epb phc 6000 epidemiology fall, 1997

Unit 15:Screening

Unit 15 Learning Objectives:

1. Understand the role of screening in the secondary prevention of disease.

2. Recognize the characteristics of diseases appropriate for screening.

3. Understand the impact of implementing screening on prevalence and incidence of disease.

4. Calculate and interpret measures of the validity of a screening test:--- Sensitivity--- Specificity

Unit 15 Learning Objectives (cont.):

5. Understand the relationship between sensitivity and specificity.

6. Calculate and interpret measures of the performance (yield) of a screening test:--- Predictive value positive (PV+)--- Predictive value negative (PV-)

7. Understand factors that influence PV+ and PV-

8. Recognize issues and sources of bias in evaluating screening programs.

Epidemiology in Disease Control:

Screening

Screening for Disease ControlScreening for Disease Control

Screening: The application of a disease-detection test to people who are as yet asymptomatic.

Purpose: To classify individuals with respect to their likelihood of having a particular disease.

Screening procedure itself does NOT formally diagnose illness.


Examination of asymptomatic people

likely Classification as

unlikely ….. to have a disease

Screening for Disease ControlScreening for Disease Control “Unlikely” referred to next

screening cycle “Likely” further testing for

diagnosis

yes no referred to next

treatment screening cycle


Screening Objective: To lower morbidity and mortality of the disease in a population (control, rather than elimination of disease).

Screening provides access to the medical care system which is not an actual goal of screening, but is a benefit.


Screening is important because:

1) Diagnostic and therapeutic advances are often slow, but screening may be a “direct solution” to modify history of a disease in a population.

2) It provides a model for studying disease mechanisms and the natural history of a disease.

Screening for Disease ControlScreening for Disease ControlPrimary requirements for screening:

1) Early detection of disease leads to a more favorable prognosis due to early treatment, as compared to delayed treatment.

2) Pre-clinical disease left untreated typically progresses to clinically-evident disease (e.g. no spontaneous regression).


Primary requirements for screening:

3) The disease should be serious (relates to cost effectiveness, ethics, and prognosis).

4) Prevalence of pre-clinical disease should be relatively high among those screened.

Diseases for which screening Diseases for which screening has been recommendedhas been recommended

Cervical cancer Breast cancer Prostate cancer Colon cancer Diabetes Hypertension


“PRICES” OF SCREENING:

1) Financial - may be very costly if screening is spread out over an entire population.

2) Anxiety - Individuals may have to be screened more often.

3) Some morbidity occurs - both in terms of the initial screening procedure, and

subsequent procedures.4) Creation of “lead time” morbidity.

Natural History of DiseaseNatural History of Disease

Birth Exposure Cells Screened Symptom Death Neoplasia Exfoliate Diagnosis Diagnosis

Age of Individual 20 30 40 45 50 55 60




TPCP: Begins at the initiation of disease; ends when thedisease is clinically manifested (25 years in this example)

Total Pre-Clinical Phase (TPCP)



Detectable Pre-Clinical Phase (DPCP)

DPCP: Begins when screening test is able to detect disease; Ends when disease is clinically evident (10 years)


Impact of Screening Impact of Screening on Epi Measureson Epi Measures

Screening

Time

Steady statePrevalence ofclinical disease(found byeither symptomsor screening)

Impact of Screening Impact of Screening on Epi Measureson Epi Measures

Screening

Time

Steady stateIncidence ofclinical disease

Note incidencerises, and thendrops sharplybecause the“pool at risk” istemporarilydepleted

Evaluating Screening TestsEvaluating Screening TestsCharacteristics of a 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) • Low cost, invasiveness, and discomfort • Performance (Yield)

Validity of Screening TestsValidity of Screening Tests

a

dc

b

True Disease Status+ - Results of

Screening Test+

-

a = true positiveb = false positivec = false negatived = true negative


How good is the screening test comparedwith the confirmatory diagnostic test?

The test will actually classify a diseased person as likely to have the condition (“sensitivity”).

The test will actually classify a non-diseased person as unlikely to have the condition (“specificity”).


a

dc

b


Screening Test+

-

Sensitivity: The probability of testing positive if the disease is truly present

Sensitivity = a / (a + c)


a

dc

b


Screening Test+

-

Specificity: The probability of screening negative if the disease is truly absent

Specificity = d / (b + d)


132

6365045

983

Breast Cancer+ -Physical Exam

and Mammo-graphy +

-

Sensitivity: a / (a + c)Sensitivity =

Specificity: d / (b + d)Specificity =


132

6365045

983

Breast Cancer+ -Physical Exam

and Mammo-graphy +

-

Sensitivity: a / (a + c)Sensitivity = 132 / (132 + 45) = 74.6%

Specificity: d / (b + d)Specificity = 63650 / (983 + 63650) = 98.5%

Validity of Screening TestsValidity of Screening TestsSensitivity: a / (a + c)

Sensitivity = 132 / (132 + 45) = 74.6%

Specificity: d / (b + d)Specificity = 63650 / (983 + 63650) = 98.5%

Sensitivity: Screening by physical exam andmammography will identify 75% of all true breast cancer cases.

Specificity: Screening by physical exam andmammography will correctly classify 98.5% of allnon-breast cancer patients as being disease free.


SETTING THE CRITERION FOR POSITIVITY

Population

Blood Sugar

Question: What is the best cutpoint?(Depends on the price for a negative outcome)

Bi-modaldistribution


RELATIONSHIP BETWEEN SENSITIVITY & SPECIFICITY:

1. Lowering the criterion of positivity results in increased sensitivity, but at

the expense of decreased specificity.

2. Making the criterion of positivity more stringent increases the specificity, but at the expense of decreased sensitivity.

Validity of Screening TestsValidity of Screening TestsRELATIONSHIP BETWEEN SENSITIVITY & SPECIFICITY:

3. The goal is to have both high sensitivity and high specificity, but this is often not possible or feasible.

4. The decision for the cutpoint involves weighing the consequences of leaving cases undetected (false negatives)

against erroneously classifying healthy persons as diseased (false positives).



5. In general, specificity must be at least 98%to be effective --- because misclassifying 2% of the population will create as many false positives as the sensitivity of the test will actually detect.



6. Sensitivity should be increased when the penalty associated with missing a case is high (e.g. minimize false negatives)--- when the disease can be spread--- when subsequent diagnostic

evaluations are are associated with minimal cost and risk



7. Specificity should be increased when the costs or risks associated with further diagnostic techniques are substantial (minimize false positives – e.g. positive screen requires that a biopsy be performed).

epb phc 6000 epidemiology fall, 1997

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