Transcript EPIB-591 Screening

EPIB-591
Screening
Jean-François Boivin
29 September 2010
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Definition
SCREENING Screening was defined in 1951 by
the US Commission on Chronic Illness as,
“The presumptive identification of
unrecognized disease or defect by the
application of tests, examinations or other
procedures which can be applied rapidly.
Screening tests sort out apparently well
persons who probably have a disease from
those who probably do not.
A screening test is not intended to be
diagnostic. Persons with positive or
suspicious findings must be referred to their
physicians for diagnosis and necessary
treatment.
Last JM. A dictionary of epidemiology. Third edition.
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Screening: criteria
1. Disease is important
(severity, frequency)
2. Pre-clinical phase
3. Test is available, valid (sensitive,
specific), reliable, acceptable
4. Early intervention effective
5. Acceptable balance harm-benefits
6. Cost-effective
7. Ethics, social acceptability
Institut national de santé publique, Québec, 2009
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NEJM, vol 339, #13, page 915
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The case fatality rate is the
proportion of people, among those
who develop a disease, who then
proceed to die from the disease.
Thus, the population at risk when a
case fatality rate is used is the
population of people who have
already developed the disease.
The event being measured is not
development of the disease but
rather death from the disease
Rothman 2002. Page 28
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Mortality
Mortality is the incidence of fatal cases of a
disease in the population at risk for dying of
the disease.
Fatality refers to the incidence of death from
a disease among persons who develop the
disease. The difference between fatality
and mortality is in their denominators.
Fatality reflects the prognosis of the
disease among cases, while mortality
reflects the burden of deaths from the
disease in the population as a whole.
Case fatality =
Number of fatal cases
Total number of cases
Koepsell-Weiss, Pages 50-51
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Case-fatality rate
See also Friis and Sellers
Page 455
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LEAD TIME BIAS:
4 years
X
No Screening
X
X
Clinical diagnosis
3 years
Screening
X
4 years
Death
X
Death
disease is detected earlier
Lead time
Survival of cases (case fatality) appears longer after screening
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Mortality analysis
No screening
Clinical
diagnosis
Death
X
X
Rate = 1 death / 20 person-years
Screening
Screen
detected
Death
X
X
Rate = 1 death / 20 person-years
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Years
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LEAD TIME BIAS:
4 years
X
No Screening
X
X
Clinical diagnosis
3 years
Screening
X
4 years
Death
X
Death
Lead time
Improved
survival
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Length biased sampling
Durations of pre-clinical cases
Screening
Years
Prevalence = f (incidence, duration)
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Background: The Mayo Lung Project (MLP) was a
randomized, controlled clinical trial of lung cancer
screening that was conducted in 9211 male smokers
between 1971 and 1983. The intervention arm was
offered chest x-ray and sputum cytology every 4
months for 6 years; the usual-care arm was advised at
trial entry to receive the same tests annually.
Results: The median follow-up time was 20.5 years.
Lung cancer mortality was 4.4 (95% confidence interval
[CI] = 3.9–4.9) deaths per 1000 person-years in the
intervention arm and 3.9 (95% CI = 3.5–4.4) in the usualcare arm.
The median survival for patients with resected earlystage disease was 16.0 years in the intervention arm
versus 5.0 years in the usual-care arm.
Conclusions: Extended follow- up of MLP participants
did not reveal a lung cancer mortality reduction for the
intervention arm.
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Case-fatality
Figure 2. Survival of patients diagnosed with lung cancer prior to July 1, 1983
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Table 2. Mortality in the Mayo Lung Project, as of December 31, 1996
Lung Cancer
All causes
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Case-fatality
The Lancet Vol 348 – November 30 1996
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Mortality rates
The Lancet Vol 348 – November 30 1996
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Equity
Ubel PA, et al. Cost-effectiveness analysis
in a setting of budget constraints. NEJM
1996; 334:1174-1177
→ 568 jurors, 74 ethicists, 73 decision
making experts
→ Screening for colon cancer
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Test # 2
Test # 1
• Cheaper
• More expensive
• Less effective
• More effective
• Applied to 100% of
population
• Applied to 50% of
population (random
selection)
• Saves 1000 lives
• Saves 1100 lives
Total cost # 1
=
Total cost # 2
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First results in a long-term investigation to
determine whether
periodic breast cancer
screening with mammography and clinical
examination leads to lowered breast cancer
mortality provide grounds for cautious optimism.
The study compares the experience in a random
sample of 31,000 women, aged 40 to 64 years,
offered screening examinations with the
experience in a similarly constituted "control"
group. There were 52 deaths due to breast cancer
in the control group, as compared with 31 breast
cancer deaths in the study group, in the period
available for follow-up.
The 3 1/2-year case fatality rates among women
with histologically confirmed breast cancers
reinforce the impression that screening leads to
lowered mortality. More time, possibly ten years of
follow-up, is needed to establish whether the
effect of the screening program is short-term or
long-term.
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Estimated Benefits and Harms Associated with a
10-Year Course of Screening Mammography for
2500 Women Who Are 50 Years of Age.
Welch HG. N Engl J Med 2010;363:1276-1278.
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