evaluating screening
TRANSCRIPT
The epidemiologic approach to evaluating screening program
Dr. RizwanAsst. Professor, department of
Epidemiology
Introduction
• Primary prevention: Prevention of Occurrence of disease• Secondary prevention: Early
detection of disease
Early detection
• The term early detection of disease means detecting a disease in earlier stage that would usually occur in standard clinical practice.
• This denotes detecting disease at a pre-symptomatic stage, at which point the patient has no clinical complaint and , therefore, no reason to seek medical care for the condition.
Benefit from early detection depends on following components
• Can the disease be detected early?• What are the sensitivity and specificity of the
test?• What is the predictive value of the test?• How serious is the problem of false positive
test results?• What is the cost of early detection in terms of
funds resources and emotional impact?
Benefit from early detection depends on following components
• Are the subjects harmed by the screening test• Do the individuals in whom disease is
detected early benefit from the early detection, and is there an overall benefit to those who are screened?
Assessing the effectiveness of screening programs
Operational measures1. Number of people screened2. Proportion of target populations screened
and number of times screened3. Detected prevalence of preclinical disease4. Total costs of the program5. Costs per case found6. Costs per previously unknown case found
Assessing the effectiveness of screening programs
7. Proportion of positive screenees brought to final diagnosis and treatment.
8. Predictive value of a positive test in population screened.
Assessing the effectiveness of screening programs
Outcome measures1. Reduction of mortality in the population screened2. Reduction of case-fatality in screened individuals3. Increase in percent of cases detected at earlier stages4. Reduction in complications5. Prevention of or reduction in recurrences or metastases6. Improvement of quality of life in screened individuals
BEFORE DISEASE STARTS DISEASE PRESENT
Biologic Onset of Disease
Symptoms First
Appear
Disease Diagnosed
No Disease
ClinicalOutcome
Therapy Given
BEFORE DISEASE STARTS DISEASE PRESENT
Biologic Onset of Disease
Symptoms First
Appear
Disease Diagnosed
No Disease
ClinicalOutcome
Therapy Given
Clinical PhasePreclinical PhaseNormal
Primaryprevention Tertiary Prevention
Biologic Onset of Disease
Symptoms First
Appear
Disease Diagnosed
No Disease
ClinicalOutcome
Therapy Given
Clinical PhasePreclinical PhaseNormal
Secondary Prevention
Remove Causes Of Disease or Immunize
Treat Clinical Disease to Prevent Death or Complications
Biologic Onset of Disease
Symptoms First
Appear
Disease Diagnosed
No Disease
ClinicalOutcome
Therapy Given
Clinical PhasePreclinical PhaseNormal
Screen for Detect and Treat Disease Early
Important assumptions
• Preclinical phase• Detectable preclinical phase• Lead time• Critical pointAll or most clinical cases of a disease first go through a detectable preclinical phase.In the absence of intervention, all or most cases in a preclinical phase progress to clinical phase
BEFORE DISEASE STARTS DISEASE PRESENT
Biologic Onset of Disease
Symptoms First
Appear
Disease Diagnosed
No Disease
ClinicalOutcome
Therapy Given
Clinical PhasePreclinical PhaseNormal
Disease Detectable by
Screening
BEFORE DISEASE STARTS
DISEASE PRESENT
Biologic Onset of Disease
Symptoms First
Appear
Disease Diagnosed
No Disease
ClinicalOutcome
Therapy Given
Clinical PhaseDetectable
Preclinical Phase
Disease Detectable by
Screening
Biologic Onset of Disease
SymptomsUsual Time of Diagnosis
No Disease
ClinicalOutcome
Therapy Given
Lead Time
Disease Detectable by
Screening
A B P C D
XBiologic Onset
Positive Detection
Test Possible
Pathologic Changes
Usual Time of Diagnosis
Critical Point
Final Outcome
X XX
B C
Positive Detection
Test Possible
Usual Time of Diagnosis
NORMAL CERVIX
CARCINOMA IN SITU
INVASIVE CERVICAL CANCER
NORMAL CERVIX
CARCINOMA IN SITU
INVASIVE CERVICAL CANCER