Screening for Diseases

Y. Vishnu Vardhan

Pg 1st Year

Community Medicine

CONTENTS

Biggest Challenge in Preventive Medicine is to distinguish between people who have the disease and those who do not..

ICEBEERG PHENOMENON

CLINICAL DISEASE

HIDDEN BURDENOF DISEASE

This gives an idea of progress of a disease from its subclinical stages to overt disease

HIDDEN: Subclinical cases, carriers, undiagnosed cases.

The Search for unrecognized disease or defect by means of rapidly applied tests, examinations or the other procedures in apparently healthy individuals.

But Today, Screening is considered a form of secondary

prevention.

It detects disease in its earlyasymptomatic phase wherebyearly treatment can be givenand disease can be cured or itsprogression can be delayed.`

Earlier it was to conserve physicians time for diagnosis, administer inexpensive lab tests etc,.

LEAD TIME:

The advantage gained by screening. (The period between diagnosis by early detection and diagnosis by other means.)

Screening programmes work better where the time lag between the disease’s onset and its final critical point are sufficiently long.

Screening test1. Done on apparently

healthy individuals2. Applied to groups3. Results are arbitrary

and final4. Based on one criteria

and cut-off5. Less accurate6. Less expensive7. Not a basis for

treatment8. Initiative comes from

investigator

Diagnostic test1. Done on sick or ill

individuals2. Applied on single patient3. Diagnosis is not final4. Based on evaluation of a

no. of signs/symptoms &lab findings

5. More accurate6. More expensive7. Used as a basis for

treatment8. Initiative comes from a

patient

SCREENING TEST vs DIAGNOSTIC TEST

1. CASE DETECTION:

Defined as “The presumptive identification of unrecognized disease, which does not arise from a patients request”.

Neonatal screening.

Prescriptive screening

The people are screened primarily for their own benefit.

Heel Prick Blood Sample

2. CONTROL OF DISEASE:

People are examined for the benefit of others.

- Screening of Immigrants from infectious diseases likeEbola, Tb & Syphilis to protect the home population.

- Screening for HIV, STD’s etc,.

Screening programme may, by leading to early diagnosis permit more effective treatment and reduce the spread of infectious disease and mortality.

Prospective screening

Ebola Check at Airports

3. RESEARCH PURPOSES:

- To know the history of many chronic diseases like cancer, HTN etc.

- Screening may aid in obtaining more basic knowledge aboutthe natural history of such diseases.

Initial screening provides a prevalence estimate and

subsequent screening provides an incidence figure.

4. EDUCATIONAL OPPORTUNITIES:

Screening programmes help in

- Acquisition of information of public health relevance.

- Providing opportunities for creating public awareness.

- For educating health professionals.

1. MASS SCREENING

Application of screening test to large, unselected population.Everyone in the group is screened regardless of the probability ofhaving the disease or condition.

a) Visual defects in all schoolchildren

b) Mammography in women

c) Colonoscopy for occultblood.

2. HIGH RISK / SELECTIVE / TARGETED SCREENING

The screening of selected high-risk groups in the population.

a) Screening fetus for Down’s syndrome in amother who already has a baby with Down’ssyndrome

b) Screening for familial cancers, HTN and DM

c) Screening for CA Cervix in low SES women

d) Screening for HIV in risk groups.

3. MULTIPURPOSE SCREENING

The screening of a population by more than one test donesimultaneously to detect more than one disease

a) screening of pregnant women for VDRL, HIV,HBV by serological tests

4. MULTIPHASIC SCREENING

The screening in which various diagnostic procedures areemployed during the same screening program.

a) DM – FBS, Glucose tolerance test

b) Sickle cell anemia – CBC, Hb electrophoresis

Before initiating a Screening Programme, a decision must be made whether it abides to all the ethical, scientific and

financial justification.

The principles that should govern the introduction of screening programmes were first enunciated by Wilson and Junger (1968)

- DISEASE

- SCREENING TEST.

The Criteria for Screening is based on two considerations:

1. DISEASE

The Disease should be important Health problem (High Prevalence)- TB

Disease should have Long & Detectable Preclinical stage.

The Natural history of disease should be adequatelyunderstood.

Appropriate test must be available for early detection of disease (before signs and symptoms appear)

Facilities must be available for diagnosis of disease (Confirmation/ Gold standard)

Early detection of disease and treatment should be able to reduce mortality & Morbidity.

The disease should be treatable, and there should be a recognized treatment for lesions identified following screening.

