the healthy people for hypertension• Screening healthy adults for diabetes• Screening of high-risk population for HIV/AIDS and Hepatitis• Screening of pregnant ladies for anemia/ Cervical cancers
etc
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Screening and diagnostic testsScreening tests Diagnostic tests
Conducted on apparently health population
Conducted on sick or with some indications
Applied to groups or communities
Applied to the patients under consideration
The initiative comes from the investigator or some agency
Initiative based on patient complaints
The objectives are predominantly preventive
The objective is to modify the treatment on basis of tests05/02/2023
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Screening and diagnostic tests
Screening tests Diagnostic tests
Based on one criterion or cut-off point
Based on clinical evaluation of signs and symptoms
Less expensive More expensive
Less accurate More accurate
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Logic of screeningApparently well population
Screening test
Negative results Positive results: Diagnostic test
Disease No diseaseDisease No disease(False negative) (True negative) (True positive) (False positive)
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True Disease Status
Screening Test
Positive Negative Total
Positive True Positives(TP)
False Positives(FP)
TP+FP
Negative False Negatives(FN)
True Negatives(TN)
FN+TN
Total TP+FN FP+TN TP+FP+FN+TN
Outcomes of a Screening Test
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• There should be an acceptable treatment for the patients
with recognized disease.
• There should be facilities for diagnosis
and treatment should be available.
• There should be an agreed policy on whom to treat as
patients.05/02/2023
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• The cost of case finding (including diagnosis and treatment of
patients diagnosed) should be economically balanced in relation to
possible expenditure on medical care as a whole.
• Case finding should be a continuing process and not a "once for all"
project.
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Uses of Screening
Case detection Objectively done to identify the unrecognized diseases e.g. neonatal screening
Control of disease Objectively done to identify the diseases to prevent transmission in the community
Epidemiology / Research
Initial screening to identify the prevalence subsequent for research purpose
Educational Opportunities
Objectively done for health education purposes e.g. screening of diabetics 05/02/2023
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Screening Strategies
Mass Screening
Screening of whole population or subgroups of population e.g. Screening of all adults for tuberculosis
High risk or Selective
Screening is applied to selectively to high-risk for a particular health problem or disease
Multiphase Screening
The people are subjected to more than one screening test. First screening for identification of suspect and second for confirmation of diseases 05/02/2023
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Types of screening
• Mass screening, no selection of population (e.g., checking all infants for hearing problems)
• Selective screening (e.g., by age and sex: mammograms for women aged over 40)
• Multiphased screening (a series of tests, as family doctors do at annual health exams)
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Latent or Incubation period
Time period lapse between the start of thedisease process up to the appearance ofsign and symptoms of disease.
Disease onset
Possible detectio
n
Final critical point
Usual time of
diagnosis
Latent/ incubation period
outcome
A B C D
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• Time between possible detection and the usual time of diagnosis by signs and symptoms is the “Lead Time”
• Time between first possible detection and the finial critical detection is the “Screening Time”
Screening time and lead time
Disease onset
Possible detectio
n
Final critical point
Usual time of
diagnosis
outcome
Screening time
Lead time
A B C D
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Concept of Latent period, Screening time and Lead time
Disease onset
Possible detection
Final critical point
Usual time of
diagnosis
Latent/ incubation period
outcome
Disease onset
Possible detection
Final critical point
Usual time of diagnosis
outcomeScreening time
Lead time
A B C D
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Summary• Screening is the testing of apparently healthy
populations to identify previously undiagnosed diseases or people at high risk of developing a disease.
• Principles of Screening: disease, test, treatment and cost.
What is the next step?
