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.
Screening for Disease ControlScreening for Disease Control
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 for Disease ControlScreening for Disease Control
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 for Disease ControlScreening for Disease Control
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).
Screening for Disease ControlScreening for Disease Control
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
Screening for Disease ControlScreening for Disease Control
“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
Birth Exposure Cells Screened Symptom Death Neoplasia Exfoliate Diagnosis Diagnosis
Age of Individual 20 30 40 45 50 55 60
Natural History of DiseaseNatural History of Disease
TPCP: Begins at the initiation of disease; ends when thedisease is clinically manifested (25 years in this example)
Total Pre-Clinical Phase (TPCP)
Age of Individual 20 30 40 45 50 55 60
Natural History of DiseaseNatural History of Disease
Detectable Pre-Clinical Phase (DPCP)
DPCP: Begins when screening test is able to detect disease; Ends when disease is clinically evident (10 years)
Birth Exposure Cells Screened Symptom Death Neoplasia Exfoliate Diagnosis Diagnosis
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
Validity of Screening TestsValidity of Screening Tests
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”).
Validity of Screening TestsValidity of Screening Tests
a
dc
b
True Disease Status+ - Results of
Screening Test+
-
Sensitivity: The probability of testing positive if the disease is truly present
Sensitivity = a / (a + c)
Validity of Screening TestsValidity of Screening Tests
a
dc
b
True Disease Status+ - Results of
Screening Test+
-
Specificity: The probability of screening negative if the disease is truly absent
Specificity = d / (b + d)
Validity of Screening TestsValidity of Screening Tests
132
6365045
983
Breast Cancer+ -Physical Exam
and Mammo-graphy +
-
Sensitivity: a / (a + c)Sensitivity =
Specificity: d / (b + d)Specificity =
Validity of Screening TestsValidity of Screening Tests
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.
Validity of Screening TestsValidity of Screening Tests
SETTING THE CRITERION FOR POSITIVITY
Population
Blood Sugar
Question: What is the best cutpoint?(Depends on the price for a negative outcome)
Bi-modaldistribution
Validity of Screening TestsValidity of Screening Tests
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).
Validity of Screening TestsValidity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY & SPECIFICITY:
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.
Validity of Screening TestsValidity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY & SPECIFICITY:
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
Validity of Screening TestsValidity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY & SPECIFICITY:
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).