Lecture 17: Screening

Question 1

Screening

Answer 1

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

Question 2

Primary, secondary, and tertiary prevention strategy(screening)

Answer 2

Primary- screening for risk factors, before the disease onset

Secondary- screening for disease

Tertiary screening for complications after the disease

Question 3

Essential components of the screening program

Answer 3

Screening test(new)

Diagnostic test(gold standard)

Intervention(or treatment)

Question 4

Screening criteria

Answer 4

1. Suitable disease
2. Suitable test
3. Suitable treatment
4. Suitable screening program

Question 5

Suitable disease

Answer 5

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)

Question 6

Suitable test

Answer 6

• 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 )

Question 7

Diagnostic test accuracy studies

Answer 7

test the accuracy of the screening test against the gold standard test

Question 8

True positives and true negatives

Answer 8

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

Question 8

True positives and true negatives

Answer 8

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

Question 9

Sensitivity

Answer 9

• 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

Question 10

Specificity

Answer 10

• 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

Question 11

What is the fixed characteristic of the test

Answer 11

specificity & sensitivity

(Accuracy)

Question 12

Positive predictive value(PPV)

Answer 12

• The proportion who really have the disease of all people who test positive
• The probability of having the disease if the test is positive

Question 12

Positive predictive value(PPV)

Answer 12

• The proportion who really have the disease of all people who test positive
• The probability of having the disease if the test is positive

Question 13

Negative predictive value( NPV)

Answer 13

• 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

Question 14

relationship between prevalence and PPV & NPV

Answer 14

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)

Question 15

predictive values

Answer 15

are not fixed characteristics of the test

-reflect both test accuracy and prevalence of the disease

Question 16

Suitable treatment

Answer 16

• 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

Question 17

suitable screening program

Answer 17

• 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)

Question 18

Lead-time bias

Answer 18

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

Question 19

Length time bias

Answer 19

• 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.