Define the validity of the screening test and
put screening to use in the population.05/02/2023
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Terms Related to Screening Tests
• Validity - relates to accuracy (correctness)
• Reliability - repeatability
• Yield - the # of tests that can be done in a time period
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Terms Related to Screening Tests (cont’d)
• Sensitivity - ability of a test to identify those who have disease
• Specificity - ability of a test to exclude those who don’t have disease
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Criteria for Evaluating a Screening Test
• Validity: provide a good indication of who does and does not have disease
-Sensitivity of the test
-Specificity of the test
• Reliability: (precision): gives consistent results when given to same person under the same conditions
• Yield: Amount of disease detected in the population, relative to the effort
-Prevalence of disease/predictive value 05/02/2023
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Validity of Screening Test (Accuracy)
- Sensitivity: Is the test detecting true cases of disease? (Ideal is 100%: 100% of cases are detected)
-Specificity: Is the test excluding those without disease? (Ideal is 100%: 100% of non-cases are negative)
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Sensitivity of a screening test
Probability (proportion) of correct classification of detectable, pre-clinical cases
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O
OO
OOO
O
O
OO
O
O
Pre-detectable pre-clinical clinical old (8) (10) (6) (14)
OO
O
O
O
OO
O
O
OO
OO
O
O
OO
OO
O
O
O OO
OO
DR Muhammad Tauseef Javed SIMS 2017
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O
OO
OOO
O
O
OO
O
O
Correctly classifiedSensitivity: ––––––––––––––––––––––––––– Total detectable pre-clinical (10)
OO
O
O
O
OO
O
O
OO
OO
O
O
OO
OO
O
O
O OO
OO
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Specificity of a screening test
Probability (proportion) of correct classification of noncases
Noncases identified / all noncases
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O
OO
OOO
O
O
OO
O
O
Pre-detectable pre-clinical clinical old (8) (10) (6) (14)
OO
O
O
O
OO
O
O
OO
OO
O
O
OO
OO
O
O
O OO
OO
DR Muhammad Tauseef Javed SIMS 2017
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O
OO
OOO
O
O
OO
O
O
Correctly classifiedSpecificity: ––––––––––––––––––––––––––––– Total non-cases (& pre-detect) (162 or 170)
OO
O
O
O
OO
O
O
OO
OO
O
O
OO
OO
O
O
O OO
OO
DR Muhammad Tauseef Javed SIMS 2017
When should we screen?Screen when:• It is an important health problem (think about how to
define ‘important’?)• There is an accepted and effective treatment• Disease has a recognizable latent or early symptomatic
stage• There are adequate facilities for diagnosis and
treatment• There is an accurate screening test• There is agreement as whom to consider as cases
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Ethics of medical care
Remember the basic ethical principles:• Autonomy• Non-maleficence• Beneficence• Justice
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Ethics in screening
• Informed consent obtained? • Implications of positive result?• Number and implications of false positives?• Ditto for false negatives?• Labelling and stigmatization
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Terms Related to Screening Tests (cont’d)
• Tests with dichotomous results – tests that give either positive or negative results
• Tests of continuous variables – tests that do not yield obvious “positive” or “negative” results, but require a cutoff level to be established as criteria for distinguishing between “positive” and “negative” groups
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How will you test the accuracy of screening test?
• Identify the screening test to be evaluated• Identify the confirmatory test for counter testing also
known as “Gold Standard Test”• Screened the population of interest by screening test• Apply counter test or Gold Standard Test to all the
positive and negative identify by screening test • Determine the accuracy by 2x2 Table analysis
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Examples of Screening and Gold Standard
Disease Screening test Gold Standard or Counter test
Diabetes Blood Glucose Glucose tolerance test
Brain tumor EEG CT Scan
Breast cancer
Mammography FNA (histopathology)
Tuberculosis
Tuberculin test Sputum for AFB05/02/2023
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0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Pre-test Probability
Post
-test
Pro
babi
lity Test=(+)
Test=(-)
X
X
X
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Sensitivity
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
Sensitivity =a
a + cTrue positive
True positive + False Negative
X 10005/02/2023
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Specificity
Specificity =D
D+ BTrue Negative
True Negative + False Positive
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Truepositive
Truenegative
Falsepositive
Falsenegative
Sensitivity = True positives
All cases
a + c b + d
= a
a + c
