Screening Topic In Psm.pdf

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  This gives an idea of progress of a disease from its subclinical stages to overt disease HIDDEN: Subclinical cases, carriers, undiagnosed cases.

CLINICAL DISEASE

HIDDEN BURDEN OF DISEASE

The Search for unrecognized disease or defect by means of rapidly applied tests, examinations or the other procedures in apparently healthy individuals.  Earlier it was to conserve physicians time for diagnosis, administer inexpensive lab tests etc,.  But Today, Screening is considered a form of secondary prevention. It detects disease in its early asymptomatic phase whereby early treatment can be given and disease can be cured or its progression can be delayed.`

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 TEST vs DIAGNOSTIC TEST

Screening test 1. Done on apparently healthy individuals 2. Applied to groups 3. Results are arbitrary and final 4. Based on one criteria and cut-off 5. Less accurate 6. Less expensive 7. Not a basis for treatment 8. Initiative comes from investigator

Diagnostic test 1. Done on sick or ill individuals 2. Applied on single patient 3. Diagnosis is not final 4. Based on evaluation of a no. of signs/symptoms & lab findings 5. More accurate 6. More expensive 7. Used as a basis for treatment 8. Initiative comes from a patient

1. CASE DETECTION: Prescriptive screening  Defined as “The presumptive identification of unrecognized disease, which does not arise from a patients request”. Neonatal screening.

The people are screened primarily for their own benefit.

Heel Prick Blood Sample

2. CONTROL OF DISEASE: Prospective screening People are examined for the benefit of others. - Screening of Immigrants from infectious diseases like Ebola, 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. 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 about the 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 of having the disease or condition. a) Visual defects in all school children b) Mammography in women c) Colonoscopy for occult blood.

2. HIGH RISK / SELECTIVE / TARGETED SCREENING

The screening of selected high-risk groups in the population. a) Screening fetus for Down’s syndrome in a mother who already has a baby with Down’s syndrome 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 done simultaneously 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 are employed 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)  The Criteria for Screening is based on two considerations: - DISEASE

- SCREENING TEST.

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 adequately understood. 

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

 A Policy should

patients.

must exceed risks and costs.

be agreed on, concerning whom to treat as

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.

WHAT IS VALID AND RELIABLE? VALIDITY IS THE ACCURACY OF A TEST. RELIABILITY IS THE PRECISION OF A TEST.

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.

Components of 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” GOLD STANDARD Ds present

Ds absent

Test positive

Test negative

Expressed as percentage….. TP/ TP+FN.

SPECIFICITY The ability of a test to correctly identify those who do not have the disease. (True Negatives) Proportion of Truly Healthy Population. GOLD STANDARD Ds present

Ds absent

Test positive

Specificity- TN/TN+FP Test negative

Calculating

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

FALSE NEGATIVES: If a Person with disease is labeled Negative: -

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

Low Sn High Sp

FALSE POSITIVES: 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”

High Sn Low Sp

BIMODAL DISTRIBUTION

Negative

UNIMODAL DISTRIBUTION

CONCEPT OF CUT-OFF POINT - Unlike in Bimodal Distribution(Dichotomous), Some diseases comes in 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 and 20 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 also identify more false negatives. 

The cut off point that identifies more true positives will also identify 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)

MULTIPLE TESTS SEQUENTIAL TESTING (Two stage)  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 positive that actually have the disease. (Are the people with disease correctly identified?)

Negative Predictive Value: The Proportion of the people who is screened negative that are actually FREE of the disease. (Are the people without disease correctly identified?) These Values are not fixated for a particular test.

Calculating…

PPV= TP/TP+FP

NPV= TN/TN+FN

 Predictive accuracy depends onPrevalence of the Disease. Specificity of the Test. More prevalent diseases has high PPV, that’s why SCREENING is more efficient & productive, If done in High risk population. Increase in Sensitivity causes a modest increase in PPV, but increase in Specificity raises PPV markedly.

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.)

3 types of Variation

- Intrasubject Variation - Intraobserver Variation - Intraobserver 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 individual over 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 sensitivity of the screening test, participation in the programme.  Hence, yield of a screening test is high in high – risk screening.

EVALUATION OF SCREENING PROGRAMMES 1. Experimental:  Conduct an RCT of the screening test to compare the disease specific cumulative mortality rate between the intervention and control group.  Problems include- Long follow up, Costs and record keeping.  Allows study of distribution of lead time, effects of early treatment and identification of prognostic factors.

2. Non – experimental:

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

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

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

POPULAR SCREENING TESTS 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%

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

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

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