Epcm l9(new) screening for diseases

EPIDEMIOLOGY & COMMUNITY MEDICINE
PREPARED BY EPCM 361 COURSE COMMTTEE
1

• Levels of prevention
• Screening: definition and types
• Characteristics of good screening test
2

Type Objective Epidemiological target Examples
Primary
prevention
the protection of health by
personal and community-
wide efforts with a focus on
the whole population
To prevent new cases of
disease occurring and
therefore reduce the
incidence of disease
Reducing exposure to
risk factors
e.g., reducing smoking
initiation in teenagers
Adding a factor that
prevents disease
e.g., vaccination, water
fluoridation
Secondary
prevention
measures available for the
early detection and prompt
treatment of health problems
To reduce the
consequences of
disease (death or
morbidity) by screening
asymptomatic patients
to identify disease in its
early stages.
mass screening
screening or case finding

Mathew J. Reeves, PhD © Dept. of Epidemiology, MSU 4
• Objective: to reduce mortality and/or morbidity by early detection and
treatment.
• Secondary prevention.
• Asymptomatic individuals are classified as either unlikely or possibly
having disease.
• Important distinction between mass or population-based screening
and case finding.
• Screening involves the examination of asymptomatic individuals who
are first classified as:
Unlikely to have disease (no further work-up required)
OR
Possibly have disease (but more diagnostic work is required
before the true disease status can be determined)

Mathew J. Reeves, PhD © Dept. of Epidemiology, MSU 5
The Pre-Clinical Phase (PCP)
• the period between when early detection by screening is
possible and when the clinical diagnosis would normally
have occurred.
Pathology
begins
Disease detectable Normal Clinical Presentation
Pre-Clinical Phase

Mathew J. Reeves, PhD
© Dept. of Epidemiology,
6
• Important to know PCP since it helps determine:
–Expected utility of screening
• Colorectal cancer = 7-10 years
• Childhood diabetes = 2-6 months
–Required minimal frequency of screening
• Mam screening women 40-49 = 1-2 years
• Mam screening women 50-69 = 3-4 years
• Prevalence of PCP indicates how much early
disease there is to detect
• Prevalence of PCP is affected by:
–disease incidence, average duration of the PCP, previous
screening, sensitivity of the test

7
Lead time = amount of time by which diagnosis is advanced
or made earlier
Pathology
begins
Disease detectable Normal Clinical Presentation
Lead Time
Screen

8
a)Noscreening-noleadtime
Clinicalphase
Preclinicalphase
5casesdevelopingover5years
0 1 2 3 4 5
Years
b)Screeningwithresultantleadtime
Leadtime
Preclinicalphase
Clinicalphase
Screen
5cases,3identifiedbyscreening
0 1 2 3 4 5
Years
(Survival = 2yrs)
(‘Survival’ = 5yrs)

9
• Equals the amount of time by which treatment is advanced
or made “early”
• Not a theory or statistical artifact but what is expected and
must occur with early detection
• Does not imply improved outcome!!
• Necessary but not sufficient condition for effective
screening.

• Screening is the process of using tests on a large scale to
identify the presence of disease in apparently healthy people.
• Screening tests do not usually establish a diagnosis, but rather
the presence or absence of an identified risk factor,
• Screening can also be used to identify high exposure to risk
factors. For instance, children’s blood samples can be screened
for lead in areas of high use of lead in paint.
10

There are different types of screening, each with
specific aims:
• Mass screening aims to screen the whole population
(or subset);
• Multiple or multiphasic screening uses several
screening tests at the same time;
• Targeted screening of groups with specific exposures,
e.g. workers in lead battery factories
• Case-finding or opportunistic screening is aimed at
patients who consult a health practitioner for some
other purpose.

• Important health problem
• High prevalence
• Natural history understood
• Long latent period
• Early detection improves prognosis

Disorder Well-defined
Prevalence Known
Natural history Long period between first signs and overt disease; medically important
disorder for which there is an effective remedy
Test choice Simple and safe
Test performance Distributions of test values in affected and unaffected individuals known
Financial Cost-effective
Facilities Available or easily provided
Acceptability Procedures following a positive result are generally agreed upon and
acceptable to both the screening authorities and to those screened.
Equity Equity of access to screening services; effective, acceptable and safe
treatment available

Evaluating a screening program is based on the following
criteria:
• Feasibility
• Reliability
• Validity
• Performance
• Effectiveness

• Acceptability
–Quick
–Easy
–Safe
• Cost effectiveness
–Screening
–Diagnosis
–Follow-up
–Intervention

Definition: the consistency of results when the screening
program is repeated on the same persons under the same
conditions.
Results may change based on:
• Biological variation
• Program method
• Intra-observer variability
• Inter-observer variability

• Cheap
• Easy to apply
• acceptable to the public
• Reliable (provides consistent results)
• Valid (it correctly categorizes people into groups with and
without disease), measured by its sensitivity and specificity.

• Sensitivity is the proportion of people with the disease in the
screened population who are identified as ill by the screening
test. (When the disease is present, how often does the test
detect it?)
–Probability to test positive among truly affected
• Specificity is the proportion of disease-free people who are so
identified by the screening test. (When the disease is absent,
how often does the test provide a negative result?)
–Probability to test negative among truly unaffected

Disease status
Present Absent Total
Positive A B A + B
Screening test Negative C D C + D
Total A + C B + D A + B + C + D
a = No. of true
positives
b = No. of false
positives
c = No. of false
negatives
d = No. of true
negatives
Sensitivity = probability of a positive test in people with the disease = a/(a+c)
Specificity = probability of a negative test in people without the disease = d/(b+d)
Positive predictive
value =
probability of the person having the disease when the test is positive = a/(a+b)
Negative predictive
value =
probability of the person not having the disease when the test is negative = d/(c+d)

Predictive value positive (PV+):- Probability to be affected among
test positives
Predictive value negative (PV-):- Probability to be unaffected
among test negatives
Percentage of correct classification PCC:- Probability to be
correctly classified

Is the screening program effective in reducing morbidity,
disability and mortality from the disease?
Outcome measures:
 Morbidity
 Disability
 Mortality
The most definitive measure is a comparison of the cause-
specific mortality rates among those discovered by screening
and those diagnosed by development of symptoms.

Epcm l9(new) screening for diseases

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