Expected benefits must exceed risks and costs.

A Policy should be agreed on, concerning whom to treat as patients.

2. SCREENING TEST

a) Inexpensive & Easy to Apply- (Simplicity)

b) Acceptable

c) Valid

d) Reliable

e) Yielding

SIMPLICITY

The test should be simple to perform, easy to interpret and, where possible, capable of use by paramedics and other personnel.

.

Ex: Blood and urine tests and ECG for early detection of hypertension

ACCEPTABILITY

• Since participation in screening is voluntary, the test must be acceptable to those undergoing it.

• In general tests that are painful, discomforting or embarrassing are not likely to be acceptable.

Ex: Screening for prostrate cancer might not be acceptable to a large proportion of the community.

VALIDITY IS THE ACCURACY OF A TEST.

RELIABILITY IS THE PRECISION OF A TEST.

WHAT IS VALID AND RELIABLE?

ACCURACY: “how close is result of a test to its true value?”

PRECISION: “how close are the results of a test on repetition?”

Validity determines the Accuracy of the Test.

- It expresses the ability of a test to separate those who have the disease from those who do not.

- A test with little systematic error is a valid test.

Componentsof

VALIDITY

Sensitivity

Specificity

Predictive Accuracy

SENSITIVITY

The ability of a test to correctly identify those who have the disease (True Positives)-“Proportion of Truly Ill Population”

Expressed as percentage….. TP/ TP+FN.

Ds present Ds absent

Test positive

Test negative

GOLD STANDARD

SPECIFICITY

The ability of a test to correctly identify those who do not have the disease. (True Negatives)

Proportion of Truly Healthy Population.

Ds present Ds absent

Test positive

Test negative

GOLD STANDARD

Specificity- TN/TN+FP

Calculating

An Ideal Screening Test should have 100% Sensitivity, and 100% Specificity. (Not Practically Possible)

FALSE NEGATIVES:

FALSE POSITIVES:

If a Person with disease is labeled Negative:

- False reassurance- Ignores any disease signs and symptoms- Postponement of treatment.- Detrimental to overall health

If a Person without disease is labeled Positive:

- Further testing with long, expensive tests.- Discomfort, inconvenience, anxiety- Burden on health facilities- Emotional trauma- Difficulty in “de-labeling”

Low SnHigh Sp

High SnLow Sp

Negative

BIMODAL DISTRIBUTION

UNIMODAL DISTRIBUTION

CONCEPT OF CUT-OFF POINT

- Unlike in Bimodal Distribution(Dichotomous), Some diseases comesin Continuous Variables (Ex: Diabetes, HTN). In these Cases, It is

difficult to calculate Sensitivity & Specificity.

-So, A Cut Off Point must be set to distinguish between Positive and Negative Result.

Consider 20 diabetics and20 Non-diabetics screened using

a blood sugar test – Vertical axis From Low to High.

Low Cut-Off Point

Sensitivity= 85%Specificity= 20%

- False Positives originate (More Non-diabetics are diagnosed positively)

High Cut-Off Point

Sensitivity= 25%Specificity= 90%

- False Negatives originate (More diabetics are not diagnosed positively)

SO,

Different Cut-off points yield different sensitivities and specificities.

The cut off point that identifies more true negatives will alsoidentify more false negatives.

The cut off point that identifies more true positives will alsoidentify more false positives.

The choice of a high or low cut off level for screening therefore depends on the importance we attach to FPs or FNs.

In case of Lethal diseases (Early Intervention possible) Cut off point must be set at Low level , as Greater sensitivity is required. (False Positives can be tolerated)

SEQUENTIAL TESTING (Two stage)

MULTIPLE TESTS

After the first (screening) test is conducted, those who tested positive are brought back for the second test to further reduce false positives.

Consequently, the overall process will increase specificity but with reduced sensitivity.

-Net Sensitivity and Net Specificity can be calculated for both the tests in sequence.

Net sensitivity falls, but Net Specificity will be gained.

SIMULTANEOUS TESTING

Two or More tests are conducted in parallel.

The goal is to maximize the probability that subjects with the disease (True Positives) are identified.- High Sensitivity.

Consequently more False Positives are also identified. (Specificity Low)

Net sensitivity is Gained, but Net Specificity will be lost- when Compared to either of the tests.

PREDICITVE ACCUARCY

Positive Predictive Value:

The Proportion of the people who is screened positivethat actually have the disease. (Are the people with disease correctly identified?)