Specificity = True negatives All non-cases = d
b + d
a + b
c + d
True Disease Status
CASES NON-CASES
Positive
Negative
ScreeningTest
Results
a d b
c
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True Disease Status
Cases Non-cases
Positive
Negative
ScreeningTest
Results
a d
1,000 b
c60
Sensitivity = True positives
All cases
200 20,000
= 140200
Specificity = True negatives All non-cases
= 19,00020,000
1,140
19,060
140
19,000
=
= 70%
95%05/02/2023
DR Muhammad Tauseef Javed SIMS 2017
Percentage of false Positive
Percentage false positive =
bb + d
FALSE POSITIVE
FALSE POSITIVE +TRUE NEGATIVE
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Percentage of false negative
Percentage false negative =
ca + c
False Negative
True positive + False Negative
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Predictive value of positive test (PPV)
Predictive Value + test =
aa + b
True Positive
TRUE POSITIVE + FALSE POSITIVE
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Predictive value of Negative test (NPV)
PREDICTIVE VALUE – VE TEST =
dc + d
True Negative
False Negative + True Negative
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Truepositive
Truenegative
Falsepositive
Falsenegative
PPV = True positivesAll positives
a + c b + d
= a
a + b
NPV = True negatives All negatives
= dc + d
a + b
c + d
True Disease Status
Cases Non-cases
Positive
Negative
ScreeningTest
Results
a d b
c
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True Disease Status
CASES NON-CASES
Positive
Negative
ScreeningTest
Results
a d
1,000 b
c60
PPV = True positivesAll positives
200 20,000
= 1401,140
NPV = True negatives All negatives
= 19,00019,060
1,140
19,060
140
19,000
=
= 12.3%
99.7%05/02/2023
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Apparent or false prevalence
False/apparent prevalence =
A +BG. total
POSITIVE BY SCREENING
TOTAL PATIENT SCREENED
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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True Prevalence
TRUE PREVALENCE =
a +cG. total
POSITIVE BY GOLD STANDARD
Total patient Screened
X 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Accuracy of the test
Accuracy =a+d
G. total
True pos + True Neg
All screenedX 100
Dis. Yes Dis. No TotalDis. yes
a (True
positive)
b(False
Positive)
a + b
Dis. No c (False
Negative)
d (True
Negative)
c + d
Total a + c b + d Grand total
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Sensitivity and Specificity
• Sensitivity and specificity has reciprocal relationship with each other
• If we increase the sensitivity of a test specificity will be decreased
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Sensitivity and specificity At cut-off 120 mg all above those will be declared as disease
Which are included as disease by the testWhich are normal but declared as disease (b/ False Positive)Which are disease but excluded by the test (d/ False Negative)Comment on Sensitivity and specificity
60 80 100 120 140 160 180 200 220 FASTING BLOOD SUGAR LEVELS AMONG NORMAL AND
DIABETIC PATIENTS
NORMAL POPULATION
CURVE
DIABETIC PATIENT CURVE
A
AB
D
C05/02/2023
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Conclusion at cut-off value 120 mg / 100 ml
• Nearly 99% of those having diabetes will be picked up by the test that means test become highly sensitive
• The test is falsely including a large number of normal persons as the diseased increasing the false positive
• The increasing false positive means that the ability of the test to exclude those not having the disease is decreasing (decrease in specificity)
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Sensitivity and specificity
60 80 100 120 140 160 180 200 220 Fasting Blood Sugar levels among normal and diabetic
patients
Normal population
curve
Diabetic Patient curve
C
AT CUT-OFF 160 MG ALL ABOVE THOSE WILL BE DECLARED AS DISEASEWHICH ARE INCLUDED AS DISEASE BY THE TESTWHICH ARE NORMAL BUT DECLARED AS DISEASE (B/FALSE POSITIVE)WHICH ARE DISEASE BUT EXCLUDED BY THE TEST (D/FALSE NEGATIVE)COMMENT ON SENSITIVITY AND SPECIFICITY
a
c
d b
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Conclusion at cut-off value 160 mg / 100 ml
• Nearly 99% of those not having the diabetes will be excluded by the test mean test become highly specific (increasing specificity)
• The test will include many of the diseased persons as the normal increasing the false negative cases
• Increasing number of false negative mean the ability of test to pick up the diseased people is decreasing (decreasing the sensitivity)
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Sensitivity and specificity
60 80 100 120 140 160 180 200 220 FASTING BLOOD SUGAR LEVELS AMONG NORMAL AND
DIABETIC PATIENTS
NORMAL POPULATION
CURVE
DIABETIC PATIENT CURVE
B
AT CUT-OFF 140 MG ALL ABOVE THOSE WILL BE DECLARED AS DISEASEWHICH ARE INCLUDED AS DISEASE BY THE TESTWHICH ARE NORMAL BUT DECLARED AS DISEASE (B/ FALSE POSITIVE)WHICH ARE DISEASE BUT EXCLUDED BY THE TEST (C/FALSE NEGATIVE)COMMENT ON SENSITIVITY AND SPECIFICITY
ab
d
c05/02/2023
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Disease pre- Test accuracy Complication? TreatmentPopulation valence (P) (Se & Sp) Screen Diag outcome?
