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EPIB-591Screening

Jean-François Boivin29 September 2010

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Definition

SCREENING Screening was defined in 1951 by the US Commission on Chronic Illness as,

Last JM. A dictionary of epidemiology. Third edition.

“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.

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Screening: criteria

1. Disease is important (severity, frequency)

2. Pre-clinical phase3. Test is available, valid (sensitive,

specific), reliable, acceptable4. Early intervention effective5. Acceptable balance harm-benefits6. Cost-effective7. 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

LEAD TIME BIAS:

X XNo Screening Clinical diagnosis Death

X X X Screening Death

disease is detected earlier

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4 years

4 years3 years

Lead time

Survival of cases (case fatality) appears longer after screening

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1 2 3 4 5 6 7 8 9 10 11 12 13 14

Years

No screening

X X

Rate = 1 death / 20 person-years

Screening

Clinical diagnosis Death

X XDeath

Screendetected

Rate = 1 death / 20 person-years

Mortality analysis

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LEAD TIME BIAS:

X XNo Screening Clinical diagnosis Death

X X X Screening Death

4 years

4 years3 years

Lead timeImproved survival

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Length biased samplingDurations of pre-clinical cases

Screening

Prevalence = f (incidence, duration)

Years

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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 usual-care arm.

The median survival for patients with resected early-stage 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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Figure 2. Survival of patients diagnosed with lung cancer prior to July 1, 1983

Case-fatality

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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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The Lancet Vol 348 – November 30 1996

Case-fatality

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The Lancet Vol 348 – November 30 1996

Mortality rates

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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 # 1

• Cheaper

• Less effective

• Applied to 100% of population

• Saves 1000 lives

Test # 2

• More expensive

• More effective

• Applied to 50% of population (random selection)

• 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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