Negative Predictive Value:

The Proportion of the people who is screened negativethat are actually FREE of the disease. (Are the people without disease correctly identified?)

These Values are not fixated for a particular test.

PPV= TP/TP+FP NPV= TN/TN+FN

Calculating…

Predictive accuracy depends on-Prevalence of the Disease.Specificity of the Test.

Increase in Sensitivity causes a modest increase in PPV, but increase in Specificity raises PPV markedly.

More prevalent diseases has high PPV, that’s why SCREENINGis more efficient & productive, If done in High risk population.

Reliability determines the Precision of the Test. (Repeatability)

It means that all the results of the test should be similar(Cluster at one place), when conducted each and every time.

This is not possible because of the Variations that cause the test to not yield same results every time. (like Lab equipment failure etc.)

- Intrasubject Variation- Intraobserver Variation- Intraobserver Variation

3 types of Variation

1. INTRA-SUBJECT VARIATION

This is the Variation in the results of the test conducted over time (Short periods) on the same individual.

The difference is due to the changes that occur to the individualover that time period.

Variation in BP during a 24 hour period.

2. INTRA-OBSERVER VARIATION

This is the Variation in the results of the test due to the same observer examining the result at different times.

EX: Two readings of Blood pressure by the Same observer.

3. INTER-OBSERVER VARIATION

This is the Variation in the results of the test due to the multiple observers examining the result.

EX: Chest X ray read by two different Radiologists.

TRUE VALUE

Yield is the amount of previously unrecognized disease that is detected and brought to treatment as a result of Screening.

YIELD = TP + FP / TP + FP + TN + FN

It depends on prevalence of the disease and sensitivityof the screening test, participation in the programme.

Hence, yield of a screening test is high in high – riskscreening.

EVALUATION OF SCREENING PROGRAMMES

1. Experimental:

Conduct an RCT of the screening test to compare thedisease specific cumulative mortality rate between theintervention and control group.

Problems include- Long follow up, Costs and

record keeping.

Allows study of distribution of lead time,effects of early treatment andidentification of prognostic factors.

2. Non – experimental:

-Cohort study (comparison of advanced disease or deathrates in those who choose to screen and those who do not)

-Case - control study (comparison of screening history inthose who have advanced disease and those who are healthy)

-Ecological study (correlation of screening pattern anddisease experience of several populations)

1. Mammography: Sensitivity 75 – 95%

Specificity 83 – 98%

2. PAP test: Sensitivity 29 – 56%

Specificity 94 – 100%

3. PSA test (4 ng/ml): Sensitivity 20 – 32%

Specificity 94 – 97%

4. FBS (5mmol/L): Sensitivity 85 – 89%

Specificity 70 – 77%

5. RAPID ELISA: Sensitivity 99.5%

Specificity 98%

POPULAR SCREENING TESTS

Screening, despite its flaws, is a major public Health determinant,measured by its effect on Mortality, Morbidity & Disability.

Establishing appropriate criteria requires considerable knowledge of the Natural history of disease, adequate facilities

for follow up & Rx.

It is necessary to ensure that the program is continuously monitored to confirm that effectiveness is maintained.

(benefits>costs)

Newer fields such as genetic screening are on the rise whichwould help the cause.

IT MAY JUST SAVE YOUR LIFE.ASK FOR IT.

1. Gordis, Leon. Epidemiology, 4th Edition, Chp 5, P71-95.

2. Park K. Textbook of preventive and social medicine; 23rd

Edition, Chp 8, P 113-119.

4. Oxford textbook of public health, 4th edition, Chp 12.6,P 3708-3731.

5. Suryakantha AH. Community medicine, 3rd Edition, Chp5, P294.

6. http://www.med.uottawa.ca/sim/data/Screening_e.htm

3. Bonita R, Beaglehole R, Kjellstrom.T, Basic Epidemiology;2nd Edition, Chp 6, P 93-96.

1. Gordis, Leon. Epidemiology, 4th Edition, Chp 5, P71-95

2. Park K. Textbook of preventive and social medicine; 23rd

Edition, Chp 8, P 113-119.

4. Oxford textbook of public health, 4th edition, Chp 12.6,P 3708-3731.

5. Suryakantha AH. Community medicine, Edition, Chp P.

6. http://www.med.uottawa.ca/sim/data/Screening_e.htm

3. Bonita R, BeagleholeR, Kjellstrom .T. Basic Epidemiology;2nd Edition, Chp 6, P 93-96.

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