Lecture 17: Screening Flashcards
Screening
involves identifying risk factors for disease or unrecognized disease or complications of disease by applying tests on a large scale to a population
Not just screening for disease. Screening can be 1,2,3ary prevention stretegies
Primary, secondary, and tertiary prevention strategy(screening)
Primary- screening for risk factors, before the disease onset
Secondary- screening for disease
Tertiary screening for complications after the disease
Essential components of the screening program
Screening test(new)
Diagnostic test(gold standard)
Intervention(or treatment)
Screening criteria
- Suitable disease
- Suitable test
- Suitable treatment
- Suitable screening program
Suitable disease
common
if uncommon -when early detection & intervention can bring about better health outcomes
Knowledge of the natural history of the disease (or relationship of risk factors to the condition) - Detectable early (detectable risk factor/ disease marker)
- Increased duration of pre-clinical phase( more time to intervene)
Suitable test
- Reliable – provides consistent results
- Safe
- Simple
- Affordable
- Acceptable
- Accuracy - The ability of a test to indicate which individuals have the disease and which do not -( Sensitivity, Specificity )
Diagnostic test accuracy studies
test the accuracy of the screening test against the gold standard test
True positives and true negatives
True positives-those who test positive to the new test amongst those who have the disease(know from gold standard)
True negatives- test negative to the new test, and don’t have the disease
True positives and true negatives
True positives-those who test positive to the new test amongst those who have the disease(know from gold standard)
True negatives- test negative to the new test, and don’t have the disease
Sensitivity
- The likelihood of a positive test in those with the disease
- The ability of the test to identify correctly those who have the disease (a) from all individuals with the disease (a+c)
true positives/all with the disease x 100
The sensitivity of a screening test is high if the proportion of true positives is high
Specificity
- The likelihood of a negative test in those without the disease
- The ability of the test to identify correctly those who do not have the disease (d) from all individuals free from the disease (b+d)
True negatives/ all without the disease x 100
The specificity is high if the proportion of true negatives is high
What is the fixed characteristic of the test
specificity & sensitivity
(Accuracy)
Positive predictive value(PPV)
- The proportion who really have the disease of all people who test positive
- The probability of having the disease if the test is positive
Positive predictive value(PPV)
- The proportion who really have the disease of all people who test positive
- The probability of having the disease if the test is positive
Negative predictive value( NPV)
- The proportion who are actually free of the disease of all people who test negative
- The probability of not having the disease if the test is negative
relationship between prevalence and PPV & NPV
in populations with a higher prevalence- PPV is higher, NPV is lower(false negatives are higher)
in populations with lower prevalence- PPV is lower and NPV is higher( false positives are higher)
predictive values
are not fixed characteristics of the test
-reflect both test accuracy and prevalence of the disease
Suitable treatment
- Evidence of early treatment leading to better outcomes
- Effective, acceptable, and accessible treatment
- Evidence-based policies covering who should be offered treatment and the appropriate treatment to be offered
suitable screening program
- Benefits must outweigh harms
- RCT evidence that the screening program will result in:
Reduced Mortality
Increased Survival time
*Need to consider lead-time bias and length time bias
- Adequate resourcing and agreed policy for testing, diagnosis, treatment and programme management
- Cost-effective
- The health care system must be able to support all elements of the screening pathway
- Needs to reach all those who are likely to benefit from it (specific initiatives for particular population groups)
Lead-time bias
patients appear to live longer after screening than after diagnosis, but the earlier detection might not have any effect. Patients live longer with the diagnosis but not longer overall.
-may give a false impression of success if the program is evaluated in survival time
Length time bias
• Calculating mean survival from screened patients gives an impression of longer average survival than occurs in the population
Slowly progressing variants of diseases remain in the preclinical phase longer than the rapidly progressing variants→ screening is more likely to identify the slowly progressing cases→ the mean from the survival data is likely to show longer survival than actually is.
