Screening lecture for sims Lahore and Post Graduatestudents 2017

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

05/02/202313 DR Muhammad Tauseef Javed SIMS 2017

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


• 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


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


Usual time of

diagnosis

Latent/ incubation period

outcome

Disease onset

Possible detection


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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31

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


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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


34

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


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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


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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http://www.courseweb.uottawa.ca/medicine_health/data/Ethics_e.htm

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


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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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


Percentage of false Positive

Percentage false positive =

bb + d

FALSE POSITIVE

FALSE POSITIVE +TRUE NEGATIVE

X 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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Percentage of false negative

Percentage false negative =

ca + c

False Negative

True positive + False Negative

X 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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Predictive value of positive test (PPV)

Predictive Value + test =

aa + b

True Positive

TRUE POSITIVE + FALSE POSITIVE

X 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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Predictive value of Negative test (NPV)

PREDICTIVE VALUE – VE TEST =

dc + d

True Negative

False Negative + True Negative

X 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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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


Apparent or false prevalence

False/apparent prevalence =

A +BG. total

POSITIVE BY SCREENING

TOTAL PATIENT SCREENED

X 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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True Prevalence

TRUE PREVALENCE =

a +cG. total

POSITIVE BY GOLD STANDARD

Total patient Screened

X 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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Accuracy of the test

Accuracy =a+d

G. total

True pos + True Neg

All screenedX 100


a (True

positive)

b(False

Positive)

a + b

Dis. No c (False

Negative)

d (True

Negative)

c + d


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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


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

Üse: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

Üse: ·Comparison of rates

Ädvantage:· Test hypothesis

¨Limitation: ·Selection ·Lead time· length

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Study designs for screening 3. Randomized Trials

Üse: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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Screening lecture for sims Lahore and Post Graduatestudents 2017

Health & Medicine