True positive No No GoodSe No Yes Good
Yes No GoodYes Yes GoodNo No Poor
Diseased No Yes PoorP Yes No Poor
Yes Yes Poor
False negative NoScreen (1 - Se) Yes
False positive No No(1 - Sp) No Yes
Yes NoHealthy Yes Yes(1 - P)
True negative NoSp Yes
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Conclusion at cut-off value 140 mg/100 ml
• The ability of the test to include or exclude the diseased person is nearly equal or critical (Balance sensitivity and specificity)
• The number of false positive and false negative are also in balance
• Therefore the point B is the suitable cut-off value for diabetic screening with sensitivity and specificity nearly above 90%
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Reliability of the Screening tests
What are the factors that determine the reliability of screening tests?
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Three type of factors effect the reliability of test
OBSERVER VARIATIONS
INSTRUMENTAL VARIATIONS VARIATIONS
BIOLOGICAL VARIATION
RELIABILITY
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Observational Variation
• Intra-observer Variations (variation in observation when a single observer repeat the same observation)
• Inter-observer Variation (Different observers when the same observation is repeated by different observers
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•Use of Multiple Screening Tests
Sequential (Two-stage) Testing
Simultaneous Testing
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Hypothetical Two-Stage Screening
Only Pos. Test 1 are given Test 2
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Hypothetical Two-Stage Screening (cont.)
TEST 2 (Glucose Tolerance Test)Sensitivity = 90%Specificity = 90%
DIABETES
+ -TEST
RESULTS + 315 190 505
- 35 1710 1745
350 1900 2250
Net Sensitivity = 315/500 = 63%Net Specificity = 7600 + 1710 = 98%
9500
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Predictive Value
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Prevalence & Predictive Value
Note: Test has 95% sensitivity and 95% specificity
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Specificity & Predictive Value
As specificity increases, positive predictive value
increases. As sensitivity increases, positive
predictive value also increases, but to a much lesser extent.
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Specificity & Predictive Value
AS SPECIFICITY INCREASES, POSITIVE PREDICTIVE VALUE INCREASES.
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RESULTS RELIABLE BUT
NOT VALID
RESULTS RELIABLE AND
VALID
RELIABILITY (REPEATABILITY) OF TESTS
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Study designs for screening 1. Correlation Studies
¨Use:Description of population
¨Strength:Suggest possibility of benefit
¨Limitation:Can’t test hypothesis 05/02/2023
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Study designs for screening 2. Analytical Studies
¨Types: ·Case-control·Cohorts
¨Use: ·Comparison of rates
¨Advantage:· Test hypothesis
¨Limitation: ·Selection ·Lead time· length
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Study designs for screening 3. Randomized Trials
¨Use:Comparison of rates
¨Strength:Most valid test of hypothesis
¨Limitation:Cost, ethics & feasibility 05/02/2023
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Review Questions (Developed by the Supercourse team)
• What is screening and what types of screening can you name?• What are the objectives of screening?• For what type of diseases would it be appropriate to set up
screening programs? List characteristics.• How is screening program evaluated?
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Thank You
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